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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #55168 (https://www.gutenberg.org/ebooks/55168)
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-The Project Gutenberg EBook of Fossils: A Story of the Rocks and Their
-Record of Prehistoric Life, by Harvey C. Markman
-
-This eBook is for the use of anyone anywhere in the United States and most
-other parts of the world at no cost and with almost no restrictions
-whatsoever.  You may copy it, give it away or re-use it under the terms of
-the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org.  If you are not located in the United States, you'll have
-to check the laws of the country where you are located before using this ebook.
-
-Title: Fossils: A Story of the Rocks and Their Record of Prehistoric Life
-       Denver Museum of Natural History, Popular Series No. 3
-
-Author: Harvey C. Markman
-
-Illustrator: Mary Chilton Gray
-
-Release Date: July 22, 2017 [EBook #55168]
-
-Language: English
-
-Character set encoding: UTF-8
-
-*** START OF THIS PROJECT GUTENBERG EBOOK FOSSILS: A STORY OF THE ROCKS ***
-
-
-
-
-Produced by Stephen Hutcheson and the Online Distributed
-Proofreading Team at http://www.pgdp.net
-
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-
-
-                                FOSSILS
-                          A Story of the Rocks
-                                  and
-                    Their Record of Prehistoric Life
-
-
-                          By Harvey C. Markman
-                  Curator of Geology and Paleontology
-
-                       Cover Design and Murals by
-                           Mary Chilton Gray
-
-
-                            DENVER MUSEUM OF
-                            NATURAL HISTORY
-
-                          Popular Series No. 3
-                        Alfred M. Bailey, Editor
-
-                        Third Edition, Reprinted
-                            October 1, 1954
-
-    [Illustration: At Work on a Dinosaur Skeleton]
-
-
-
-
-                                CONTENTS
-
-
-                                                                     Page
-  Introduction                                                          5
-  The Prehistoric Record                                                5
-      Varieties of Fossils                                              8
-      Fossilization                                                     9
-      Floras and Faunas                                                13
-      Formations                                                       16
-      Geological Time                                                  18
-      Explanation of the Time Chart                                    23
-      The Geological Section                                           25
-  Before the Age of Reptiles                                           31
-      The Pre-Cambrian Complex                                         31
-      Cambrian Life                                                    33
-      The Ordovician Record                                            35
-      Silurian Events                                                  36
-      Devonian Progress                                                37
-      Carboniferous Forests                                            40
-      Permian Hardships                                                43
-  The Age of Reptiles                                                  47
-      Dinosaurs                                                        48
-      Plant Life and Climate                                           56
-      Coal and Fossil Footprints                                       57
-      Mesozoic Invertebrates                                           58
-      Extinct Birds                                                    60
-      Ancestors of the Mammals                                         61
-  The Age of Mammals                                                   64
-      Prehistoric Horses                                               70
-      Mastodons and Mammoths                                           80
-      The Rancho La Brea Fossil Pits                                   88
-  The Age of Man                                                       93
-  Supplementary Reading                                                95
-
-
-
-
-                         LIST OF ILLUSTRATIONS
-
-
-                                                                     Page
-  At Work on a Dinosaur Skeleton                                        2
-  Fossil Bones in Rock Formation                                        7
-  Insect Fossils                                                       10
-  Restoration of Rhinoceros                                            12
-  Dinosaur Tracks                                                      17
-  Time Chart                                                           22
-  Geological Section Showing Positions of Formations                   26
-  Marine Beds of the Benton Formation                                  28
-  Plesiosaur Bones in Place                                            28
-  Invertebrate Fossils                                                 34
-  Modernized Fishes                                                    38
-  Prehistoric Plants                                                   41
-  Marine Reptiles                                                      46
-  Diplodocus                                                           49
-  Trachodon                                                            51
-  Stegosaur                                                            52
-  Sea Turtle                                                           54
-  Murals, Hall of Mammals                                              63
-  Uintatheres and Contemporary Life                                    65
-  Moropus                                                              67
-  Titanotheres                                                         69
-  Oligocene Mammals                                                    71
-  Pleistocene Horse                                                    73
-  Structure of Molar Teeth                                             75
-  Grazing Type of Molar Teeth                                          77
-  American Mastodon                                                    79
-  Long-Jawed Mastodont                                                 81
-  Molar Tooth of Mammoth                                               83
-  Nebraska Mammoth                                                     85
-  Rancho La Brea Fossils                                               87
-  Folsom Bison                                                         90
-  Man and Mammoth                                                      92
-
-
-
-
-                                FOSSILS
-
-
-
-
-                              INTRODUCTION
-
-
-In the recent growth of knowledge there has been rapid progress in two
-directions. The commercial exploitation of natural resources, being
-fundamental to modern civilization, attracts a liberal share of the
-talents and energies of workers trained for the industrial professions.
-A second trend has specialized in the further development of the
-sciences which are characteristic of our time. Such activities, in the
-natural history field, deal largely with the refinements of exact
-definition, nomenclature and classification, all of which means little
-or nothing to the layman who is otherwise engaged.
-
-For the latter, however, there is a quality of interest which may be
-described as a wholesome curiosity about what has happened, how it
-happened, how we know it happened, and what it may signify to one who is
-neither industrialist nor scientist. This booklet is intended for the
-many who feel that there is more to be obtained from a natural history
-museum than an occasional glimpse of a bewildering “marvel.” In addition
-to being a guide to fossil exhibits it supplies parts of a great story
-which specimens alone can not relate.
-
-
-
-
-                         THE PREHISTORIC RECORD
-
-
-All that is known of the extinct plants and animals which inhabited the
-earth before man began the practice of recording his observations has
-been obtained from a study of the rocks. The few possible exceptions to
-this rule, in which animal and plant remains have been preserved by
-freezing or drying, are so unusual as to be hardly worth mentioning.
-
-Explanation of this is that plant and animal tissues quickly decay under
-ordinary conditions when life ceases. Unless protected from destructive
-agencies which are especially active at the surface of the ground, even
-the heavier bones of animals and the large trunks of fallen trees will
-soon crumble into shapeless masses. The usual method employed by nature
-to prepare a fossil specimen is so closely related to the natural
-process of rock making that a little knowledge of that subject will be
-necessary in order to know what fossils are and how they are preserved
-for so long a time.
-
-It should be understood first that a fossil is some record, commonly
-preserved in rock, of a kind of plant or animal which no longer exists
-as a living type. This, at least, is the ordinary sense of the word and
-more elaborate definitions are of small service to anyone. It may be
-necessary to add, however, that all things which have lived at any time
-are regarded as either plants or animals.
-
-Nature’s way of producing rocks and fossils remains a mystery to many of
-us because we are so wrapped up with the importance of finding names for
-things and materials that we frequently neglect the consideration of
-sources and histories. Everyone knows a rock when he sees it in a large
-mass, but when he looks at sand, mud, dust, or soil, he seldom thinks of
-it as related in any way to rocks. Although the difference is almost
-entirely a matter of size, our use of words makes it seem unreasonable
-to speak of the finer particles as rock.
-
-There can be no reality or meaning in the natural record for an
-individual who has failed to observe a few simple facts which involve
-changes going on in all parts of the world at the present time. With
-regard to rocks, it is supposed that what happens in our day also
-occurred under like circumstances ages ago. Anyone wishing to do so may
-see for himself that rock masses break down wherever they are exposed to
-the elements, that the larger pieces are reduced to smaller fragments,
-and that the final product is sand or dust.
-
-He may also note that this finely ground material is being moved and
-sorted, by rain, wind, and streams, transported to lower levels and
-accumulated in great quantities wherever it finds a resting place. Along
-with it go sticks and leaves, bugs, shells, bones and carcasses of
-animals, some of which in time may become fossils. In large lakes and
-seas there is a steady distribution of such materials over broad areas,
-yesterday’s accumulation of sediments being buried by the contributions
-of today, the most recent of the settlings always resting upon older
-ones until something happens to disturb that arrangement.
-
-Not so readily observed are other parts of the process, such as the
-consolidation of sands and muds into the firm sandstones and shales
-which we again recognize as rocks. Much of this requires more than the
-few score years of a human lifetime for its accomplishment, but many of
-us have seen muds become so solidified, by merely drying, that they
-could hardly be distinguished from prehistoric shales. It is to be noted
-also that some ancient fossils come from sandstones which are scarcely
-more rock-like than the loose sands of an ocean beach. Thus we learn
-that firm consolidation of rock-making materials is not always a sign of
-great antiquity, and that hardness of rock is not always essential to
-the preservation of imbedded plants and animals.
-
-    [Illustration: Rhinocerous Bones as Found in the Rock
-
-    This exhibit was taken from the famous fossil quarry at Agate,
-    Nebraska. The fossilized remains are still partially imbedded in the
-    sandstone which preserved them for millions of years.]
-
-The rocks themselves must explain the many things which have happened
-during the course of millions of years, and this they do remarkably well
-when carefully studied, for many of the factors involved in their
-histories leave characteristic marks. Changing climates, the draining of
-seas, the uplifting of mountain ranges, all have ways of registering
-their occurrence which are as convincing and reliable as anything ever
-written by man. Piece by piece the story has been patched together
-through the efforts of thousands of investigators. Parts of the
-narrative remain buried at inaccessible depths, and whole chapters, no
-doubt, have been destroyed by the same forces that composed this
-tremendous record of prehistoric times.
-
-
-VARIETIES OF FOSSILS
-
-It would be a serious mistake to regard nature as divided into a number
-of distinct and independent schools of fossil making, each refusing to
-use the methods and devices of another. There are, however, certain
-features which stand out so prominently that a little classification
-becomes helpful. While this procedure brings out differences it should
-be understood that processes actually work together, several of them
-usually being involved in the production of any individual specimen.
-
-(1) Impressions of animals and plants, or parts of these, are frequently
-left in soft sand or mud which later becomes converted into more durable
-rock. This type of fossils is represented by animal foot-prints and the
-imprints of leaves, flowers, insects, and like objects which may be
-mingled with the finely ground materials of the common sedimentary
-rocks.
-
-(2) Parts of plants and animals may be gradually replaced by mineral
-matter with little or no change from original form and texture. Fossils
-of this class are said to be petrified or turned to stone. They are also
-known as replacements. The fleshy parts of animals do not petrify.
-
-(3) Many animals among the invertebrates use mineral substances for
-protective or supporting structures. Small plants of various kinds
-follow a similar practice. These structures, being produced in stony
-materials, are readily converted into fossils. The shells of mollusks
-are the best known illustrations in this field, and all that is required
-for a shell to become a fossil is the extinction of the species of
-animal that produced it. Fossils of this type are extremely abundant.
-
-(4) Preservative substances other than those which produce common rocks
-may be mentioned among fossil-making possibilities. Bones are known to
-have been preserved in asphalt, and insects in resins, but such cases
-are few in comparison with the products of other methods.
-
-(5) In rare instances there has been preservation of extinct creatures
-by the process of drying or by refrigeration. Occasional mummies are
-found with shriveled flesh and skin still in place, but better
-preservation of all tissues occurs when the temperature is quickly
-reduced below freezing point and held there without interruption. This
-can happen only in the colder parts of the earth and is always subject
-to climatic change. The effect of drying also may be undone at any time
-by a slight increase in the amount of moisture.
-
-(6) Coal beds often produce fossils of an unusual sort. In the formation
-of coal, plant material gradually loses some of its more perishable
-substances but retains carbon which has better lasting qualities and
-slowly accumulates to produce the seams and beds that are mined. In the
-early stages of the process the original vegetation undergoes little
-change in appearance but eventually practically all of its character is
-lost. Many fossil leaves are found as thin layers of carbon, bedded in
-the clays which are commonly associated with coal deposits.
-
-(7) Concretions, which are hardened lumps of mineral substances
-occurring commonly in sandstones and shales, are often mistaken for
-fossils because of their peculiar shapes. However, there are localities
-in which the mineral solutions have been concentrated and deposited
-around shells, leaves, seeds, or similar objects, thus producing an
-abundance of fossils which may be obtained by opening the concretions.
-Fossils of this type are well known from Mazon Creek and other districts
-in Illinois, Kansas, Colorado, and elsewhere.
-
-
-FOSSILIZATION
-
-Footprints need little explanation other than a consideration of the
-factors which make it possible for them to be preserved. The sand or mud
-must be neither too soft nor too hard to take the form of the foot and
-retain its shape when the foot is withdrawn. Then in some manner the
-impression must be protected while the rock-making process goes on. When
-such protection is obtained it is usually in the form of more mud and
-sand, deposited over the surface which received the impression. At a
-later time the covering may be separated from the lower part of the
-deposit, which serves as a mold, and if the separation be accomplished
-successfully a natural cast of the foot will be obtained as well as the
-mold in which it was produced. Since conditions for perfect work are not
-always present in a laboratory of this kind, it is not surprising that
-fossil footprints are very rare considering the number and variety of
-tracks left by wandering animals.
-
-Impressions of leaves are explained in much the same way except that the
-leaf remains under its protective covering until it decays. Similar
-impressions may be obtained from the bodies of delicate invertebrate
-animals but they are seldom preserved because of the softness of the
-tissues. The smaller fishes provide much better material for the
-production of fossils according to this method. While the fish is being
-flattened by the weight of surrounding sediments, scales, fins, and soft
-bones retain their positions and provide the necessary resistance to
-leave an impression of the body form when the flesh is gone.
-
-    [Illustration: Insect Fossils (enlarged)
-
-    Fine specimens of this type are obtained from an old lake bed at
-    Florissant, Colorado.]
-
-The larger and more spectacular fossils, such as skeletons, skulls, and
-detached bones are nearly always of the replacement type. Replacement of
-plant and animal substances by mineral matter is a slow process and in
-younger fossils the change is rarely completed, some of the original
-material being present in a partially altered condition or not modified
-at all. Since air does not often carry the necessary materials and
-provide other essential conditions, replacement may be regarded as
-something which happens underground or in water. It is perhaps best
-explained in connection with limestones, because calcite or “lime” is
-frequently the replacing substance although other minerals, especially
-quartz, may serve the purpose.
-
-Besides converting bony or woody objects into rock substance, mineral
-replacements may assist in the production and preservation of fossils in
-another manner. It often results in the filing of cavities with some
-rock-making substance which retards destruction through crushing or
-other injury. In many cases, so-called fossil shells are not shells at
-all; instead, they are merely a stony filling which was once surrounded
-by shell substance. In other instances the original shell remains as it
-was during the life of its former occupant, preservation of the shell
-being due largely to the substitution of a mineral filler for the soft
-animal tissues once present.
-
-Limestone comes into existence through a more elaborate process than
-that which produces sandstone and shales. It is one of the three types
-of common rocks, known collectively as the sedimentaries, in which
-fossils are found. It differs from sandstones and shales, however, in
-that much of its substance has been dissolved in water instead of being
-transported in the form of finely ground rock particles. Lime occurs in
-many varieties of rock which are exposed to the wear and tear of the
-elements throughout the world. Slowly but more or less continuously it
-is taken from this source by ground and surface waters coming in contact
-with it. Particularly active is carbonated water, moving underground
-through pores and crevices.
-
-This underground circulation of mineral matter in a dissolved condition
-explains the occurrence of fossils in land areas which have not
-necessarily been submerged during any great length of time, for it is
-well known that plant and animal remains are not invariably washed into
-lakes or seas, and that all sedimentary deposits have not been built up
-in large bodies of water. Here we are dealing with what is known as the
-continental type of sedimentation and such fossils as dinosaurs,
-mastodons, three-toed horses, and other former inhabitants of land
-areas.
-
-In order to become properly fossilized, certain conditions are
-absolutely necessary, and only a small percentage of the once-living
-multitude secures the required treatment. There must be present, soon
-after death, some protection from the activities of the carnivorous
-birds and beasts that would separate and scatter the parts of a carcass,
-also from the smaller gnawing animals that would continue the
-destruction, and finally from wind, sun, rain, frost, and bacterial and
-chemical activities which in the course of only a few years would remove
-everything but possibly a few scraps of tooth enamel, which is the
-hardest of animal tissues.
-
-A slight covering of earth substance in any form serves to check the
-disintegration, and this may be acquired in several ways. Animals that
-perish in bogs or quicksands are soon covered over; in many localities
-wind-blown dust and sand do the work; and flooded river valleys provide
-an abundance of mud for the necessary burial of others. Even
-underground, the decay of soft tissues is too rapid to permit of
-replacement by mineral substance in a manner that would reproduce form
-and texture. Skin and flesh are almost invariably lost, although in a
-few instances the thick scaly hides of dinosaurs are known to have
-produced natural molds and casts by the method explained in connection
-with footprints and other impressions.
-
-    [Illustration: The skeleton of this prehistoric American rhinoceros
-    is mounted in a “half shell” which was modeled over the bones to
-    show the form of the living animal. The artist’s reconstruction
-    appears in the painting above the fossil exhibit.]
-
-With regard to the more durable tissues found in the teeth, bones, and
-shells of animals, or the woody parts of plants, the case is different.
-These parts become firmly imbedded in the ground, but moisture still has
-access, and it begins to work immediately; for all water moving
-underground finds soluble substances which it picks up and carries with
-it wherever it goes, and much of the load consists of mineral matter
-which may be unloaded again when the necessary conditions are found.
-
-Mineral-laden waters will drop one kind of substance to take up another
-which dissolves more readily, and this happens sooner or later when a
-buried bone or log is encountered. Complications of various sorts enter
-into the process, but the final outcome frequently is a complete change
-from one chemical composition to another which is more enduring, the
-transformation being brought about so gradually and thoroughly that in
-many fossils the inner structure of the original tissue is as accurately
-reproduced as the fine detail of surface features.
-
-Converted into stone, however, the result is still far from permanent.
-While yet underground the fossil is subjected to distortion and breakage
-due to earth movements which bend and dislocate the rock deposits. What
-causes these upheavals and depressions of the earth’s surface remains
-the subject of much discussion, but that they have occurred on a large
-scale and continue to occur is clearly evident. At higher altitudes the
-surface rocks and fossils are exposed to a larger variety of destructive
-activities than at lower levels where protective coverings are more
-likely to be provided and retained. Once stripped of that protection
-there is little chance for a fossil to survive. Beyond a doubt there are
-many thousands of tons of prehistoric remains damaged or destroyed each
-year, by weather and stream erosion.
-
-
-FLORAS AND FAUNAS
-
-As the various types of sediments continue to accumulate on land and in
-water they produce deposits of sandstones, claystones, and limestones
-which in time may acquire great thickness and cover wide areas of sea
-floor, or continental surface. Usually there is more or less mixing of
-sediments resulting in sandy limestones, limy clays, and other
-combinations. Quite commonly, however, the types remain fairly pure but
-become arranged in layers which alternate from one kind of material to
-another. At all times the character of the deposit will depend upon the
-nature of the rocks which supply the materials, and any fossils that may
-be produced will consist of such plants and animals as live and die
-during the time the rock is in the making.
-
-Some of the rock layers will be rich in plant and animal remains, others
-quite barren, the difference being due partly to conditions influencing
-the life of the region. In addition, the character and amount of
-rock-making materials at the time may be favorable or unfavorable to the
-preservation of fossils. Seas, lakes, and valleys may at any time be
-drained, or enlarged and deepened, by changes in the elevation of
-underlying rocks. The amount and variety of mineral substances dissolved
-in the waters of a region not only affect the character of rock deposits
-but also the plants and animals living in the water. Some of these
-chemical solutions provide cementing materials which bind together the
-grains of sands and mud; others have a detrimental effect upon cementing
-material previously deposited, and so construction and destruction go on
-continuously, more or less hand in hand, to produce complicated and
-often puzzling results.
-
-A little more salt, or a little less of it, may change completely the
-variety of life inhabiting a body of water. A slight change in the depth
-of the water often accomplishes the same thing, for plants and animals
-are so delicately adjusted to their environments that conditions fatal
-to one race of creatures may provide the exact life requirement of
-another. This is a matter of practical knowledge which is being used
-today in the cultivation of plants and animals for market purposes. It
-is being demonstrated continuously, also, upon living subjects in
-experimental laboratories throughout the world; and, in a bigger way,
-the facts are observable wherever life is considered in relation to
-habitat. That anything so obvious should be regarded as guesswork or
-theorizing, or opposed to truth, when applied to former inhabitants of
-the earth, is somewhat surprising. And, it may be added, the cultural
-worth of fossil study comes to a focus on this very point, for men and
-women are now meddling, consciously or unconsciously, wisely or
-unwisely, with an all-important environment about which they have
-learned very little—one called, among other things, “civilization.”
-
-For any portion of the world a complete-list of the different kinds of
-plant inhabitants comprises the _flora_ of that region, and a like
-summary for the animal life is known as the _fauna_ of the district. It
-is generally understood that different species of both plants and
-animals inhabit different regions of the earth, but outside of
-professional circles it is only beginning to be recognized that changes
-in floras and faunas occur from time to time, that slight differences
-may be noted in the course of observations extending over a period of
-only a few years, and that everything in a fauna or flora eventually may
-be displaced by new forms.
-
-It is, however, a convenient practice to use these terms in connection
-with time periods, rock beds, and types of environment, as well as
-geographical areas. Thus we have such phrases as a “Cretaceous fauna”
-(attaching the name of a geologic period), a “Benton fauna” (with
-reference to the fossils of a rock formation), a “marine flora” (using
-the name of an environment), an “Arctic flora” (which applies to a
-definite portion of the earth surface and its plant inhabitants).
-
-Faunas include animals which many persons do not recognize as such.
-Sponges, corals, insects, worms, crabs, oysters, and a host of other
-boneless creatures are grouped together as _invertebrate_ animals, while
-another group includes the fishes, amphibians (toads, frogs, and
-salamanders of today), reptiles (crocodiles, lizards, snakes, and
-turtles being well known varieties), birds, and mammals. This second
-lot, provided with backbones and skeletons, comprise the great division
-of _vertebrate_ animals.
-
-Floras also include types which are commonly seen but not popularly
-identified as plants. The algae are perhaps best known as seaweeds,
-water-silk, and pond scums; fungi as toadstools and moulds. Both groups
-are large and of important rank in the vegetable kingdom; only the
-algae, however, are recognized as important fossil producers. Better
-known types of plants are the mosses, ferns, evergreens, grasses, and
-the more conspicuous flower-bearing forms, from weed size to tree size.
-
-Many rocks owe their character to the work of large colonies of plants
-or animals, for the living organisms are frequently the active agency
-which takes dissolved mineral substance from the solvent liquid and gets
-it back into solid form. The liquid is, of course, the water in which
-the creatures live, while the mineral substance often becomes a
-commodity required by a plant or animal in its mode of living. Mollusks
-have a way of using lime in the production of shells, and many a bed of
-limestone consists almost entirely of this by-product of molluscan life.
-Tiny coral polyps build complicated and beautiful structures from the
-same mineral substance. Either intact or in broken condition, these
-structures contribute in a large way to the making of limestones. Algae,
-among the lowliest of plants, have done extensive work along similar
-lines, and numerous invertebrate animals could be named as important
-factors in the production of rocks. Many of the shells and other
-fabrications retain their peculiar patterns long after the extermination
-of their makers, and a highly informative part of the fossil record is
-provided in this manner. It is also by far the larger portion of the
-record, for the earlier ages of prehistoric time failed to produce a
-vertebrate animal of any kind, while the invertebrate record dates back
-to pre-Cambrian time.
-
-
-FORMATIONS
-
-If in some part of North America there had been steady accumulation of
-sedimentary materials under constantly favorable conditions since the
-beginning of Cambrian time, the result would have been a deposit of
-sandstones, claystones, and limestones measuring nearly fifty miles from
-bottom to top. These figures are based on actual production in North
-America where extensive measurements have been made in many localities.
-When other parts of the world are as thoroughly investigated and older
-deposits included in the calculations, the total thickness of such beds
-will probably be more than one hundred miles.
-
-No single pile of rocks offering a complete cross section of the
-geological record has ever been produced, but portions of the section
-are exposed to view on all the continents. In order to carry on
-desirable investigations and make comparisons, it has been necessary to
-divide this great composite section into small units which may be named
-in some way and placed definitely with relation to lower and higher, or
-older and younger, layers. To serve this purpose there has been
-developed the idea of rock _formations_, and here we have a word which
-is not defined readily, even for the use of those who are familiar with
-it. Nevertheless it is used so commonly that some understanding of its
-meaning becomes desirable.
-
-A _formation_ may be regarded as an extensive rock mass, variable, in
-thickness and other proportions, as well as in composition, but
-representing a period of time during which there was no great change in
-the character of plant and animal life, and no serious interruption in
-the depositing of the rock-making materials. Occasionally the lower and
-upper limits of a formation are well defined and readily located.
-Frequently, however, the transition is gradual, one formation merging
-into another with no apparent mark of separation. In such event the
-original description serves to establish more or less definitely the
-boundaries of a formation.
-
-Descriptions are published whenever a worker believes he has discovered
-a significant part of the great section which has not previously been
-named. The usual practice is to apply a name taken from the locality in
-which the beds were investigated, and in this manner the names of
-formations become associated with towns, rivers, counties, mountains,
-states and other geographical features. The locality which supplies the
-name is then regarded as the “type locality” for the formation, but
-wherever these same beds may be traced or otherwise identified the one
-formation name applies.
-
-    [Illustration: Dinosaur Tracks
-
-    An ancient trail in sandstone of the Dakota formation. East slope of
-    the hogback, west of Denver.]
-
-The “Dakota formation,” to use a convenient illustration, is mentioned
-in scores of reports bearing on the geology of Colorado, Iowa, Kansas,
-Nebraska, New Mexico, Texas, Utah, and Wyoming, as well as the Dakotas.
-On the geological map of Colorado it appears on both sides of the
-Rockies, scattered in strips and patches from north to south boundary
-lines. The beds are easily located in the foothills district west of
-Denver because of their tendency to produce the prominent ridges known
-as hogbacks.
-
-Many formations are exposed over much less territory, some have even
-greater extent. Thickness may vary from a few inches to thousands of
-feet, and no two exposures will be exactly alike though some similarity
-necessarily prevails throughout. “Exposures” are simply portions of the
-beds which are not concealed by loose rock, soil and vegetation, or
-overlying formations. Canyon walls, steep cliffs and mountain slopes,
-gullies, and badlands provide a large variety of natural exposures. In
-such places rocks and fossils may be studied to best advantage.
-
-Since a formation may contain a variety of beds, including sandstones,
-shales, limestones, and all sorts of mixtures, there is sometimes need
-of subdividing it; but formations are the smallest units commonly shown
-on geological maps. They are actual rocks which fit into a historical
-scheme of things and may be regarded aptly as the pages of a book which
-nature has done in stone.
-
-
-GEOLOGICAL TIME
-
-“How old are they?” “How can you learn their names from the rocks?”
-These are typical examples of questions most frequently asked concerning
-fossils. The second question follows the usual reply to the first, for
-prehistoric plants and animals are as old as the rocks in which they are
-found. The answer, as to age, must come from the rocks and what we have
-learned about them through many years of hard work, thoughtful
-observation, and careful study. Names, however, come from a different
-source. Nature, apparently, managed for a long time to carry on without
-the use of words. Since man began talking he has had no trouble
-inventing names for things which interest him.
-
-Early students of rocks and fossils likewise accomplished a great deal
-without being able to date events in terms of years although many of
-their efforts and interests centered on the problem of discovering a
-continuous sequence of events in the fragments of evidence that had been
-uncovered. This relatively simple problem has not been fully worked out,
-and some of the breaks in the record are recognized as “time gaps” which
-may never be converted into history.
-
-The question of time, expressed in years, has been a puzzle which
-attracted some attention even in the earliest days of investigation. Its
-solution was attempted by several methods long before there was
-sufficient information to make them work satisfactorily, which accounts
-in part for the extreme variation in results of the calculations. Even
-now it is to be expected that changes will have to be made as long as
-pertinent studies are continued. Two of the most promising methods of
-investigation in late years have been producing figures which are
-surprisingly large. More accuracy than ever before is probably present
-in modern estimates but, except for comparatively recent time, there is
-yet no way of knowing within a range of millions of years when a
-creature lived.
-
-Astronomy and physics were used in early calculations but, although
-taken seriously by some geologists, it was soon recognized by others
-that certain events revealed by earth history could not be explained
-with so short a time allowance as these methods indicated. One of the
-first estimates provided a total of only twenty-five millions of years
-and included a great stretch of time during which the earth, according
-to prevailing theory, was more sun-like than rock-like, a time when
-planets were being born and the earth could not have been in its present
-physical condition, which is the chief concern of the geologist. Since
-those earlier conditions could not have supported life as we know it,
-our knowledge of cosmic history renders small service in the study of
-fossils.
-
-Among the methods suggested by astronomy and the laws of physics is one
-which is based on the probable rate at which the earth cooled from its
-molten condition to present temperature. It is believed now that the
-heat of the earth is not necessarily due to an original molten state and
-that a steady rate of cooling cannot be ascertained. Any figures based
-on such procedure, therefore, are discredited today.
-
-The amount of salt in the oceans, and the time required for its
-concentration there by natural processes, offers another way of
-attacking the problem. It is a well known fact that salt is being added
-to the seas at a fairly constant rate; sea water, then, must become
-saltier from year to year. The salt comes from rocks exposed on land
-surfaces and is transported by the rivers which drain these areas. By
-analyzing the river waters it is possible to estimate the amount of salt
-annually dumped into the oceans and, also by chemical analysis, it is a
-comparatively simple matter to figure the total amount now present in
-the oceans. Some recent calculations indicate that thirty-five million
-tons of salt are being added each year, and this figure divided into the
-total amount for all the years places the age of the oceans at three
-hundred sixty millions of years.
-
-However, there are certain other factors which complicate the problem.
-For instance, it is known that land areas exposed to surface drainage
-have not always been of their present size, and the annual production of
-salt by the different types of rocks exposed at various times in the
-history of the earth has not always been as it is now. The rocks also
-must be older than the oceans, but how much older cannot be determined
-by means of figures obtained in this way.
-
-Until the beginning of this century there was little anticipation of a
-better measuring stick than one in use at the time which placed its
-reliance on the total thickness of the sedimentary deposits and the
-length of time required to produce this great accumulation of material
-which is known as the geological column. Since the total thickness, or
-height of the column, was not accurately known, and with recognized time
-gaps to bridge, there was little hope of working out a complete
-chronology by this device, but it has supplied highly desirable and
-reliable information concerning parts of the record.
-
-The system has been somewhat improved since its earliest use, and one of
-its latest applications gives us an age, for known sedimentary rocks, of
-nearly half a billion years, this being based on a total thickness of
-one hundred miles and an average rate of 880 years for the building up
-of one foot of sediments. Its greatest weakness is due to the absence of
-a reliable factor to take care of long stretches of time in which the
-sedimentary rocks are known to have been subjected to destructive
-processes. A yardstick of this character cannot be applied to rocks that
-have been destroyed, and there are excellent reasons for believing that
-these interruptions may account for several times the lapse of years
-indicated by the amount of rock remaining in the column which has been
-pieced together.
-
-Following the discovery of radium, however, the present century provided
-a new field of knowledge which has contributed greatly to the
-measurement of geologic time. The penetrating rays produced by radium
-and other radioactive substances are due to extremely slow but violent
-disintegration of the material. Uranium and thorium are radioactive
-elements which occur in the rocks of many parts of the world. There is
-little or no loss of material as the so-called disintegration proceeds;
-instead there is a complicated series of transformations in which other
-elements are produced, radium itself being one of these. Helium and lead
-eventually take the place of the less stable elements and the known rate
-at which these products accumulate provides the highly desired key to
-the age of the rocks.
-
-Part of the gas, helium, may escape, but except in rare instances where
-chemical alteration might occur, there probably is no loss of lead.
-Fortunately, when this metal is produced by radioactivity it differs
-slightly in atomic weight from ordinary lead; otherwise the presence of
-the latter would introduce a misleading factor. Since the speed at which
-the change goes on cannot be increased or decreased, it is assumed that
-throughout past ages it has never been faster or slower. The amount of
-such change that has been completed in any body of radioactive minerals
-may be measured by techniques employed in physics and chemistry. If it
-is found that the amount of helium or lead present requires a hundred
-million years for its production at the working speed of the parent
-elements, the mineral deposit must be at least that old.
-
-Certain conditions of course complicate the problem seriously: knowing
-the age of a piece of rock which happens to contain some radioactive
-element is of small service in historical studies unless the rock can be
-definitely associated with a flora or fauna, or some outstanding event
-disclosed by geological investigations. But there have been a few
-instances in which most of the necessary conditions were present, and
-more and better opportunities to apply this method will no doubt appear.
-Other elements, or their radioactive isotopes, are already being
-employed with good results. Some of these, such as carbon 14, are more
-sensitive indicators for the accurate dating of events in comparatively
-recent time.
-
-When it can be used, this type of measurement is far less subject to
-uncertainties than any other. It promises to eliminate all need for
-guessing, and comes close to a degree of accuracy which is satisfactory
-to the scientist, a person who thoroughly dislikes uncertainties of any
-kind. If suitable material can be found in just the right places it
-should accomplish what the preceding method cannot do—the accurate
-measurement of the great time breaks which interrupt the geological
-record in many places. Something along this line already has been
-accomplished, for radioactive material has been found in some of the
-oldest of the rocks. Regardless of the destruction going on in other
-localities, these rocks have continued to register the passing of time,
-and a tremendous antiquity for the earth and some of its first
-inhabitants has been indicated.
-
-Tests made on radioactive minerals from Gilpin County, Colorado, have
-established the age of late Cretaceous or early Cenozoic rocks at sixty
-million years, providing a convenient and reasonably accurate date for
-the beginning of the Age of Mammals. In Russia, one of the oldest
-mineral deposits yet studied in this way and regarded as early
-Pre-Cambrian, produced the astonishing figure of 1,850,000,000 years;
-what we commonly refer to as geological history may therefore be
-regarded as covering a range of approximately two billions of years. The
-earth, in some form or other, has in all probability passed through an
-earlier history of another billion years or more.
-
-Wherever we may roam, a portion of the prehistoric record is to be found
-in the rocks underfoot and not far from the surface. Formations as
-already mentioned may be regarded as the pages—often torn and badly
-scattered—of nature’s own book, in which the geological periods are
-chapters. But instead of numbering these pages and chapters we have
-_named_ them, in order to get the parts reassembled in orderly fashion
-and restored to a condition which makes the book legible. However, the
-names cannot render the service intended except in connection with a
-time chart and an outline of earth history.
-
-    [Illustration: GEOLOGICAL TIME
-    Figures to the left denote millions of years that have elapsed up to
-    recent time]
-
-  CENOZOIC
-     Age of Man
-             RECENT              Man and his Culture
-          1  PLEISTOCENE         Last of Mammoths & Mastodons
-     Age of Mammals
-          7  PLIOCENE            Horses modernized
-         20  MIOCENE             Grasses and Grazing Animals
-                                 Three-toed Horses, Rhinos, Camels
-         35  OLIGOCENE           Specialization of Primitive Ancestors
-         60  EOCENE              Decline of archaic types
-                                 Mammals flourishing
-  MESOZOIC
-     Age of Reptiles
-        125  CRETACEOUS          Last of Great Reptiles
-                                 Specialization of Dinosaurs
-        160  JURASSIC            Bony Fishes thriving
-                                 Flowering plants advance
-                                 Cycads
-                                 Birds and Flying Reptiles
-        200  TRIASSIC            Few small mammals of lower orders
-                                 Dinosaurs become prominent
-  PALEOZOIC
-     Age of Amphibians
-        225  PERMIAN             Reptiles advancing
-                                 Amphibians dominant insects
-        300  CARBONIFEROUS       Dense forests of spore-bearing plants
-     Age of Fishes
-        350  DEVONIAN            Shark-like Fishes
-                                 Land floras established
-        375  SILURIAN            First land animals (scorpions)
-                                 Armored Fishes prominent
-     Age of Invertebrates
-        425  ORDOVICIAN          Corals and Bryozoa
-                                 Progress among Mollusks
-        500  CAMBRIAN            Brachiopods gaining
-                                 Trilobites dominant
-                                 Advance of shelled animals
-  PROTEROZOIC
-     EARLIEST LIFE
-       1000  UPPER PRE-CAMBRIAN  Small marine invertebrates
-                                 Lowest Forms of Plant and Animal Life
-                                 Few Fossils
-  ARCHEOZOIC
-       2000  LOWER PRE-CAMBRIAN  Some chemical evidence of life
-                                 No fossils
-
-Such aids have been devised and revised from time to time. No figures
-have been offered as final or absolutely “right” since the beginning of
-scientific investigations. Time divisions have been proposed that are
-not yet in common use while others have been abandoned or modified.
-Sources of information are so numerous that appropriate credit cannot be
-given fairly for anything that is up-to-date. The combined chart and
-outline here provided is based on time calculations of recent date but
-with figures slightly rounded off for the sole purpose of making them
-easier to remember. In view of the still existent probability of error
-it is felt that the slight alteration of figures may justify itself. It
-need not be regarded as misleading if the present purpose be
-considered—the stimulation of a natural history interest which is not
-vitally concerned with the little difference between a thousand million
-years and nine hundred ninety-nine million years.
-
-
-EXPLANATION OF THE TIME CHART
-
-The whole of geological time has been divided and subdivided according
-to varying practices. The development of life is perhaps the one
-outstanding feature of the time divisions, but for the most part the
-changes in floras and faunas have been gradual rather than abrupt, and
-this makes it very difficult to draw sharp lines or to visualize
-beginnings and endings of the various stages of development.
-Occasionally there is good excuse for drawing a line, where the record
-is broken and resumed again after a long lapse of time. The principal
-cause of such breaks is the elevation of great land masses, which brings
-on an interval of erosion and surface destruction for the areas
-uplifted.
-
-These movements of parts of the earth’s crust have been exceptionally
-pronounced at certain times, often culminating in the production of
-mountain systems, and because of the extreme changes they introduce are
-known as revolutions. The major divisions of prehistoric time have been
-established, at least in part, by such _revolutions_; crustal, climatic,
-or other _disturbances_, on a smaller scale and recurring with greater
-frequency, may be regarded as establishing boundaries for the minor
-divisions. Hence we have five great Eras of geological history, and
-these are divided again into Periods. The time chart shows an
-arrangement commonly used in America. In the first column the names of
-the Eras are stated in technical form. Closely coinciding with these
-terms are the popular names of the Ages which appear in the second
-column. These names, describing the dominant life of each age, are very
-convenient. The more scientific terms used for the eras, while serving
-essentially the same purpose, are a little more systematic and
-generalized in that they refer to ancient life (Paleozoic), middle life
-(Mesozoic), and recent life (Cenozoic), without being specific as to any
-class of animals or plants for any one division of time.
-
-The period names, in the central column, have been derived from
-miscellaneous sources, some of them from geographical districts, some
-from descriptive references to prominent features of the rocks, others
-indicating a degree of approach to recent time. In paleontology (fossil
-study) it has long been a practice to cut the periods into lower,
-middle, and upper divisions, and in a few cases it has been found
-desirable to make two periods out of an old one. What was once known as
-the Lower Carboniferous is now commonly recognized as the Mississippian
-period while the upper portion has become the Pennsylvanian. The Lower
-Cretaceous is now the Comanchean of some authors.
-
-Both old and new practices are responsible for a little confusion at the
-present time. A former division into Primary, Secondary, Tertiary, and
-Quaternary eras has been partly abandoned, but the term “Quaternary”
-still applies to the Age of Man, while “Tertiary time” remains in good
-usage for the balance of the Cenozoic era. Among the newer introductions
-may be mentioned the use of a Paleocene period which precedes the
-Eocene. Geologists are not entirely in agreement as to the necessity for
-this addition and many would not give it equivalent rank with other
-periods. In the interest of simplicity these modern refinements have
-been omitted from the chart.
-
-The figures appearing in the third column, between the Ages and Periods,
-indicate the millions of years that have elapsed up to present time.
-They denote the age of the rocks at the beginning of each period. The
-age of a plant or animal which lived in Eocene time would be, according
-to this scale, somewhere between 35 million and 60 million years. In
-practice it is usually possible to determine whether a fossil was
-embedded in the rocks during an early or late portion of the period, and
-thus its age may be established within a shorter range, but it is
-impossible to be exact, even in terms of millions of years, with regard
-to anything as far back in prehistory as the Eocene period.
-
-The period in which we are living today is known as Recent. It began at
-the close of the Ice Age or Pleistocene period about ten thousand years
-ago and represents so little of earth history since the beginning of
-life that a chart many times the length of this page would be required
-to show the rest of the periods in proportion. The Cambrian period is an
-early chapter in which the story of prehistoric life suddenly becomes
-clear and richly varied. It is, however, much farther from the beginning
-of the record than it is from the present, and the Pre-Cambrian eras
-would require a great deal more space in order to show their relative
-lengths. The Archeozoic and Proterozoic eras have to some extent been
-divided into periods, but the great antiquity of the rocks has obscured
-much of their history, and divisions established for one locality have
-been of little service elsewhere. Consequently, the period names are in
-less general use and the common practice is to refer to all this great
-stretch of time as Pre-Cambrian.
-
-In the last column, at the right of the chart, some of the historical
-features are indicated. This column should be read from bottom to top in
-order to get the proper development of the story, and at best this
-sketchy outline of events can be no more than suggestive of the progress
-and decline through which the earth’s inhabitants have passed.
-
-Rocks of every period except probably the Silurian are known to have
-been deposited somewhere in the Colorado area, although in most cases
-the record for each period is far from complete. Formations are too
-numerous and too varied locally to be shown on a chart of this type.
-
-
-THE GEOLOGICAL SECTION
-
-In the study of fossils there are two important field aids usually
-available. For any locality there should be a geological map and a
-section showing the sequence and character of the strata. On a
-small-scale map many of the local details have to be omitted, but the
-position of the larger exposures is indicated and, with this information
-at hand, the fossil-bearing strata may be located with the help of a
-geological section. The latter is frequently obtained from technical
-reports published by State and National Geological Surveys. Frequently,
-however, it is possible to obtain only a general plan for a given
-locality, and a great deal of literature may have to be scanned in order
-to get that. Excellent geological maps of Colorado have been published
-by the Colorado Geological Survey and the United States Geological
-Survey.
-
-It often happens that a formation is not where we expect to find it,
-this being due to several possible factors. The sediments may not have
-been deposited there, or they may have been removed by erosion. Where
-the structure has been disturbed by folding and faulting, a multitude of
-complications is introduced. The expected sequence is sometimes inverted
-and repeated through a series of folds. Formations also may be moved
-miles out of place by faulting. Both thickness and character of
-sediments may vary considerably within a formation. In some regions the
-geology is very simple, in others extremely difficult to understand.
-
-    [Illustration: FORMATIONS of the DENVER-FOOTHILLS REGION
-    A GENERALIZED SECTION SHOWING SOME OF THE SURFACE FEATURES
-    RED BEDS    HOGBACKS    TABLE MOUNTAIN    DENVER DISTRICT
-
-REGION OF MOUNTAIN-MAKING UPLIFT
-
-Formations bordering the mountains have been bent into upright
-positions.]
-
-                PERIODS
-
-  RECENT
-  PLEISTOCENE
-  PLIOCENE
-  MIOCENE
-  OLIGOCENE
-  EOCENE
-  CRETACEOUS                                              THICKNESS
-  SOFT SANDSTONES GRITS & CLAYS        DENVER & ARAPAHOE   2000 ft.
-  SANDSTONES, SHALES & LIGNITE         LARAMIE             1000 ft.
-  YELLOWISH SANDS & SHALES             FOX HILLS           1000 ft.
-  SOFT DARK GRAY OR RUSTY SHALE        PIERRE              5000 ft.
-  LIMESTONES & SHALES                  NIOBRARA             500 ft.
-  DARK SHALES & LIME                   BENTON               400 ft.
-  GRAY OR BUFF SANDSTONES & CLAYS      DAKOTA               300 ft.
-  SHALES, SANDSTONE & LIME             MORRISON             200 ft.
-  JURASSIC
-  TRIASSIC
-  PERMIAN
-  DEEP-RED SANDY SHALES, LIME, GYPSUM  LYKINS               700 ft.
-  CARBONIFEROUS
-  MASSIVE PINK OR WHITE SANDSTONE      LYONS                200 ft.
-  RED OR BROWN SANDSTONE &             FOUNTAIN            1500 ft.
-  CONGLOMERATE
-  DEVONIAN
-  SILURIAN
-  ORDOVICIAN
-  CAMBRIAN
-  PRE-CAMBRIAN
-  METAMORPHIC & INTRUDED ROCKS         IDAHO SPRINGS
-  SCHIST, GNEISS, QUARTZITE            (PART)
-  BASEMENT ROCKS of IGNEOUS ORIGIN
-
-A generalized section for the western part of the Denver Basin is
-introduced here for the use of local students. The formations normally
-present in this region are shown in their usual position. They are
-briefly described on the chart, and their thickness is indicated by
-figures which may be regarded as near the maximum for the district. The
-section will apply to most of the foothills area between Morrison and
-Boulder though surface features and thickness of beds will vary
-considerably from place to place.
-
-Certain of the formations are known to be fossil bearing, others barren
-or nearly so. When fossils are present they are usually restricted to
-certain localities, and these may be widely scattered. The following
-remarks apply to the possibilities for finding fossils in the formations
-named.
-
-
-                         _Denver and Arapahoe._
-
-Leaf impressions of palms, ferns, and numerous species of well-known
-trees and shrubs are common in many localities. Petrified wood is fairly
-abundant, and a few scattered bones of reptiles and mammals have been
-found. The two formations are treated as a unit because the Arapahoe is
-neither conspicuous nor sharply defined. Denver beds are well exposed on
-the slopes of Table Mountain at Golden; fossils, however, have been
-obtained from several localities nearer the city of Denver, notably from
-the hills just west of Overland Park.
-
-
-                               _Laramie._
-
-Plant material is locally abundant, principally the leaves of familiar
-deciduous trees, palms, and ferns. Many of the clay pits being worked
-near Golden are in this formation. Oysters and a few other mollusks may
-be found in some places.
-
-
-                              _Fox Hills._
-
-Better exposures of this formation are located to the north of Denver.
-Marine mollusks are most frequently found.
-
-
-                               _Pierre._
-
-In addition to the characteristic dark shales, this formation includes
-some limy material and sandstone beds, both of which are fossiliferous
-in places. Two types of marine mollusks are characteristic:
-_Inoceramus_, generic name for several species of clam-like bivalves
-readily identified by concentric elevations which produce a rippled
-effect on the shell surfaces; and _Baculites_, cephalopods with
-straight, chambered shells which often break at the suture lines, where
-the fossil is weakened by the chamber walls. Small oyster shells are
-fairly common also. The formation is to be found some distance to the
-east of the prominent hogback where it weathers into smooth surfaces in
-the form of broad valleys and flats, with rounded contours on the few
-elevations that may be present. It forms a soft, flaky soil when dry, is
-a sticky “gumbo” when wet. The clay is generally of a rather dark
-grayish color when freshly exposed but it takes on a rusty appearance
-after weathering. At various levels there are numerous iron-cemented
-concretions, many of which contain fossil shells.
-
-    [Illustration: Marine Beds of the Benton Formation, Northeastern
-    Colorado
-
-    A stream channel has cut deeply into the formation, uncovering and
-    partly destroying a plesiosaur skeleton which was found at the level
-    where the men are standing.]
-
-    [Illustration: Plesiosaur Bones in Place
-
-    Benton formation. Surface rubble has been cleared away, and several
-    vertebrae are partially uncovered in the area at the right of the
-    hammer.]
-
-
-                              _Niobrara._
-
-The formation contains fossils rather similar to those of the Pierre.
-Shark’s teeth have been found in some of the lower beds. Limestone is a
-prominent feature, often forming a well defined ridge near the foot of
-the eastern slope of the main hogback. The limestones commonly have a
-chalky character.
-
-
-                               _Benton._
-
-The formation is not especially productive in this region. Marine shells
-are numerous in some localities, and bones of marine reptiles have been
-found at various places. As usually seen, it is almost entirely composed
-of impure clay shales, very dark, brownish-gray to almost black, and
-commonly interbedded with thin patches of white bentonite, yellow ochre,
-gypsum, and limestone.
-
-
-                               _Dakota._
-
-This formation produces the high hogback which is usually present some
-distance east of the Red Rocks. There are generally two or three layers
-of massive, light-colored sandstone separated by clays which are used
-extensively in the making of bricks and pottery. Leaf impressions and
-some fish scales are found in the clays and occasionally in the
-sandstone. The hogback is a good marker from which to locate other
-formations, because of its prominence in the foothills landscape.
-
-
-                              _Morrison._
-
-Good dinosaur material has been taken from the Canon City and Morrison
-districts. The formation is to be found on the lower west slope of the
-Dakota hogback. It consists of continental deposits of the stream and
-lake types. There is considerable sandstone in this formation and a
-little limestone is to be found here and there, but the most
-characteristic feature is in the shales. When freshly exposed, the
-shales are delicately tinted with gray, green, and maroon, a
-bronze-green being rather prominent. This formation is highly variable
-in character, with much of the clay often buried under the valley floor.
-In addition to the bones of reptiles, there are plant fossils, usually
-of poor quality, and fresh-water gastropods more or less abundant in
-some localities.
-
-
-                               _Lykins._
-
-Outcrops are not prominent, owing to the small amount of
-weather-resisting materials. The sandy clays are commonly of a deep red
-color mottled with spots of light gray. A white limestone is sometimes
-present near the middle of these deposits, and gypsum beds are included
-locally. The formation is often indicated only by red soil in the
-depressions between ridges. Few fossils have been reported.
-
-
-                                _Lyons._
-
-This formation is usually prominent as the eastern wall of the uplifted
-Red Rocks series. In some localities it forms a ridge of pink or white
-sandstone distinctly separated from the older sediments to the west.
-Very few fossils are found.
-
-
-                              _Fountain._
-
-Exposures usually are brown to red in color, though sometimes a dirty
-white. The prominent rocks are rather coarse sandstone, commonly with a
-gritty texture due to the angular character of the sand or gravel from
-which they were made. These are the westernmost of the Red Beds and the
-oldest of the uplifted sedimentary rocks bordering the foothills in most
-of our area. Fossils have been found in the formation, but it is
-practically barren for the territory here considered.
-
-                            * * * * * * * *
-
-This geological section also illustrates a method of dating crustal
-movements and the birth of mountain ranges, for the folding of the
-strata along the flanks of the Rocky Mountains has a great deal of
-significance in this connection. The sedimentary layers were originally
-deposited over much of the present mountain area in a horizontal
-position, and only those formations in existence at the time could be
-distorted by the upheavals which produced the new elevations. Of the
-series generally involved in the movement the Laramie beds are the
-youngest. Since these beds had not been formed until near the close of
-the Cretaceous period it is to be assumed that the mountains must be of
-more recent date, younger than the topmost of the deformed beds and at
-least as old as the lowermost of the undisturbed formations overlying
-them.
-
-Some disturbance is evident also in the Arapahoe and Denver beds which
-overlie the Laramie, but this is believed to have occurred sometime
-after the occasion of the first great uplift. Volcanic materials in
-these beds lead to the belief that the sediments were deposited during a
-period of volcanic activity brought on by the crustal folding which
-terminated the Mesozoic era. Hence the conclusion arises that the age of
-the Denver and Arapahoe beds must coincide closely with some of the
-earlier stages in the history of the mountain system. This interval is
-often referred to as Post-Laramie time.
-
-
-
-
-                       BEFORE THE AGE OF REPTILES
-
-
-THE PRE-CAMBRIAN COMPLEX
-
-The rocks of Pre-Cambrian time have been buried deeply under the
-accumulation of younger sediments, and the resulting pressure in many
-places has been tremendous. In addition to the effects of pressure there
-also is recorded in these ancient formations the repeated movements of
-the materials since they were first deposited. Vertical and side
-adjustments of parts, with relation to other parts, have distorted the
-original arrangement of the rock particles to such an extent that
-ordinary fossils would eventually become unrecognizable. These crushing,
-grinding, and kneading forces working through millions of years alone
-would account for the absence of fossils from the older deposits.
-Frequently the rocks have become so changed in form that their original
-character can only be conjectured, and because of this change they are
-known as metamorphic rocks.
-
-A few beds of Archeozoic age remain in nearly their original condition,
-but they are either without fossils or they have produced very
-questionable and unsatisfactory specimens. The existence of life during
-these early stages of earth history is indicated largely by chemical
-rather than fossil evidence. Much of the ancient limestone has been
-converted into marble, but it is not unreasonable to believe that plants
-and animals were instrumental in the production of this type of rock as
-they are today. Certain varieties of iron ore deposits are now being
-built up by the aid of plants, and similar ores in the ancient rocks may
-have had a like origin. The presence of great quantities of carbon, in
-the form of graphite, may be regarded also as a sign of life, for this
-substance is accumulated on a large scale by living plants, and may be
-retained in a solid form after the partial decay of the plant tissues.
-
-So far as the direct evidence goes, there is no sign of any creature of
-large size or of such complicated structure as the common plants and
-animals of today. The chemistry of the mineral deposits is not entirely
-convincing as to the presence of life, but it is regarded as highly
-probable that microscopic, single-celled plants and animals, comparable
-to modern algae and protozoa, were in existence during Archean time.
-Throughout later eras there is unmistakable evidence of gradual
-development from simpler to more elaborate life-forms and the Archeozoic
-is commonly regarded as a time of preparation during which simple
-organisms of some kind were becoming adapted to early conditions which
-could not support life on a higher plane. The importance of the work
-done by such lowly creatures in the preparation of suitable environments
-for more advanced modes of living is overlooked almost entirely.
-
-During the next era, the Proterozoic, the record of life becomes
-somewhat clearer. Fossils are hardly to be regarded as abundant but
-there were several well-defined types of animals which left shells and
-other parts composed of mineral matter. Among these may be mentioned the
-Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced
-delicate, often lace-like shells of many patterns adorned with the
-radiating filaments or spines which have suggested the name for this
-group. Foraminifera produced minute shells, sometimes many chambered,
-and often bearing a confusing resemblance to the work of snails. Common
-chalk is composed almost entirely of such shells and fragments of them.
-
-Sponges and Bryozoa are animals of slightly higher organization. They
-are many-celled instead of one-celled and the cells have special work to
-perform, which is a most important step in the direction of the
-specialization which characterizes the structural and life pattern of
-later arrivals. The Bryozoa lived in moss-like colonies which have been
-important rock-makers; the fossil forms bear some resemblance to corals.
-Sponges are too well known to require description although the familiar
-article of commerce is merely the framework of once-living animals. They
-represent the earliest organization of true animal bodies even though in
-appearance they may have a resemblance to plants.
-
-Actual plants of this era were of the algae class, aquatic in habit as
-were their animal neighbors, the first to leave a record in the form of
-fossils. This record, obscure and distorted, has long been a source of
-perplexity to investigators. Without well-defined floras and faunas to
-guide them, and with rocks frequently in chaotic relationships, early
-geologists were content to regard it all as a “Pre-Cambrian complex.”
-Recent studies have contributed a great deal of information not
-available some years ago. It is quite possible that more advanced types
-of life were in abundance before the close of the second era, but
-material on which to base sound opinion is still scarce.
-
-Rocks of Pre-Cambrian age are plentiful in the foothills region west of
-Denver. The schists, gneisses, and quartzites exposed for some miles
-immediately beyond the red-beds are part of this great complex. The
-Idaho Springs formation is known to be one of the oldest in this
-district, although its exact age has not been determined. Other
-formations are recognized among the metamorphic rocks of the region but
-none has contributed to our knowledge of early life.
-
-
-CAMBRIAN LIFE
-
-There can be no mistake as to the prolific development of life in
-Cambrian seas, for fossils of this age are to be found in many parts of
-the world, where ancient sea bottoms now form part of the land surface.
-Invertebrate animals appear to have made much progress, but plants were
-either scarce or too small and delicate to be productive of fossils. It
-is probable, however, that seaweeds and other algae were flourishing
-along with the invertebrates, because animal life is directly or
-indirectly dependent on the existence of plants. The latter sustain
-themselves by taking carbon and nitrogen from air, water, and soil, but
-animals must obtain their requirements by eating plants or eating each
-other. They cannot obtain what they need from the inorganic world
-without this help from the vegetable kingdom.
-
-One group of animals stands out prominently above all its
-contemporaries. Known as the trilobites they were by far the most
-distinguished and most characteristic of Cambrian invertebrates.
-Trilobites inhabited the warmer seas of this period and several later
-ones, but were extinct by the end of the Paleozoic era. Hundreds of
-species have been described, most of them under four inches in length.
-Well-known distant relatives now living are the shrimps, and other
-crustaceans. The name Trilobite has reference to the three lobes which
-are apparent in the form of the upper surface, the central lobe forming
-a broad ridge extending along the back. Beneath the outer lobes on each
-side there was, during life, a row of short, jointed legs used for
-swimming and walking, but these delicate appendages are seldom preserved
-in the fossils.
-
-Second in importance among the animals of the period were the
-brachiopods or lamp-shells, not true mollusks although they were
-provided with similar shells composed of calcium phosphate or calcium
-carbonate. Shells are of two parts (bivalved) as in the case of clams,
-but the valves are above and beneath the body instead of on the right
-and left sides, which is the arrangement among mollusks. Although
-abundant as individuals, there were only a few species during the
-earlier part of the period; the number of species increased, however,
-and the race became very persistent. About seven thousand species have
-been described, and the race is not yet extinct although the number of
-living species is relatively small.
-
-Cambrian life evidently included representatives of all the great
-divisions of invertebrates; sponges, jelly-fishes, worms, and primitive
-corals have been reported. At the end of the period there was an
-elaborate molluscan fauna. The closing of the period in North America
-was apparently a gentle elevation of continental areas and a consequent
-withdrawal of the sea.
-
-    [Illustration: Invertebrate Fossils
-
-    Only a few prominent types have been selected from thousands of
-    invertebrates known to zoologists. The forms illustrated are of
-    frequent occurrence as fossils.]
-
-  CRINOIDS
-  CEPHALOPODS
-    Coiled types
-    Ammonite
-    Scaphite
-    Straight-shell type
-    Baculite
-  TRILOBITE
-  BRACHIOPODS
-  BIVALVES
-    Inoceramus
-    Oyster
-  GASTROPODS
-    Snail-like Univalves
-  PROTOZOA
-    UNICELLULAR FORMS
-      Radiolaria (Microscopic)
-      Fusulina limestone
-      Foraminifera (Enlarged)
-    MULTICELLULAR FORMS
-      Cup coral
-      Reef coral
-      Sponge
-      Bryozoa
-
-
-THE ORDOVICIAN RECORD
-
-Extensive land areas must have subsided again early in the Ordovician
-period for marine sediments were laid down over a large part of the
-North American interior, and three epochs or subdivisions of the period
-have been based on as many invasions of the sea. In these ancient
-deposits the record of life continues to show new forms. Nothing of a
-very spectacular sort is recorded other than a great increase in the
-number of species among types that were established in earlier periods.
-
-Trilobites were at their best, brachiopods continued to flourish, and
-the mollusks made new progress, especially the cephalopods, a group
-which includes our cuttle fishes and squids. Some of these predatory
-creatures attained large size and were no doubt masters of the sea.
-Typical forms were provided with tapering chambered shells that
-occasionally reached a length of twelve or more feet. Most of the shells
-were straight and trumpet-like or but slightly curved. Some were closely
-coiled and in this respect more like the well-known nautilus now in
-existence.
-
-The bryozoans became very common in the later part of the period and
-corals made slight advances. Somewhat of a novelty at this time were the
-crinoids, commonly known as “stone lilies” although not plants at all.
-They have been described as starfishes with back turned downward and a
-thick stem attached beneath. Where they lived in great abundance the
-limestone deposits may consist almost entirely of their stems. Crinoids
-continued to produce a variety of forms throughout several of the
-succeeding periods.
-
-The brachiopods were commonest of all animals representing this period,
-however, and their wide distribution, together with their known
-preference for warm waters, is taken to be an indication of mild
-temperatures prevailing over a large portion of the earth. Land plants
-are indicated by spore-bearing forms related to the ferns and mosses.
-Impressions of such plants have been found in Europe but, since most of
-the known rocks of this age were formed in seas, the marine algae are
-more abundant as fossils.
-
-In the Colorado area, rocks of Ordovician time are exposed only in
-mountainous areas where they have been lifted high above their original
-levels. They are not especially rich in fossils although they have
-produced some fish remains which are of interest in that they suggest an
-age of vertebrates which is just ahead.
-
-
-SILURIAN EVENTS
-
-Since land floras and faunas had not yet become conspicuous the fossil
-record for this period is limited to those areas which were invaded by
-the sea. Apparently there was no such invasion of the present Colorado
-region, for rocks of this age are not in evidence. If they exist at all
-they are restricted to localized districts which are deeply buried under
-sediments of later periods. There may have been no Silurian deposition
-in this area, or such rocks may have been produced only to be destroyed
-by elevation and consequent subjection to weathering and erosion during
-a long interval of time, in which they were completely removed. In the
-region of the Colorado Rockies there is no evidence of returning seas
-until late Devonian time.
-
-In other parts of the world, however, there was extensive deposition of
-rock-making sediments in seas which were inhabited by algae and
-invertebrates of the types previously described. Among the common
-animals of the time there were still numerous species of brachiopods,
-trilobites, corals, crinoids, and bryozoans. In addition to the
-primitive cone-shaped, cup corals there were several advanced types but
-the habit of building large reefs was not yet established.
-
-“Sea scorpions,” really large crustaceans, flourished during Silurian
-time, and late in the period there appeared a race of true scorpions
-which lived on dry land or between high and low tides along the
-seashore. These were smaller and much like modern descendants but
-probably they did not wander far from the ocean shores where an
-abundance of food was available. These little scorpions, the largest
-measuring only two and a half inches in length, are the oldest
-air-breathing land animals of the fossil record.
-
-It was not until the period was well advanced that fishes became
-numerous, and much of our knowledge of the beginning of an “Age of
-Fishes” has been obtained from European fossils. Although fishes are
-classed with the vertebrate or backboned animals there are large groups
-which do not have bony skeletons but are provided instead with a simple
-framework of cartilage. Among the earlier and more primitive types were
-the ostracoderms or bony-skinned fishes with no internal bones and only
-a small amount of bony substance in the armor-like plates and scales
-which covered the forward portion of the body.
-
-The ostracoderms comprise a small group of fishes about which very
-little is known. They appear to have been inhabitants of fresh-water
-streams as well as lagoons bordering the seas, and may have been related
-to the small sharks of the time. They lived during the Ordovician,
-Silurian, and Devonian periods, and left no descendants now recognized
-among living creatures. A much larger type of armored fishes is known as
-the arthrodires, a name which refers to a pointed neck and an
-arrangement of the armor plates to permit a movement of the head. These
-were the most ferocious fishes of the Silurian and Devonian seas, some
-of them reaching a length of twenty feet though most were much smaller.
-Their jaws were provided with formidable shearing and crushing plates
-instead of teeth.
-
-
-DEVONIAN PROGRESS
-
-The Devonian is one of the most outstanding of all periods from the
-viewpoint of life development. Dominance of the fishes is its greatest
-achievement, the invertebrates remaining about as they were and the
-higher vertebrates barely in evidence, but life on a large scale was no
-longer confined to the seas. Fresh-water fishes became prominent and
-land plants well established. The first forests appeared, with fern-like
-plants predominating although woody trees of several types and
-considerable size were included. It is quite possible that extensive
-land areas had been well supplied with vegetation during earlier times,
-but the delicate tissues of plants are far less likely to be preserved
-than the limy parts of animals. The fossil record, therefore, cannot be
-expected to reveal more than a suggestion of the progress made at this
-level of living. The story of plant life becomes much clearer in the
-next period when conditions were more favorable for the production and
-preservation of plant fossils.
-
-Land animals of the time are almost unknown. A few snails and scorpions
-have been found, and some footprints made by early amphibians. Insects
-probably were in existence although the evidence is not quite clear on
-this point. The increasing number of fresh-water fishes, however, may be
-regarded as a sure indication that inland conditions were becoming more
-favorable for plant and animal inhabitants of all kinds.
-
-The extent of development among the fishes cannot be accurately
-indicated by naming a few types, for it is mainly in the number of
-species and genera within the larger groups that progress is seen. In
-general it may be stated that the fishes of the period had not yet
-acquired the bony skeleton and typical form of familiar modern species.
-Skeletons were of cartilage, partly hardened in some instances by lime.
-Armor plates were customary with certain races but were not present
-among all fishes. Neither were these armored forms exceptionally large,
-as compared with living sharks. Although occasional giants appeared, the
-majority were small. Many were sluggish creatures with poorly-developed
-jaws, living as scavengers on sea and stream bottoms. Tail fins were
-usually unbalanced as in the sharks, or pointed and rounded rather than
-evenly forked.
-
-    [Illustration: Modernized Types of Fishes from Eocene Shales of
-    Southwestern Wyoming]
-
-The great tribe of true bony fishes, such as the cod and perch, which
-includes more than ninety percent of the fishes living today, was not
-yet in existence. About one-third of the many kinds of fishes then
-living were related to the sharks, a group which is relatively
-insignificant in recent years. Nearly one-fourth of the total belonged
-to a tribe of enamel-scaled fishes, now represented only by a few
-sturgeon and gar-pike.
-
-Lung fishes have never been a large group but it is noteworthy that they
-have had existence since Middle Devonian time. Living members of the
-race, inhabitants of Africa and South America, make a practice of
-burrowing into the mud of stream channels during dry seasons and are
-provided with lungs which enable them to breathe air in the manner of
-higher vertebrates. They survive the complete drying-up of the streams
-and live for months without water. Other forms, with less development of
-lungs, frequent stagnant pools and come to the surface occasionally for
-a breath of air. All are provided with gills also, which enables them to
-obtain their oxygen as other fishes do. They are believed to be a
-connecting link between the fishes and the early amphibians. More
-accurately, perhaps, they should be regarded as holding an intermediate
-position without being directly ancestral to any higher type of
-vertebrate animal.
-
-Still dominant among the invertebrates were the brachiopods, on the
-whole averaging a little larger in size, and otherwise indicating
-congenial times for that type of organism. They reached the peak of
-their development during this period. Trilobites were declining although
-a few new and strangely ornamented varieties made a brief appearance.
-Crinoids apparently found living conditions less favorable during
-Devonian time, but in a later era they again became prominent. Corals
-were favored only at times and in certain localities. Along with the
-crinoids they appear to have suffered from the presence of an unusual
-amount of mud in the waters of their customary habitats. Both had a
-preference for clear water as indicated by the absence of fossils from
-limestones containing more than a very small percentage of muddy
-sediments. Crustaceans, similar to the sea-scorpions and better known as
-eurypterids, became prominent among fresh-water animals. Some were
-unusually large for creatures of this class, lengths of several feet
-being recorded from fragments. Gastropod mollusks came into prominence
-in localities where living conditions were favorable. Bivalves continued
-to thrive but the cephalopods had a rather meager development
-considering the heights they were to achieve in subsequent periods.
-
-In western North America the large expanse of territory known as the
-Great Plains was evidently well above sea level during this entire
-period, for no beds of this age are found in eastern Colorado. West of
-the Front Range, however, there was some deposition of marine sediments
-during late Devonian time. Formations of this age are exposed near
-Salida and Glenwood Springs, on the White River Plateau, and in the San
-Juan region.
-
-The Carboniferous period gets its name from the vast deposits of coal
-which were developed during that time in many parts of the northern
-hemisphere. Depressed land surfaces bordering the continents, and
-extending well into the interior of present boundaries, supported dense
-growths of vegetation and provided the swampy conditions most favorable
-to coal production. Varieties of plants which are now of small size and
-lowly position in the botanical world acquired the proportions of large
-trees.
-
-
-CARBONIFEROUS FORESTS
-
-Best-known fossils of the period are carbonized portions of the larger
-trees, and impressions left in the muds and sands of ancient bogs.
-Forest trees of several kinds reached the height of a hundred feet, with
-a trunk diameter of two to six feet. This size often is exceeded in
-modern forests, but by trees of an entirely different type. Considering
-the amount of development among the plants of earlier periods,
-Carboniferous forests provide an outstanding spectacle of advancing
-life.
-
-Quite common among the larger trees were two varieties of club-mosses,
-also known as scale trees. They were cone-bearing evergreens with only
-slight resemblance to modern conifers. Instead of seeds they produced
-spores, a method of reproduction which is practiced among ferns. The
-trunks were marked from bottom to top with uniform patterns of cushions
-and scars indicating the points at which leaves were attached during the
-earlier stages of growth. In the Lepidodendrons the rows of scale-like
-cushions wind spirally upward while among the Sigillaria there is a
-vertical arrangement of leaf-scars which resemble the imprints of a
-seal, these impressions being in straight and parallel rows on a surface
-which may be either ribbed or smooth. The leaves of scale trees were
-stiff and slender, and arranged in grass-like tufts at the top.
-
-Calamites, related to our horsetail rushes, were somewhat smaller than
-the scale trees. Their trunks consisted of a thin, woody cylinder with a
-pithy interior, and were marked at intervals by nodes which gave them
-the “jointed” appearance of a bamboo stem. Leaves were arranged in
-circles around the nodes of main stem or branches. Spore-bearing cones
-appeared at the tips of the stems.
-
-    [Illustration: Prehistoric Plants
-
-    Some of the larger and better known plants of past ages are shown as
-    reconstructed by artists. Finer details of the reconstructions often
-    have to be neglected because of uncertainties due to the scattered
-    and fragmentary character of the fossil record.]
-
-  LEAF IMPRESSIONS
-    Carboniferous Ferns
-    Strap-leaf Conifer (Cordaites)
-  MODERNIZED TYPES
-    Sequoia Cone and foliage
-    Miocene Fossils (Florissant Shales)
-      Maple
-      Willow
-    Eocene palm (Denver Beds)
-  HORSE TAIL RUSHES
-    Restoration (Calamites)
-    Fossils Leaves and Stem
-  CYCADS
-    Restoration
-    Fossil Trunk
-  CLUB MOSSES
-    Restoration (Sigillaria)
-    Fossils Trunk Impressions
-  SCALE TREES
-    Restoration (Lepidodendron)
-    Fossil Leaf scars
-
-Also included among the larger trees were the Cordaites or large-leaved
-evergreens, tall and slender, seed-bearing but not true conifers as yet.
-Leaves were strap-shaped or grass-like, the larger ones having a length
-of six feet and a width of six inches. Trunks were woody, resembling
-pine, but with a central pith. The flowers were small and resembled
-catkins in form.
-
-Ferns and fernlike plants were so numerous that the period has been
-known as an age of ferns. Earlier knowledge of these forests was based
-on fossils of a fragmentary character from which an accurate association
-of parts could rarely be obtained. A general relationship with the ferns
-was apparent, but careful study of additional material has given us a
-rather different view of Carboniferous plant life and we note a highly
-diversified array of forms with many suggestions of modern tendencies.
-The flora as a whole may be regarded as highly specialized for the
-conditions which prevailed at the time and were not to continue through
-subsequent periods. Warm temperatures and abundant moisture were
-essential especially to spore-bearing types, and the cold, arid
-conditions of the next period put an end to many of the groups, or
-greatly reduced their prominence.
-
-This could be regarded equally well as an age of insects, for some of
-these invertebrates acquired the greatest size they have ever had,
-particularly the dragonflies with a wing-spread of more than two feet in
-one of the largest fossils so far discovered. Cockroaches numbering
-upwards of five hundred species have been named. Though large they are
-hardly to be regarded as giants, lengths of three or four inches being
-about the limit.
-
-Some of the insect types of today quite evidently existed among the
-inhabitants of Carboniferous forests, but it is apparent that there were
-also some antiquated forms which may have descended from the trilobites.
-Although some authorities regard this as the period in which insects
-originated, there are others who maintain that definite beginnings are
-not established so readily on present evidence. Spiders are believed to
-have made their appearance at this time.
-
-Four-footed vertebrates resembling salamanders were prominent animals of
-the Carboniferous swamps. At first adapted to a life in water and later
-to land conditions, they are known as amphibians, the name being based
-on the ability to live in two different kinds of environment. Common
-living representatives of this group are the toads and frogs, but these
-tailless forms are not known among fossils of the Paleozoic era and are
-almost unknown throughout the Mesozoic. The Age of Amphibians, as we
-apply that phrase, was definitely not an age of toads and frogs.
-
-These primitive land animals were of different types, ranging from much
-smaller sizes up to the length of a crocodile. Most of them had short
-legs, and feet which were suitable for locomotion upon land, but many of
-the creatures probably spent most of their lives in the water. Tails
-were usually high and flattened as if for swimming, sometimes long, at
-other times greatly shortened in proportion to the body. Heads were
-generally large, jaws long, and mouths wide.
-
-Before the close of the period true reptiles appear, but this race of
-animals is destined to make a more spectacular advance than the
-amphibians and will be discussed in connection with Mesozoic life. The
-amphibians, however, are regarded as being the ancestors of the reptiles
-as well as the higher quadrupeds which follow them. Although living
-reptiles are readily distinguished from living amphibians there is a
-different situation with regard to these primitive forms, for among the
-fossils it becomes increasingly difficult to separate the two groups as
-new material is investigated.
-
-Invertebrates had their ups and downs during the period. Trilobites
-became scarce, and brachiopods for a time were the most abundant of the
-shelled animals but later declined rapidly. In favorable localities the
-crinoids established a wonderful record for new species before the
-period had advanced very far. Hundreds of species of Carboniferous
-invertebrates are known, and in many of the rocks of the period they are
-the only fossils to be found, for the vertebrates were still unable to
-venture far from the swampy districts, and much inland territory was too
-well drained to support either the floras or faunas then existing.
-
-In the Colorado area there are both marine and continental formations
-but the great coal-making forests and their inhabitants were limited to
-other localities. As a consequence this region is not famous for
-Carboniferous fossils.
-
-
-PERMIAN HARDSHIPS
-
-For a time there was no great change in North America following the
-opening of the Permian. Then began a series of mountain-making movements
-and continental uplifts which drained the swamps, lakes, and inland
-seas. With the passing of the vegetation which had established itself in
-and around these areas much of the animal life followed. It is probable
-that a considerable proportion of the marine life survived, much more
-than is indicated by the fossil record, but the receding seas carried
-the survivors into territory which is now inaccessible to fossil
-hunters.
-
-After Middle Permian time the climate everywhere seems to have been cold
-and dry. By the end of the period there had been accomplished more
-geographical change throughout the world than at any time since the
-beginning of the Paleozoic era. Traces of the crustal movements which
-produced new mountain ranges can be followed in Europe, Asia, and North
-America. The Appalachian region was raised to a great height, possibly
-in excess of three miles. A major disturbance of this character is known
-among geologists as a revolution, and to this particular one the name
-“Appalachian Revolution” has been given.
-
-The elevation of continents necessarily changes their coast lines. This,
-in turn, influences ocean currents which have an important bearing on
-climatic conditions. In addition to this, the elevation of mountain
-systems not only rearranges the distribution of hot and cold winds over
-the land areas but it may produce barriers to the migrations of floras
-and faunas, confining them to areas in which it is no longer possible to
-live. When the effect upon plants and animals is considered, it is
-easier to understand why a line is drawn across the geological time
-chart at such a point and an era of prehistory is regarded as closed.
-
-During the Permian period there was recorded in the rocks more
-widespread glacial action than ever before or since. With less inland
-water to provide the necessary evaporation there was a marked decrease
-in rainfall, and arid or semi-arid conditions replaced the hospitable
-climate that had been such an important factor in the prolific life of
-the Carboniferous. The struggle for existence became intense, but
-hardier types of plants and animals, with greater ability to adapt
-themselves to adverse conditions, established themselves here and there,
-as ancestral forms became extinct. Most of the large spore-bearing trees
-died out and seed-producing varieties began to acquire prominence, among
-them the coniferous evergreens. Ferns, however, proved their
-adaptability by producing some new forms which became prominent in
-Permian floras.
-
-The prehistoric amphibians have been divided into three orders, one of
-which includes all the larger forms. This group, known as the
-labyrinthodonts, continued on through Permian time but began to show
-backward tendencies, with dwindling limbs and a return to life in the
-water. Among the larger land varieties are typical fossils ranging from
-about fifteen inches to five feet in length. In outward appearance they
-differed from Carboniferous amphibians. One of the other orders,
-including a great diversity of smaller forms, became extinct during this
-period, leaving no known descendants. The third order is regarded as the
-oldest, and probably the ancestral group from which the modern newts and
-salamanders originated.
-
-The most successful of Permian land animals were the peculiar reptiles
-that learned to live in drier regions. Like the horned toad and Gila
-monster of our arid southwestern United States, the larger Permian
-reptiles were four-footed animals. In size and shape they were not
-greatly different from amphibians then living. An exception to this
-rule, among some of them, is the development of long, bony spines above
-the vertebrae of the back. A fairly common fossil of this type, found in
-Texas and known as _Dimetrodon_, had a total length of six feet, about
-half of this being in the tail. The tips of the spines adorning the back
-reached a height of three feet or more and there was probably a covering
-of skin over these bones, which would produce a sail-like structure or
-“fin” of large size. Its use has not been explained but it provides an
-easy name for these odd creatures—the “fin-back lizards.”
-
-Rock deposits produced in arid regions usually have characters which are
-not difficult to recognize. Gritty texture, irregular bedding, red
-color, and gypsum are common features. Formations of Permian age are to
-be found in Colorado but better fossil deposits have been discovered in
-Kansas and Texas.
-
-    [Illustration: Marine Reptiles: Plesiosaur (Lower Skeleton) and
-    Mosasaur
-
-    The Mesozoic Era produced many types of reptiles besides the
-    dinosaurs. Two of the marine forms are shown in this illustration,
-    both from Cretaceous beds of the western United States.  Plesiosaurs
-    were the giants of the seas in their time, lengths of forty to fifty
-    feet being not uncommon. A long flat tail provided locomotion for
-    the mosasaurs whereas the plesiosaurs resorted to the peculiar limb
-    structures known as flippers or paddles.]
-
-
-
-
-                          THE AGE OF REPTILES
-
-
-The Mesozoic, or era of middle life, was a long stretch of time during
-which there was marvelous development among the reptiles. Many strange
-types were produced and most of them became extinct before the end of
-the era. The reptilian stock branched out in many directions. Types
-emerged which differed from one another so widely that their mutual
-relationships have become obscure. Hideous and fantastic creatures
-suggesting sea serpents and dragons were worldwide in distribution.
-Reptiles of the air and seas acquired large size and weird forms, but
-greater advances were made upon land.
-
-The flying reptiles or pterosaurs flourished in Jurassic times with some
-of the larger varieties surviving until near the close of the
-Cretaceous. Although these winged lizards were the first of the
-vertebrates to fly they are not to be confused with birds. They were
-without feathers, and the earlier forms were provided with long tails
-bearing a flattened rudder-like tip. One of the best known of this type
-had a length of about eighteen inches. Its jaws were long and provided
-with sharp teeth. The wings were membranes attached to body and legs,
-stretched and manipulated by means of greatly elongated fingers. In
-later types there was a reduction in tooth equipment and length of tail.
-_Pteranodon_, found in Kansas, had a wing spread of twenty-five feet, a
-large toothless beak, a short body, and a mere stub of a tail. It was
-one of the last of these winged monsters.
-
-Several types of marine reptiles appeared during this era, among them
-the plesiosaurs which first appeared in Triassic seas. These peculiar
-animals were serpent-like with regard to the character of head, neck,
-and tail, but in other respects were quite different, the short
-barrel-shaped body being provided with four large paddles corresponding
-to the usual limbs of quadrupeds. Fossil remains of these animals are
-common in many Jurassic and Cretaceous deposits, some of the largest
-exceeding forty feet in length. Mosasaurs, also marine carnivores,
-inhabited shallow Cretaceous seas throughout the world and are
-especially abundant as fossils in the Kansas chalk beds. These were
-elongated forms with a resemblance to salamanders in some respects but
-provided with long pointed jaws and sharp teeth. Swimming was
-accomplished largely by the tail though probably aided to some extent by
-four webbed paddles or flippers. The ichthyosaurs were more fish-like in
-construction, as the name implies. The limbs were short and broad, and
-there was usually present a well-developed tail-fin as well as a large
-fin on the back. They were especially abundant in Jurassic time. Fossils
-are fairly common in marine deposits of western North America. Mosasaurs
-and ichthyosaurs were about half as long as the plesiosaurs.
-
-
-DINOSAURS
-
-Most spectacular of the prehistoric reptiles were the dinosaurs, a large
-group of animals varying greatly as to size, form, and habits. They were
-adapted for a life on land though many of them probably spent much of
-their time partly submerged in the waters of lakes and streams. There is
-little that can be said of the group as a whole other than that all of
-them were reptiles. Further than that it is necessary to regard them as
-belonging to several different subdivisions of the Reptilia.
-Classification has been difficult and the names used for the various
-subdivisions are often misleading to the layman who tries to understand
-the terminology.
-
-Ancestral reptiles were five-toed and five-fingered but among the
-dinosaurs there were many departures from the standard formula. Three or
-four of the digits were commonly well developed, the others when present
-being shortened or reduced to mere rudiments. Early in the history of
-dinosaurs there was a division of the stock into two main branches, each
-of which includes a variety of types and sizes, and is again subdivided.
-The two main groups are best recognized by the construction of the bony
-framework which comprises the pelvic girdle or hip region of the
-skeleton. In order to avoid technical difficulties, however, the
-remaining discussion of these interesting reptiles will be confined to a
-few names and descriptions which serve to illustrate roughly the great
-amount of variation that developed from the comparatively simple
-ancestral pattern. The plan according to which the dinosaurs are usually
-classified is barely suggested by the types described.
-
-The meat-eaters were active creatures provided with powerful jaws and
-teeth. They were unarmored, moved about on their hind feet, and during
-their time were the most highly advanced of all animals. _Tyrannosaurus_
-with a length of forty-five feet or more, and _Deinodon_, nearly as
-large, were among the greatest of these. Both lived in the Cretaceous
-period. Their teeth were simple but strong, knife-like, curved, and
-finely serrated. Skulls were large and the forelimbs were reduced almost
-to a state of uselessness. Large carnivores lived also during Jurassic
-time and even as far back as late Triassic. Early Triassic forms were of
-smaller size.
-
-More primitive flesh-eating dinosaurs of the Triassic and Jurassic
-periods were delicately proportioned and lightly built bipeds bearing
-some resemblance to birds. _Struthiomimus_, which means
-ostrich-resembling, was about the size of the bird which provides the
-name. It was slender in the limbs, three-toed, long necked, long tailed.
-The skull was small, forelegs long for a biped. Unlike most dinosaurs it
-was toothless. All these bird-like carnivores were small as compared
-with other contemporary forms. Compsognathus, of Germany, and one of the
-smallest of all dinosaurs, had a length of less than three feet,
-including the long tail.
-
-    [Illustration: One of the Large Jurassic Dinosaurs (_Diplodocus
-    longus_)
-
-    This magnificent specimen, exhibited by the Denver Museum of Natural
-    History, has a length of seventy-five feet six inches. Two years
-    were required to complete the task of removing the bones from the
-    matrix rock and preparing them for mounting. Diplodocus was one of
-    many large reptiles which inhabited western North America a hundred
-    and fifty million years ago. The skeleton was obtained from the
-    Morrison beds of eastern Utah. The same formation is exposed in many
-    Colorado localities, including the foothills west of Denver, where
-    it acquired its name from the town of Morrison.]
-
-In Jurassic time there became prominent a group of large dinosaurs which
-were more equally developed as to fore and hind limbs. They were
-sluggish creatures, quadrupedal in their manner of locomotion,
-vegetarians in regard to their diet. Some of them reached enormous
-proportions and it is believed that they resorted to life in the water
-in order to get part of the weight off their feet. _Diplodocus_ and
-_Brontosaurus_ are the names of well-known giants in this group. They
-had long necks and tails, very small skulls, were the largest of all
-land animals and are known to have reached a length of eighty feet or
-more. Some estimates, based on measurements of incomplete skeletons,
-have exceeded one hundred feet, but these extremes are somewhat
-questionable. _Diplodocus_ was the more elongated of the two, with much
-of its length in the whip-like tail. Our mounted skeleton has a length
-of seventy-five feet six inches, measured along the vertebrae. Its
-height at the pelvis is twelve feet six inches.
-
-The teeth of these large quadrupeds are of a slightly broadened and
-blunted form which has caused some speculation as to their possible use.
-It has even been suggested that the animals were fish-eaters but this
-seems impossible in view of the great size and general characteristics
-of the group. Although they differ extremely in some respects, they are
-regarded as being more closely related to the carnivores than to the
-herbivores of the second great branch of the tribe.
-
-The unquestioned herbivores, constituting this second branch of the
-dinosaurian race, also include both bipeds and quadrupeds. The better
-known plant-eaters were large animals but not such monsters as
-_Tyrannosaurus_ or _Brontosaurus_. Of the bipeds, _Trachodon_ is perhaps
-best known. It is one of the duck-billed dinosaurs which had an average
-length of about thirty feet. The duckbills were unarmored, active
-animals, good swimmers as well as runners. They were prominent and
-widely distributed during late Cretaceous time. Many skeletons have been
-found in western North America. Natural casts and impressions of
-mummified remains indicate that the hides were scaly and the feet
-provided with webs between the toes. The bill was broad, flat, and
-toothless, but the sides of the mouth were provided with a large number
-of simple teeth closely arranged in parallel rows. The fine skeleton
-exhibited in our hall is thirty feet six inches in length. Near
-relatives of _Trachodon_, such as _Corythosaurus_ had hollow, bony
-crests, combs, or tubular structures on top of the head. These may have
-been of some service in connection with breathing while feeding under
-water.
-
-    [Illustration: A Duck-billed Dinosaur of the Cretaceous Period
-    (_Trachodon mirabilis_)]
-
-    [Illustration: Stegosaur (_Stegosaurus stenops_)]
-
-Among the quadrupedal vegetarians an interesting family is represented
-by _Stegosaurus_, a late Jurassic dinosaur having a length of about
-twenty feet. These creatures had heavy limbs, all used in walking, an
-arched back, and almost no brain at all. A double row of large flattened
-plates standing upright and extending from the rear of the skull nearly
-to the tip of the tail provided some protection for the back of the
-animal, but otherwise there was no defensive armor. Several long spikes
-at the end of the tail probably served as weapons. The mounted skeleton
-in our collection was obtained from Garden Park, near Canon City,
-Colorado, a district which has long been famous for dinosaur remains.
-
-The ankylosaurs were more completely armored with closely set bony
-plates fitting neatly over the body. They were of about the same size as
-the stegosaurs but the body was broad and somewhat flattened. These
-armored quadrupeds apparently lived only during the Cretaceous period,
-after the disappearance of the stegosaurs. Their tooth equipment was
-very poor and in a few cases entirely lacking. _Ankylosaurus_ and
-_Nodosaurus_ are good examples of the type. They have been described as
-animated tanks and are sometimes referred to as having the appearance of
-enormous horned toads.
-
-Among the last of the dinosaurs to come and go were the horned
-quadrupeds known as the Ceratopsia. Their entire history appears to have
-been confined to the Upper Cretaceous and the closing stages of the
-reptilian era in America. _Triceratops_ and _Monoclonius_ are well-known
-representatives of the group. Besides the horns, which appeared above
-the eyes or near the center of the nose, there was a broad, flattened,
-backward extension of some of the skull bones which produced a great
-frill or collar reaching over the neck as far back as the shoulders.
-This frill, combined with the large skull, gave the animal the
-appearance of being nearly one-third head. _Triceratops_ had three
-horns, _Monoclonius_ only one. The average length of the animals was
-slightly under twenty feet.
-
-Although very little is known about the ancestry of the horned dinosaurs
-a valuable discovery in Mongolia may throw some light on the subject. A
-small dinosaur with a well-developed frill, but no horns, once inhabited
-the region of the present Gobi desert, and in recognition of the
-apparent relationship it has been named _Protoceratops_. In addition to
-numerous skeletons, several nests of eggs were found in association with
-the bones. Until this discovery was made, dinosaur eggs had been
-practically unknown. A reproduction of one of these nests is among our
-exhibits.
-
-    [Illustration: A Sea Turtle of Cretaceous Time (_Protostega gigas_)
-
-    This marine animal belongs to a group which became extinct near the
-    close of the great reptilian era, but a few related forms still
-    survive. Their weight is greatly reduced by the peculiar
-    construction of the shell, and the front feet are enlarged for use
-    as oars, an excellent illustration of the manner in which a land
-    type can become adapted to life in the sea.]
-
-With the possible exception of a very few short-lived survivals
-dinosaurs were extinct before the opening of the Age of Mammals, many of
-them for millions of years. Along with them went other types of ancient
-reptiles, and the cause of their extinction is a problem which may never
-be solved. Conditions remained favorable for the turtles, which made
-their first appearance during Triassic time, and for the crocodiles,
-which date back to the Jurassic period. Snakes were only at the
-beginning of their history as the era closed. The survival of these
-modern forms suggests that they were favored to a greater extent than
-the dinosaurs during a prolonged period of changing conditions the full
-details of which are unknown to us.
-
-In general it is to be expected that disaster would first overcome the
-highly specialized creatures, such as the dinosaurs, which had become
-more delicately adjusted to the particular environments in which they
-lived. It appears that some of them had been too progressive up to a
-certain point, but not sufficiently adaptable to get beyond that stage,
-or fortunate enough to make their advances in directions that could be
-followed, through fluctuations in the matter of food supply, predatory
-enemies, climate, and other factors which bear upon success and failure.
-
-The reptilian era closed with exceptional volcanic activities in many
-parts of the world, but these cannot account for the disappearance of
-the highly diversified and abundant reptilian life. The eruptions were
-merely incidental to movements and readjustments in large masses of rock
-comprising the earth’s crust or surface. Such crustal folding and
-elevations always have been of serious consequences to both plants and
-animals because of their effect upon drainage and climate. There were
-disturbances of this kind in western North America in late Jurassic
-time, with folding and uplift in the region of the Sierras and probably
-extending from Mexico to southern Alaska. A great trough to the east of
-this elevated district was produced in the course of these movements and
-provided access to the sea from south to north. During the Cretaceous
-period there were repeated invasions and retreats of the sea by way of
-this great depression, consequent upon slight changes in the elevation
-of the floor. Hence there are numerous marine formations in Colorado and
-adjoining states, some of them rich in fossils.
-
-Before the close of the Cretaceous period the sea had made its final
-departure from this region, and the Mesozoic era was terminated by
-revolutionary disturbances which brought about the uplifting of a new
-mountain system. The Rocky Mountains may be regarded as part of this
-system and to have had their birth at this time. The Rockies, however,
-show unmistakable signs of repeated elevation, with intervals of erosion
-during which there was great reduction of their total height. What we
-see of them today is the result of more than fifty million years of
-continuous geological activity.
-
-
-PLANT LIFE AND CLIMATE
-
-Some idea of the Mesozoic climate is obtained from the character and
-distribution of the plant life. Triassic floras are not large and there
-is very little fossil evidence for the earlier half of the period. It is
-quite possible that arid or desert conditions prevailed for a time in
-much of North America, as at the close of the Paleozoic era. Plant life
-was at first not abundant, and conditions were unfavorable for the
-production of fossils. In Upper Triassic rocks of Virginia, however,
-there are signs of swampy conditions, with rushes and ferns
-predominating. Adjoining forest areas were well timbered with large
-coniferous evergreens which show no annual growth rings, as similar
-trees do in regions where cold winters alternate with warm summers. This
-suggests, for that time and place at least, a uniformly warm climate,
-lacking seasonal variations. Warm temperature or subtropical climates
-are indicated again by some of the Jurassic and Cretaceous plants, but
-intervals of lower temperatures and variable climates are also apparent.
-Palms, figs, and other trees, very similar to modern types now living
-only in warmer regions, were widely distributed in late Cretaceous time,
-and their range was extended into regions which have since become too
-cold to support such growths.
-
-The trend toward modern forms in the plant world was gradual, but
-throughout the era there were occasional novelties that attract the
-attention of botanists. Ferns and horsetail rushes, reminiscent of the
-Paleozoic forests, soon began to lose their prominence as the
-seed-bearing trees gained the ascendency. Mesozoic time could well be
-called the age of cycads, because of the striking performance of this
-plant group. Different varieties flourished in the three periods, with
-the Jurassic standing out as the time of greatest abundance.
-
-To the uninitiated, the usual cycad fossils resemble “petrified
-pineapples,” but these are merely the scarred stems or trunks of small
-to medium-sized trees with a tufted arrangement of leaves at the top,
-and usually without branches. Foliage and habit of growth suggest
-something more like large ferns or low-growing palms, with short, thick
-trunks seldom more than fifteen feet tall and many of them under three
-feet. The leaves are rarely found entire or attached to the trunks, but
-occasional discoveries indicate a leaf-length of about ten feet.
-Although they are classed among the first and lowest of seed-bearing
-plants, and in this respect are related to the conifers, their
-appearance was quite unlike that of the modern cone-bearing evergreens.
-
-More nearly resembling the common conifers of today were the sequoias,
-of early Mesozoic origin and far more abundant during Cretaceous time
-than they are at present. The maidenhair trees, now represented by a
-single species of _Ginkgo_ which is cultivated principally in China and
-Japan, were never very prominent but are of interest as an ancient
-family that persisted throughout the Mesozoic and down to our own time.
-Before the close of the Cretaceous period the flowering plants had
-greatly outnumbered the spore-bearing groups, such as the ferns and
-horsetails which were formerly so abundant. We know little of early
-flowers, however, except in connection with trees, the large gayly
-colored blossoms of the type now conspicuous in woodlands, meadows, and
-gardens being later arrivals and poor subjects for preservation as
-fossils.
-
-Cretaceous floras were surprisingly modern in character, far in advance
-of the animal life. Poplars, plane trees, magnolias, palms, figs, oaks,
-and buckthorns were abundant at the close of the Cretaceous, as
-indicated by fossils of the Laramie formation, which is the surface rock
-in many localities near Denver. Also abundant in various places at this
-time were walnut, hazelnut, laurel, tulip, maple, beech, birch,
-breadfruit, ivy, holly, and many other well-known trees and shrubs.
-Sedges and grasses, which became so important to the herbivorous mammals
-of the next era, made their first appearance in Cretaceous time but were
-then inconspicuous.
-
-
-COAL AND FOSSIL FOOTPRINTS
-
-The abundance of plant life in the Colorado area during the Cretaceous
-period is indicated by the extent of coal deposits of this age. About
-one-fourth the area of the state is underlain by coal seams varying in
-thickness from a few inches to fifty feet or more, most of it being
-Cretaceous. In the northern Colorado district the coal-bearing formation
-is the Laramie. Near Denver there is some coal in the Arapahoe formation
-which overlies the Laramie and is of later age.
-
-Coal mines often produce excellent plant fossils, and occasionally other
-evidence of prehistoric life. In a mine near Canon City, Colorado, a
-series of natural casts of dinosaur feet was taken from the overlying
-rock after the coal had been removed. One of these, in the Denver Museum
-of Natural History, is seen to consist of sandstone inside a very thin
-layer of dark clay. Flattened against the lower surface is the
-carbonized stem of a Cretaceous plant which grew in the swamp where the
-coal deposit was formed.
-
-Since the shape of dinosaur feet is unmistakable we can only assume that
-a large reptile of this type walked over the surface of swampy ground in
-which a great thickness of decaying vegetation had accumulated. A layer
-of mud settled over the top and became sufficiently firm to retain the
-mold of the feet as the animal moved along. Any plant material either on
-the mud or included in it was pushed to the bottom of the impressions
-and flattened out by the weight of the huge creature. Then sand was
-washed into the footprints from some nearby source during a heavy
-rainstorm.
-
-Following these events there was probably a subsidence of the area, and
-a great thickness of rock-making sediments was built over the ancient
-swamp. The buried vegetation gradually became converted into coal, the
-sand consolidated into a firm sandstone, and the mud produced the shales
-forming the roof of the present mine, which is now at an elevation of a
-mile above sea level as a consequence of the general uplifting of the
-Rocky Mountain region during late Cretaceous and subsequent time.
-
-When the coal was removed, the hard sandstone casts separated readily
-from the softer shales surrounding them. A small amount of the shale
-adheres to the sandstone, and some of the flattened vegetation, now in
-the condition of coal, still remains attached.
-
-
-MESOZOIC INVERTEBRATES
-
-As in other eras, the invertebrates fluctuate with the periods.
-Characteristic forms appear, become more or less prominent, then in many
-cases decline or disappear. Variations among the mollusks are
-particularly helpful in the identification of rocks which originated in
-the Cretaceous seas. Clam-like bivalves of the genus _Inoceramus_, the
-straight-shelled ammonids known as _Baculites_, and oysters, are locally
-common in some of the formations exposed a few miles west of Denver.
-
-The ammonids, or “ammonites,” were extremely abundant throughout the
-world during Mesozoic time. Their shells were chambered like those of
-the pearly nautilus, a related cephalopod mollusk inhabiting tropical
-seas at the present time. While only four species of the Nautilus tribe
-are living today, thousands of species of ammonids swarmed the
-prehistoric seas. Many new forms came into existence in Triassic time
-but few survived the period. A pronounced revival occurred in the
-Jurassic, only to be followed by a decline and eventual extinction at
-the close of the Cretaceous. Ammonites measuring three or four inches
-across the diameter of the coiled shell were about average size, but
-diameters up to three or four feet were not uncommon. Externally the
-shells were ornamented with ribs, knobs, and spines; inside was a pearly
-lining. The partitions were thin and composed of the same pearl-like
-substance as the lining. Each partition becomes wavy as it approaches
-the shell, and the line of union has a distinct pattern which is seen in
-specimens which have lost the outer shell layer. This wavy suture line
-becomes more complicated in the later members of the race, and the
-peculiar markings developed by the repeated partitions provide a
-convenient method of identification.
-
-The belemnites or ink-fishes, regarded as ancestors of the cuttlefishes
-now living, comprise another group of carnivorous mollusks. These,
-however, had lost the external shell, and the usual fossil is part of an
-internal shell or “skeleton,” known as the guard. This limy structure
-has the form of a pointed cigar, and is seldom over a foot long although
-the total body-length of the larger animals was commonly about six to
-eight feet. Several hundred species have been described, the majority
-being of Jurassic age. They declined rapidly toward the close of the
-era.
-
-The invertebrate life of the Mesozoic was strongly dominated by
-mollusks, with cephalopods in first place, the bivalve pelecypods and
-the single-valve gastropods or snail-like forms sharing subordinate
-positions. The dominating trilobites, sea-scorpions, and tetracorals of
-the Paleozoic had disappeared, while the brachiopods and crinoids were
-greatly modified and more like the forms which live today.
-
-Crinoids became moderately abundant at various times, but in many ways
-different from their relatives of the preceding era. Some of the largest
-known species, with stems estimated as fifty feet long, have been found
-in lower Jurassic rocks. A great abundance of microscopic life is
-indicated by the frequent occurrence of chalk in the Cretaceous
-formations. Corals of the modern reef-building type (hexacorals) were
-common in the warm seas of a large part of the world.
-
-The insects of the early Mesozoic are represented by few fossils
-although it is evident some new forms were becoming established. The
-warm climates prevailing throughout much of the world appear to have
-been a favorable factor in the progress of insect life. In addition to
-the older cockroach and dragon-fly types may be mentioned the arrival of
-grasshoppers, cicadas, caddis-flies, beetles, and ants.
-
-Several hundred species are found in Jurassic rocks, and by the end of
-the Cretaceous period most of the insect families now known to us were
-probably in existence. The record is seriously obscured by the erosion
-of rocks which so frequently marks the end of a period, also by the
-small size of the subjects, and by the exceptional conditions required
-for the production of such delicate fossils. Among the last of the
-familiar insects to appear were the bees and butterflies. These
-evidently came in with the more advanced types of flowering plants that
-produce the nectar on which many insects feed. It is probable, too, that
-without the arrival of these insects and their service in the
-pollination of flowers, the floras of today would be rather different
-from what they are.
-
-
-EXTINCT BIRDS
-
-It is not surprising to find that birds made their first appearance in
-the Mesozoic era, for of all animals they are most like the reptiles as
-a class. Feathers are about the only dependable characteristic of the
-entire group, nearly every other feature being matched by some reptilian
-creature of great antiquity. The nesting habit, which includes care of
-the young as well as the eggs, is a matter of progress which relates to
-flight and to warmer body temperature. It appears to have been developed
-by forest-dwelling types living among trees and nesting there in
-comparative safety from enemies prowling on the ground.
-
-The oldest known prehistoric birds were found in lithographic stone of
-Upper Jurassic age. _Archaeopterix_ was discovered in 1861 at
-Solenhofen, Germany. Sixteen years later a similar bird in a better
-state of preservation was found in Bavaria. The latter was named
-_Archaeornis_. These Jurassic fossils are regarded as true birds by some
-authorities, while others believe them to be more nearly related to the
-reptiles, the opinions being based on careful studies of the skulls and
-other skeletal features. Both birds had teeth of reptilian character,
-and it is evident that there was no beak, for the jaws were covered with
-scaly skin. The bony construction of the long tail would suggest lizards
-rather than birds, were it not for the presence of feathers which were
-attached at each side. Head, neck, and parts of the body were covered
-with scales. Wings were well provided with stout feathers but the
-skeletal framework indicates that the birds were gliders rather than
-true flyers. Claws on the wings served like fingers to aid in climbing
-among the branches of trees, a practice which is occasionally noted
-among the young of living birds. In adult birds of today, however, the
-claw-like appendages of the fore limbs are greatly reduced and of little
-service.
-
-The next fossil birds of importance have been found in Cretaceous rocks
-of Kansas, both of them fishers of the seas instead of forest
-inhabitants. _Ichthyornis_ was a small bird, standing about eight inches
-in height, a powerful flyer with reptilian jaws and teeth. _Hesperornis_
-was built for diving and swimming, like the loon, but was somewhat
-larger and provided with teeth. Its wings were too poorly developed to
-be of use in flying.
-
-Toothed birds became extinct with the close of Cretaceous time, and the
-ancestors of modern types were in existence before the Age of Mammals,
-but fossil remains are few and poorly preserved. Large ostrich-like
-birds, however, are known to have lived in North America during the
-Eocene period. One of these, named _Diatryma_, stands nearly seven feet
-tall in the reconstructed skeleton. Its legs are heavy, wings greatly
-reduced, beak massive. In its relation to modern birds it is possibly
-nearer to the cranes than the ostriches.
-
-Flightless birds of large size are known from many parts of the world
-and seem to have been prominent throughout the Cenozoic era, as they are
-today in the southern hemisphere. _Aepyornis_ lived in Madagascar during
-the Pleistocene period and may have become extinct quite recently. Its
-eggs are the largest known among fossils, several times the size of an
-ostrich egg. Also in this period the moas were living in New Zealand
-where their remains are still abundant. One of the largest, known as
-_Dinornis_, had about the same form as _Diatryma_ but the neck was
-longer, head and beak smaller, legs better fitted for running, height
-about eleven feet.
-
-A much smaller flightless bird, the dodo, became extinct in modern time.
-This former inhabitant of Mauritius and other islands of the Indian
-Ocean was related to the doves and pigeons, and had lost its power of
-flight through disuse of the wings. It was a clumsy, defenseless bird
-weighing possibly as much as fifty pounds. Actual remains are few and
-incomplete, and descriptions published by the explorers who knew the
-bird two centuries ago are not entirely trustworthy. In the Pleistocene
-Rancho la Brea beds of California the largest of all prehistoric flying
-birds has been found, a vulture bearing the name of _Teratornis_.
-Re-assembled skeletons show them to be slightly bigger than existing
-condors.
-
-
-ANCESTORS OF THE MAMMALS
-
-The monotremes or egg-laying types of mammals such as the duck-bill and
-spiny anteaters which now inhabit Australia are almost unknown as
-fossils. Marsupials, the next higher living group, which includes the
-opossum and kangaroo, appeared at the end of Cretaceous time along with
-the placentals or higher mammals which dominate the history of the
-Cenozoic era. Nevertheless, there are a few teeth and jaws from rocks of
-Triassic and Jurassic age to indicate that small mammals, from the size
-of mice to slightly larger than rats, existed throughout most of the Age
-of Reptiles. There is no complete skeleton of any of the earlier forms,
-and little is known of their relationships either with living orders of
-mammals or with probable ancestors among the reptiles. The record
-becomes somewhat clearer toward the end of the era but it is obscured
-again by the great disturbances which followed.
-
-Looking back among earlier land animals for the origin of the first
-mammalian stock it is necessary to go as far as Permian or even
-Carboniferous time. Reptiles then living had many structural features in
-common with mammals, and mammal-like forms continued to flourish until
-late in the Triassic. An interesting group of such animals, named
-therapsids, was one of the earliest reptilian stocks to appear, and is
-well known from fossils found in the Red Beds of Texas and New Mexico,
-in Europe, South Africa, and Asia. Quite a variety of types is included
-in this group, with many advances in dentition, and modifications of the
-skull, limbs and pelvic construction which strongly suggests a
-relationship to the mammals.
-
-    [Illustration: Murals Over Fossil Exhibits, Hall of Mammals
-    Top: Eocene; Protylopus, Tanyorhinus, Patriofelis, Uintatherium,
-    Turtle, Crocodile, Eohippus.
-    Middle: Upper Oligocene; Mesohippus, Merycoidodon, Hoplophoneus,
-    Metamynodon, Poebrotherium, Trigonias.
-    Bottom: Pliocene; Teleoceras, Turtle, Synthetoceras, Amebelodon,
-    Teleoceras.]
-
-
-
-
-                           THE AGE OF MAMMALS
-
-
-The striking feature of life development in the Cenozoic era is the
-great progress and expansion over the earth of the mammalian races. The
-division of the era into periods, however, was based largely on a study
-of fossil mollusks. In the Paris basin of France, it was noticed by the
-geologists of a century ago that the youngest of the sedimentary beds
-contained the greatest number of recent or still living species.
-Successively downward into the older beds the percentage of recent
-species decreased until there were practically no living species
-represented in the oldest rocks of the series. From the percentage of
-recent forms among prehistoric ones it was proposed that the following
-division be made: Eocene, meaning _dawn of the recent_; Miocene, meaning
-less recent; and Pliocene, meaning _more recent_. Sometime later it was
-suggested that another period be added, and to this was given the name
-Pleistocene, meaning _most recent_. In 1854, the older Miocene
-formations were segregated and referred to a newly provided Oligocene
-period, this name meaning _little of the recent_.
-
-Early geologists grouped the rocks in three great divisions, applying
-the names Primary, Secondary, and Tertiary. To these was added
-afterwards the name Quaternary, which applied to the youngest formations
-of the earth. Only two of these terms remain in common use at present:
-it is a frequent practice to refer to the combined Eocene, Oligocene,
-Miocene and Pliocene periods as the “Tertiary” division of Cenozoic
-time; to the Pleistocene and Recent periods as the “Quaternary”
-division. The geology of some remote future may be clearer with regard
-to the full significance of this subdivision of the Age of Mammals into
-two parts. It may be that a great era was concluded at the end of
-Pliocene time as others have been concluded, by the usual earth
-disturbances and climatic changes and by the decline of animals once
-prominent in the faunas of the world. Events of such character have
-registered their occurrence but may eventually prove to have been a
-series of minor events not comparable with the revolutionary changes
-that terminated other great time divisions. The favored practice of
-including ourselves and our times in the Cenozoic is based on a trend of
-opinion which holds that no great era has been ended since the Age of
-Reptiles was concluded.
-
-    [Illustration: Skulls of the clumsy, six-horned uintathere and the
-    early, hornless titanothere form part of this Eocene display. In the
-    mural these animals and the little “three-toed” Eohippus, smallest
-    of horses, are pictured with a contemporary turtle and crocodile.]
-
-The oldest of Eocene rocks show a great variety of mammals and a strange
-assortment of forms far in advance of the Mesozoic record. Ancestries
-and successive stages of development have been only partially worked
-out, though details have been better preserved for some of the groups
-than for others. Some oddly shaped creatures such as the uintatheres
-reached their full development in a relatively short time and passed out
-of the picture before the end of the period. These animals, represented
-in our collection by _Uintacolotherium_, acquired large bodies and many
-horns, but a peculiar tooth equipment fitted them for a special diet
-which apparently failed to be supplied in sufficient abundance at a
-critical time.
-
-On the other hand, we find in this period the ancestors of more
-successful groups, some of which continue on into modern times. Only a
-few of these histories can be traced in a brief sketch, but in a general
-way it may be stated that the successful races had modest beginnings and
-that they developed very slowly into what they are now, by a process of
-adjusting themselves, or by becoming better adapted to new or previously
-unused conditions in their respective environments. In their early
-stages the various types had much in common; they were generalized,
-rather than specialized for any particular kind of existence. What they
-were fitted for is best indicated by their teeth and feet, though other
-structural features frequently contribute valuable information. The rise
-of mentality is indicated by skull capacities and the increasing
-development of the upper lobes of the brain, as revealed by casts taken
-from the interior of skulls.
-
-The creodonts were the earliest and most primitive of the flesh-eating
-mammals or carnivores. Many of them were small of body and brain, and
-equipped with teeth that indicate a mixed or largely insectivorous diet,
-or possibly the habit of feeding on carrion. Although there was
-considerable variation among them, and some tendency toward
-specialization, there was little to suggest the coming of more
-progressive groups such as the cats and dogs, with teeth perfected for
-the tearing and cutting of flesh, and feet especially fitted for the
-life of hunters. The ancestry of the cats cannot be traced farther back
-than the Oligocene but it probably connects somewhere prior to that time
-with the creodonts.
-
-_Cynodictis_, an Oligocene carnivore slightly under two feet in length
-is commonly regarded as a primitive dog, but its characters are so
-generalized that it probably differs but little from the ancestors of
-many other carnivores. The skeleton of this animal suggests a slender
-and flexible body like that of the weasel, with somewhat shortened limbs
-and a long tail. It lived in forested regions and was probably more or
-less of a tree dweller. The more advanced carnivores required longer
-legs, better adapted for running and overtaking the prey, which is the
-dog’s way of hunting, or for stalking and springing upon the quarry,
-which is the method of the cat.
-
-    [Illustration: Moropus (_Moropus cooki_)
-
-    Though its teeth clearly indicate a diet of plant material, this
-    strange animal had claws on its toes, like the carnivores. It is
-    probable that the claws were used in digging for roots, as indicated
-    by the artist.]
-
-A prominent group of mammals today is that known as the ungulates, or
-hoofed animals, which includes the horses, cattle, deer, swine,
-rhinoceroses, tapirs, and other types both living and extinct. Their
-probable ancestors were the condylarths, primitive ungulates of the
-Eocene period. One of these, known as _Phenacodus_, serves well to
-illustrate the general character of the early hoofed mammals. It was
-about five and one-half feet long, rather large for its time, with long
-tail and short limbs, low elongated skull and small brain, very similar
-in many respects to the creodonts or ancestral carnivores. The teeth,
-however, were partially of the grinding type so essential to the welfare
-of plant feeders.
-
-The condylarths were five-toed animals and evidently provided with small
-hoofs, but the more progressive ungulates soon lost one or more of the
-toes, and a division of the group into odd-toed and even-toed branches
-became firmly established. Consequently, the families of ungulates
-having one, three, or five toes are classed together as being closely
-related to one another, and those having two or four toes are segregated
-in a second lot. The odd-toed clan, known as perissodactyls, included
-such animals as the horses, rhinoceroses, tapirs, and titanotheres, each
-of these types being placed in a separate family. The even-toed clan has
-been treated in a similar way and named the artiodactyls. In this
-division are such families as swine, cattle, deer, camels, oreodonts,
-and others. The odd-toed group dominated among the larger animals of
-North America for a time but has been completely replaced by the
-even-toed division which is still flourishing, although some of the
-older families have become extinct.
-
-Among the exhibits of the Denver Museum of Natural History may be seen
-complete skeletons of extinct horses, rhinoceroses, titanotheres, and
-chalicotheres representing the perissodactyls. _Moropus_ was one of the
-chalicotheres, an exceptional family which never became very prominent
-although it had a prolonged history and persisted in Europe and Asia
-after its extinction in North America. The family is grouped with the
-ungulates because of many similarities found in the molar teeth, skulls,
-and other parts of the skeletons, but the toes were provided with claws
-instead of hoofs. The use of these claws is somewhat of a puzzle:
-possibly for defense against carnivorous enemies, for dragging down
-branches in order to obtain food, or for digging roots which may have
-been an important part of the diet.
-
-Titanotheres are represented in our collection by the skeletons of the
-large, horned type which was the last of the race and destined to
-extinction by the middle of the Oligocene period. Smaller hornless
-varieties of Eocene time are illustrated by skulls. This family of
-ungulates had an unprogressive dental equipment, and a small brain in a
-flattened skull. The molar teeth readily distinguish the group from
-other ungulates and enable us to trace the relationship between earlier
-and later varieties. These teeth were of a type which is soon destroyed
-by wear, and it is evident that the animals survived only so long as
-their environment provided them with an abundance of soft vegetation.
-
-    [Illustration: Titanotheres of Oligocene Time
-
-    The name of these animals refers to the large size though they were
-    greatly exceeded in bulk by the mastodons and mammoths of later
-    periods. Ancestral titanotheres, dating back to the Eocene, were
-    hornless animals of much smaller size. These splendid specimens were
-    obtained in Weld County, Colorado.]
-
-The large assortment of rhinoceros material provides an idea of the
-great abundance and variety of forms in this family which was once
-prominent in North America but no longer among the inhabitants of that
-continent. Some of the mounted skeletons have been restored on one side
-to show how these animals appeared in the flesh.
-
-Of the even-toed ungulates there are also several types illustrated by
-complete skeletons. _Merycochoerus_, the subject of one of our mounted
-groups, represents the oreodonts, a large family of mammals whose
-history begins with the Upper Eocene and ends in the Lower Pliocene. The
-oreodonts were small animals, rather pig-like in form and quite common
-in the western plains region shortly after the time of the titanotheres.
-Ancient swine are represented in our exhibits by two mounted skeletons
-which were obtained from northeastern Colorado, where the bones were
-found associated with rhinoceros and titanothere remains. Some of these
-animals were of very large proportions, and the entire family is
-commonly known as the “giant pigs.”
-
-Camels and closely related forms were quite abundant in North America
-from early Oligocene to comparatively recent time. Numerous types were
-developed during the course of their history, some small and delicately
-formed, others tall and clumsy and much like the giraffe in structure.
-Parts of many of these creatures have been found but the only completely
-prepared skeletons in our collection are of the little gazelle-camel,
-_Stenomylus_, from Lower Miocene deposits in northwestern Nebraska.
-Pleistocene bisons are represented by several complete skeletons and
-numerous skulls and horncores, some of the species showing an extreme
-development in the length of horns. With two of the bison skeletons are
-shown prehistoric weapon points, found with the bones and indicating
-that these animals were hunted by primitive men at some time near the
-close of the Ice Age. The artifacts first discovered near Folsom, New
-Mexico, by field workers of our Museum, have become known to
-archeologists as Folsom points.
-
-
-PREHISTORIC HORSES
-
-The past history of horses is well known from an abundance of fossil
-material, ranging in age from the Eocene down to the present. Modern
-horses have only one toe in each foot, but there are remnants of two
-additional toes which may be seen only in the bony structure underlying
-the skin. Most of their ancestral relatives were three-toed as far back
-as the Oligocene period. During Eocene time, however, there was a stage
-which may be regarded as four-toed although it was evidently a temporary
-condition, linking known horses with more remote forms having five toes.
-
-    [Illustration: Oligocene Mammals From Weld County, Colorado
-
-    The giant pigs (_Archaeotherium mortoni_) at the left of the group,
-    and the rhinoceros (_Trigonias osborni_) were common animals of the
-    western plains region at one time.]
-
-_Eohippus_, the “dawn horse” as it has been called, is one of the oldest
-and best known of the American horses. Its relation to existing members
-of the family can be traced by means of changes in tooth structure as
-well as in the gradual reduction in the number of toes that is seen
-among intermediate forms. Its ancestors some day may be positively
-identified in that group of generalized, primitive, five-toed, hoofed
-mammals which are known to have lived at the beginning of the mammalian
-era, but such identification has not yet been established. Even
-_Eohippus_ bore little resemblance to the familiar horse of today. Its
-height was only eleven inches, and in body form it had much of the
-appearance of a modern dog. There were four toes on the front foot, one
-of them decidedly shorter than the others but complete in all its parts,
-and evidently capable of service in carrying a portion of the animal’s
-weight. The hind foot had three complete toes and a tiny remnant of a
-fourth which could not have been apparent externally.
-
-As changes in the structure of the feet progressed, the central toe of
-the original five continued to increase in size while the adjacent
-digits became relatively shorter and eventually so reduced in length
-that they could touch the ground no longer. The smaller bones at the
-extremities, corresponding to the joints of our fingers and toes,
-eventually disappeared from the side toes. Then the longer bones of the
-outer digits lost the broadened supporting surface, where the missing
-toes had been attached, and became reduced to pointed remnants known as
-splints. Extreme shortening of the splint bones eventually leaves only a
-small knob which is often referred to as a rudimentary toe. In the
-skeleton of a large horse the splints are readily seen, but in some of
-the earlier species they are so small that they may easily be destroyed
-or overlooked by the collector who removes the fossilized material from
-the surrounding rocks. Even then, the bones of the wrist and ankle may
-indicate in an unmistakable manner that an additional toe once was
-present, for each bone is supported by another, and at the point of
-attachment there is a characteristic surface whose purpose is usually
-obvious.
-
-Throughout the Cenozoic era the changes continued. Among the horses of
-the North American Oligocene were _Mesohippus_, approximately the size
-of a collie dog, and _Miohippus_ which was slightly larger. Both were
-three-toed, but the rudimentary splint of a fourth toe was still present
-in the front foot. _Parahippus_ and _Merychippus_ carried on during the
-Miocene period, the latter being characteristic of the time, and
-showing, in addition to other progress, a decided trend toward the
-modern structure of molar teeth. There was some increase in size but the
-largest horse of that period was hardly more than a small pony.
-
-    [Illustration: A Pleistocene Horse of the Texas Plains (_Equus
-    scotti_)]
-
-_Hipparion_ and _Protohippus_, living during Upper Miocene and Pliocene
-time, represent later stages of the three-toed condition. The side toes
-were completely formed but greatly shortened, only the central toe
-touching the ground. In some of the species the outer toes had also
-become very slender, approaching the splint condition. By this time the
-molar teeth were longer and better adapted for feeding on grasses which
-were becoming sufficiently abundant to attract some of the forest
-dwellers into the open country.
-
-During the Pliocene period, in the genus _Pliohippus_ and also in
-_Hipparion_, the feet were far advanced in structure, with most of the
-species single-toed, the side digits having reached the splint stage.
-Pleistocene horses of the genus _Equus_, like living species of that
-genus, were strictly one-toed animals, ranging over grassy areas and
-highly specialized for a life in that kind of environment.
-
-Specialization is to be noted partly in the foot and leg structures
-where the modifications have contributed to greater speed and travelling
-ability. This is of great service to an animal of the plains where food
-and water are often scarce, and great distances frequently have to be
-covered in order to obtain sustenance. The horse, as we know it, is
-built for speed, its limbs and feet being elongated to permit a greater
-stride, and also modified to decrease the weight without loss of
-strength. The ordinary ball-and-socket joint is replaced by a
-pulley-like construction which limits the direction of movement but
-provides an excellent mechanism for locomotion, especially over flat,
-open ground. Flexibility in other directions is sacrificed for greater
-strength, and the foot incidentally becomes less suited for other
-purposes.
-
-This is what is meant by “specialization”—a departure from
-“generalization.” The study of fossils provides numerous illustrations
-of specialized development which contributes greatly to an interest in
-prehistoric life. Any specialized structure or habit which increases
-fitness for a particular way of living is also known as an “adaptation.”
-Quite in line with the idea of specialization and adaptation is the
-change which occurred in the construction of the horses’ teeth, for the
-dental equipment of the modern grazing animals differs widely from that
-of the browsing creatures which lived on the soft leaves and other plant
-substances of the forests.
-
-    [Illustration: The Structure of Molar Teeth
-
-    The large lower molar of a long-jawed mastodon shows worn and unworn
-    cusps, with the enamel layer forming a heavy border around the
-    central dentine where the surface covering has been worn through. In
-    the grinding teeth of rhinoceroses (illustrated at the right) the
-    crown pattern is quite different, but both types are adapted for
-    softer foods and are similar in having the protective enamel on the
-    outside only. The central tooth shows the condition after the
-    shallow surface depressions have been removed by wear.]
-
-The cheek teeth or grinding equipment of the horses underwent as
-complete a change as the feet. Modification resulted in a new type of
-tooth which enabled herbivorous animals to take advantage of a kind of
-vegetation which was late in arriving and has since become the principal
-diet of the ungulates. The grasses are coarse and harsh as compared with
-the leaves of forest shrubbery, requiring more thorough grinding to make
-them digestible. In addition they contain minute particles of silica,
-which is a highly abrasive mineral that quickly wears down the tooth
-substance, especially the softer materials found in tooth construction.
-An increase in the length of the tooth would offset the excessive wear
-but would not necessarily produce a better mechanism for grinding.
-
-The fulfillment of the new requirements is to be seen in the change from
-what is known as the low-crowned, browsing type of molar, to the
-high-crowned, grazing type. Details of the changes that may be traced
-through millions of years of gradual adjustment become apparent only
-from the examination of a great deal of fossil material. As compared
-with earlier types of construction, a modern molar tooth may appear
-extremely complicated, but the process which brought about the improved
-quality is very simple. A little discussion of tooth structure, however,
-is required to make this clear.
-
-A tooth, as everyone knows, is partly imbedded in the jaw, partly
-exposed outside the gum. In a short-crowned tooth the exposed portion is
-known as the crown, and the part imbedded in the jaw consists of one or
-more roots which are comparatively long. The crown is nearly always
-protected by a thin layer of hard enamel. In a grinding tooth, the
-working surface has a number of more or less prominent elevations known
-as cusps. The enamel layer completely covers this surface until wear
-begins. As the tooth goes into service the signs of use begin to appear;
-the enamel is soon worn from the tops of the cusps, and the underlying
-substance, called dentine, becomes exposed. This is far less resistant
-to wear, and as the enamel continues to be reduced the tooth becomes
-less efficient as a grinding device, partly because of the smoothing off
-of the surface, partly because of the relative softness of the inner
-material which is being exposed in increasing quantity. A very old molar
-tooth of the low-crowned type has a smooth surface from which almost the
-last trace of the enamel has been removed. In many prehistoric animals
-the enamel is of a darker color than the dentine or cement, this
-difference in color enabling one to see at a glance how the teeth are
-constructed.
-
-    [Illustration: Grazing Type of Molar Teeth
-
-    The side view of the bison’s molar and premolar equipment
-    illustrates the elongated construction which is common among grazing
-    animals. In the pattern of the grinding surface may be seen a cross
-    section of the enamel layers. One layer surrounds each tooth while
-    two folded “cylinders” of the same material occupy the interior.]
-
-In a long-crowned tooth the roots are usually very short, for much of
-the crown itself is imbedded in jaw bone, and the longer roots are not
-required. Growth of the tooth is usually completed after a few years;
-then as it is gradually worn away it is continuously moved upward by the
-production of new bone under the roots, which slowly fills the bottom of
-the socket and continues to provide the necessary support. An equally
-important difference between the two types of teeth, however, is to be
-seen in the arrangement of the enamel, the long-crowned type being
-provided with this durable substance on the inside of the crown instead
-of having a mere protective cap on the outside.
-
-The more complicated structure was developed from the simpler form by
-the easy method of deepening certain depressions located between cusps
-at the top of the tooth. As the crown of the tooth increased its length
-these depressions remained tucked in, and eventually became deep pits
-roughly cylindrical in shape. In addition to the enamel and dentine, a
-third tooth substance, known as the cement, made its appearance at about
-this time, and we find that quantities of this new material were
-deposited outside the crown enamel and also inside the enamel walls of
-the pit, in this way producing a firmly consolidated structure otherwise
-weakened by deep channels and hollow pockets. The cement differs only
-slightly from the dentine but is deposited while the uncut tooth is in
-the gum tissues of the mouth, the enamel and dentine elements being
-formed earlier in the embryonic tooth before it emerges from the jaw
-bone.
-
-A tooth constructed by such a process, if cross-sectioned through the
-crown, will be found to consist of successive layers of hard and softer
-materials. In living animals the top of the tooth soon wears off and the
-enamel layers stand in higher relief because of their greater resistance
-to wear. A roughened surface of excellent grinding quality is thus
-provided, and as long as the wear continues there remains the same
-relative amount of enamel to retain the roughness, and resist abrasion.
-
-Among the various types of grazing animals there is a marked difference
-in the arrangement and form of the enamel layers. Within a species of
-genus, however, the complicated enamel patterns of the molar teeth are
-consistently similar. In the case of horses especially, these patterns
-provide a most helpful key to the identification of extinct forms. The
-general pattern, in any of the more modern horses, may be understood
-more readily if the wavy enamel layers be regarded as forming a set of
-cylinders with deeply crinkled walls. Near the outer border of the
-tooth, surrounded by a thin layer of cement, is the enclosing cylinder
-which represents the enamel cap of the old-fashioned, low-crowned tooth.
-Inside of this is the central mass of dentine which has been penetrated
-by two of the deep pits previously mentioned. The original enamel cap
-has been depressed into these pits, forming two inner cylinders which
-are filled with cement. Instead of being circular in outline, when the
-cap is worn through at the grinding surface these inner cylinder walls
-are seen to be wrinkled and folded so as to produce a most irregular
-pattern. However, if several teeth of the same kind of horse are
-compared, it will be found that the edges of these cylinders produce
-figures which are remarkably uniform and characteristic for that
-species.
-
-    [Illustration: American Mastodon (_Mastodon americanus_)
-    A true mastodon of the short-jawed type.]
-
-
-MASTODONS AND MAMMOTHS
-
-Elephant-like mammals both living and extinct are classed together in a
-single order bearing the name Proboscidea. Living members of the group
-are the elephants, of which the large Indian and African species are
-best known. Among prehistoric representatives the most frequently
-mentioned in the popular literature of North American animals are the
-following:
-
-The American Mastodon, an immigrant from Siberia which ranged over
-nearly all of the United States and Canada. It was principally a forest
-dweller, rarely found in plains regions, was abundant during the
-Pleistocene period and may have been known to the early American
-Indians;
-
-The Woolly Mammoth, which was about nine feet tall. It ranged over
-British Columbia into the United States and across to the Atlantic,
-disappearing in late Pleistocene time;
-
-The Columbian Mammoth, about eleven feet tall, lived in the early half
-of the Pleistocene period, ranging over the warmer portions of North
-America, including practically all of the United States and much of
-Mexico;
-
-The Imperial Mammoth, reaching a height of more than thirteen feet, and
-becoming extinct in the Middle Pleistocene. It was a western form,
-remains being found from Nebraska to Mexico City.
-
-Originally placed in the genus _Elephas_, the mammoths are referred to
-commonly as elephants, though technically they should not be regarded as
-such. Recent explorations and researches have added greatly to our
-knowledge of these animals but have also caused much confusion with
-regard to scientific names, for many new subdivisions of the larger
-group are now recognized, and it has become necessary to change some of
-the older nomenclature.
-
-    [Illustration: A Long-Jawed Mastodont (_Trilophodon phippsi_).
-    One of the Early American Proboscideans]
-
-The large mammoth exhibited by the Museum bears the impressive name of
-_Archidiskodon meridionalis nebrascensis_. Fifty years ago it might have
-been identified simply as a specimen of the imperial elephant and in
-such case would have received the old name of that species, which was
-_Elephas imperator_. But late in the last century it was proposed that
-the mammoths be recognized by some other name to distinguish them more
-sharply from living elephants. The name suggested for the new genus thus
-established was _Archidiskodon_, in recognition of the more archaic or
-primitive construction of the enamel plates in the mammoths molar teeth.
-The specific name, _meridionalis_, had been given to a kind of mammoth
-which is well known from the southern part of Europe, and the Latin
-name, signifying “southern,” had been applied to differentiate this
-species from the northern or woolly mammoth.
-
-This mammoth, however, had disappeared from southern Europe and for many
-years its subsequent history remained a mystery. The late Dr. Henry
-Fairfield Osborn had been engaged in an extensive study of the subject,
-and when the nearly perfect skeleton from Angus, Nebraska, was brought
-to his attention he recognized it as being closely related to
-_meridionalis_, and considered it to be a record of the migration of
-that species into North America. Because of minor variations from the
-typical mammoth of southern Europe he regarded it as a variety or
-subspecies which had descended from the latter, and the subspecific
-name, _nebrascensis_, was added to take care of this situation, using a
-Latinized form of the name of the State in which the skeleton was found.
-With the knowledge we now have of these mammoths it becomes apparent
-that _Archidiskodon meridionalis nebrascensis_ is an ancestor of the
-imperial mammoth, currently known as _Archidiskodon imperator_, and not
-identical with it.
-
-This instance is typical of the manner in which prehistoric animals
-obtain their names. Although given a Latin form, these technical names
-are derived from many languages, and the root words are applied with
-reference to anything that happens to appeal to the author as
-significant. Consequently there is seldom a name of this kind which may
-be translated directly into natural history or science. It is a mistake
-to believe that these strange phrases conceal important technical
-information which is available only to those who are familiar with dead
-and foreign languages. Actually they contain nothing of the sort, and
-the most enlightened of the Greeks and Romans could not find it there.
-When a name is needed there is none better than the one provided by the
-specialist who is skilled in the business of naming things. Some
-technical ability is required, to apply the name where it properly
-belongs, but technical knowledge is not obtained from such sources.
-Names, in any form, have another purpose to serve. There is no magic in
-them and there need be no mystery about them.
-
-    [Illustration: Molar Tooth of Mammoth
-
-    This type of tooth is constructed for long continued use and will
-    withstand the wear of more abrasive foods. The position of the white
-    enamel plates is seen in this view of the grinding surface. These
-    plates extend all the way to the base of the tooth, which is of the
-    long-crowned variety and not to be destroyed by the wearing away of
-    a single outside layer of enamel.]
-
-Other specimens in the Museum collection are the long-jawed mastodonts,
-so named because of the elongated jaws and protruding chin which is
-often mistaken for a tusk. Early members of this group had more cheek
-teeth than later types of mastodons, and longer jaws were required for
-their accommodation. Some of them had flattened lower tusks which
-evidently were used for digging purposes. These are popularly known as
-“shovel tuskers.” The more modern American mastodon had shorter jaws
-and, like the mammoths and elephants, only one pair of tusks. Both the
-long-jawed and short-jawed types are represented by complete skeletons,
-and also by tusks, jaws, and teeth of many individuals. The American
-mastodons and mastodonts were of about the same size as the smaller
-mammoths.
-
-The difference between mastodons and mammoths is most readily recognized
-in the structure of the grinding teeth, the molars and pre-molars. In
-the mastodon these teeth are of the short-crowned type, while in the
-mammoths, as in the modern elephants, they are long-crowned. The
-difference between these two types of molars has been described with
-reference to horses, and the change from the older to the modern form
-may be regarded as coming about in the same general way, through a
-series of gradual modifications. In both horse and mammoth the final
-development shows internal enamel extending from the grinding surface
-nearly to the roots. Otherwise, however, there is almost no resemblance,
-for the mammoth tooth is made up of flattened enamel plates, the number
-of which is variable for different species. In the jaws of a very young
-individual these plates may be seen as separate parts. As the tooth
-continues to grow, the plates become cemented together, and when the
-ends of the plate are worn down it may be observed that each consists of
-a layer of enamel surrounding a flat central core of dentine. The type
-of construction is rather more obvious in the mammoth tooth than in that
-of a horse, partly because of the larger size, and partly because of the
-relative simplicity of construction.
-
-The earlier history of the Proboscidea is not recorded in the rocks of
-North America, for the group was of African origin and its migrations
-did not extend as far as the New World until middle Cenozoic times. The
-mastodons and mammoths were the largest of land animals since the Age of
-Reptiles, but their Old World ancestors were not conspicuous because of
-their bulk. Many of these ancient forms, even in the earliest stages,
-reveal some of the prominent characters that dominate the entire group.
-None of them, however, should be regarded as a miniature mammoth or
-mastodon, for these highly specialized types were perfected only at a
-comparatively recent date, and by a process that works very slowly.
-Among the earlier forms there were also some oddities which failed to
-survive or to produce a successful branch of the stock such as the
-elephants.
-
-    [Illustration: Nebraska Mammoth
-    (_Archidiskodon meridionalis nebrascensis_)]
-
-The earliest known member of the order was _Moeritherium_, an animal of
-the size of a tapir, living in Egypt during the late Eocene and early
-Oligocene time. At this stage the characteristic specializations leading
-to the mastodons and mammoths were apparent but not far advanced. The
-proboscis was probably much like the flexible snout of modern tapirs,
-for the need of a long trunk had not yet arrived. In upper and lower
-jaws the second pair of incisor teeth were becoming large and prominent.
-The enormous tusks of the mammoths later developed from the enlargement
-of the same pair of upper incisors, and in some of the long-jawed
-mastodonts the lower pair also produced large tusks, though frequently
-the lower tusks were not prominent.
-
-_Dinotherium_ had downward-growing tusks in the lower jaws, none in the
-upper. This genus was fairly common in the Miocene of Europe, Asia, and
-Africa. In the tropics it survived throughout the Pliocene and possibly
-into the Pleistocene. Some of the species acquired the size of
-elephants, but it is apparent that they were not ancestral to any of the
-more progressive types. They are to be regarded rather as an offshoot
-from the main line of descent.
-
-In 1859 only ten species of the elephant-like mammals were known, and
-all were referred to a single genus. At the present time eleven genera
-appear to be well founded, and the number of recognized species has
-reached a hundred, if it has not already passed that figure. New
-discoveries are expected to add to the existing total. With this mass of
-material before us we note certain definite trends among the more
-progressive types. The increasing weight was accompanied by the
-development of strong, upright limbs in which the bones have a columnar
-position instead of the angular assembly which prevails among most of
-the mammals. As the tusks increased in size there was a shortening of
-both skull and neck to bring the weight closer to the point of support.
-The front teeth disappeared except the second pair of upper incisors
-which remain as tusks in the modern elephant. The cheek teeth present in
-the shortened jaws of the mammoth were reduced to one pair at a time in
-the upper set and another pair below. From a simple, low-crowned origin
-these grinding teeth developed into the more successful high-crowned
-pattern with numerous plates of enamel inside. A prehensile upper lip
-acquired the length and usefulness of the elephants trunk.
-
-    [Illustration: Rancho la Brea Fossils
-
-    One of the most unusual of the many animals that have been taken
-    from the tar pits is the large ground sloth, seen at the left in
-    this group. Such sloths were very abundant during Pleistocene time,
-    and some may have lived up to a few thousand years ago.
-    Archaeologists have found indications that these creatures may have
-    been hunted by cave-dwelling peoples of the American Southwest.
-
-    Other skeletons include the saber-tooth tiger, characterized by the
-    long curved upper canine teeth which undoubtedly were used for
-    stabbing and slashing, and the dire wolf, the smaller of the two
-    which are facing the sloth. The artist’s reconstruction of this
-    scene also shows the great vulture, Teratornis, which is the largest
-    known bird of flight.]
-
-Over-specialization in the production of tusks appears to have been the
-principal factor in the downfall of the mammoths. The large size of the
-animals and the difficulties of finding sufficient food to sustain life
-must have been a serious handicap at times, but their ability and
-inclination to travel over long distances enabled some of them to find
-tolerable living conditions until the end of the Glacial Period. They
-are now extinct and the nearest living relatives are the elephants,
-somewhat reduced in size of tusks and body but otherwise very similar.
-
-There are many other tribes of mammals whose ancient history is
-partially known though broken by periods of time for which there is no
-fossil evidence. All have undergone changes in which various forms and
-degrees of specialization are featured; this general process is best
-revealed by the horses and elephant-like animals which have left a
-clearer record. For other groups the story would differ but little
-except as to names and specific details.
-
-
-THE RANCHO LA BREA FOSSIL PITS
-
-The La Brea tar pits, as they are often called, provide a remarkable
-record of Pleistocene life in southwestern North America. Scattered over
-an area of about thirty acres just off Wilshire Boulevard in Los
-Angeles, these bone deposits were known, as far back as 1875, to contain
-the remains of prehistoric animals. It was not until 1905, however, that
-their value was recognized by paleontologists. In that year the
-University of California began an investigation, and excavations were
-carried on at intervals by various institutions during the next ten
-years. A great deal of material was acquired by the Los Angeles Museum
-of History, Science, and Art, where many skeletons, skulls, and other
-interesting specimens have been placed on exhibition.
-
-The pits have the form of small craters formed by the seeping of oil
-from the underlying rocks. The seeps appear to have been active during
-part of the Pleistocene period but apparently not at the beginning. The
-oil is rich in asphalt which has served as a preservative for the bones,
-and owing to its sticky properties has been an effective animal trap for
-thousands of years.
-
-The fossil beds at present are of oil-soaked earth and sand. In past
-times there must have been a greater percentage of oil, often concealed
-by a layer of dust or pools of water. The large number of carnivorous
-animals found in the deposits suggests that they were attracted by the
-cries and struggles of creatures wandering carelessly into the asphalt
-and serving as live bait to keep the traps in continuous operation.
-
-Animals found there include many species still living in the locality,
-some that have migrated to other territory, and a large number that have
-become extinct. Among the latter may be mentioned species that differ
-but slightly from living relatives, others that have left no
-descendants. Horses, bison, and wolves, though extinct species, were of
-relatively modern types. On the other hand the large sloths and
-saber-tooth cats seem rather out of place. True cats are represented by
-the mountain-lion, bob-cat, and a species of lion which is nearly
-one-fourth larger than any of the great cats of the Old World. A
-long-legged camel, with a height of approximately eight feet to the top
-of the head, was among the native animals of the district. Skunks,
-weasels, badgers, squirrels, rabbits, bear, deer, and antelope were more
-or less abundant.
-
-The La Brea group exhibited by the Denver Museum of Natural History
-includes the following species: horse (_Equus occidentalis_), bison
-(_Bison antiquus_), wolf (_Aenocyon dirus_), saber tooth (_Smilodon
-californicus_), sloth (_Mylodon harlani_). Horses had entirely
-disappeared from the North American continent by the time the first
-white man arrived. _Equus occidentalis_ was one of the several species
-living during the Pleistocene period, this one apparently being
-restricted to California and perhaps adjacent states. _Bison antiquus_
-was slightly larger than the plains bison of recent times and had it
-horns set at a characteristic different angle. The species was first
-described from Kentucky and appears to have had a wide distribution.
-
-The wolves in this group are about the size of timber wolves, but have
-heavier skulls with less brain capacity, massive teeth especially
-adapted to biting and crushing large bones, and limbs of rather light
-construction. They probably assembled in packs where meat was abundant
-and, hunting in this fashion, were able to attack and overcome the
-larger ungulates and edentates. To most visitors the large ground sloth
-is the most interesting animal of the group. This edentate animal is
-shown at the edge of the pool with one foot stuck in the “tar.”
-
-The edentates are a group of primitive animals with very simple teeth,
-if any. Teeth are usually lacking in the front part of the mouth,
-sometimes entirely absent, as among anteaters. Better known living
-representatives of the group are the tree sloths, armadillos, and
-anteaters of South America. Ground-sloths were prominent among South
-American mammals during much of Cenozoic time. During Pliocene and
-Miocene time there was a marked tendency to large size, and it was
-principally during these two periods that they appeared in the United
-States area.
-
-_Mylodon_ was one of the larger North American ground-sloths. Its teeth,
-without the protective enamel which is present among higher mammals, are
-restricted to the cheek region, and have the form of simple pegs;
-instead of being specialized they stand close to the extreme of
-generalization. The construction of the entire skeleton is massive,
-suggesting great strength with slow movements. The hands are well
-developed, provided with stout claws, and must have served the creature
-well as protection against attacks by predatory neighbors. We have some
-idea as to what caused the extermination of the ground-sloths in this
-particular region, but the complete disappearance of such a large and
-widely distributed group at the close of the Pleistocene period is a
-mystery that may never be explained.
-
-    [Illustration: The Folsom, New Mexico, Bison (_Bison taylori_)]
-
-The saber-tooth cat, sometimes referred to as a tiger, was specialized
-as a meat eater though hardly as a hunting animal. In the La Brea region
-its principal food was probably the flesh of the sluggish ground-sloths.
-The size was equal to that of the African lion, with hind limbs slightly
-longer and the front legs more powerfully developed. The most remarkable
-characteristic is to be found in the development of the upper canine
-teeth and modifications of the skull which were necessary to enable the
-animal to use these teeth as weapons.
-
-In order to make the “sabers” effective it was necessary to get the
-lower jaws out of the way, and this was provided for in an unusual type
-of hinge which enabled the mouth to open wider than is possible in the
-case of the less specialized carnivores. Judging by all the structural
-features of the skeleton, _Smilodon_ could not have lived well on small
-animals, for it was not equipped to capture that kind of prey. It is
-evident that large mammals were preferred, and that the method of attack
-was to spring upon the victim and cling there with the powerfully
-developed fore limbs until the kill was completed by stabbing into a
-vulnerable spot. That the position of the large sabers near the front of
-the mouth interfered with normal feeding, is a reasonable conclusion.
-There are also anatomical features which lead to the belief that this
-carnivore was a blood sucker, perhaps more than it was meat-eater.
-
-If most of these conclusions are correct we have here another case of
-over-specialization and a possible explanation of the extinction of two
-species. Such evidence as we have is far from conclusive, for there is
-no proof that Rancho La Brea was the last stand of either the
-saber-tooth or the ground-sloth. Both races were widely distributed and
-their living conditions could not have been exactly duplicated in other
-localities. It has been suggested, however, that _Smilodon_ ate the last
-of _Mylodon_, and starved soon afterward because it had become unable to
-partake of other foods. The conjecture is offered for what it is worth,
-together with the facts on which the story has been based.
-
-The geological record for Pleistocene time is not as complete as one
-might imagine. Numerous localities have produced representative fossils
-but the yield is rarely large enough to solve many of the riddles which
-are constantly arising as investigation proceeds. Aside from those areas
-which bordered the retreating ice cap and where living conditions were
-far from favorable, the sedimentary deposits of this period are not
-continuous over large areas. Many Pleistocene fossils are found in
-stream channel beds which are always subject to removal by subsequent
-floods.
-
-    [Illustration: Early Man in North America
-
-    There is abundant evidence to indicate that the great elephants of
-    Pleistocene time were hunted by primitive Americans whose only
-    weapons were darts or spears tipped with points of stone. A skull
-    and the lower jaws of several mammoths are shown here.]
-
-Isolated patches of fossil-bearing sediments frequently record the
-migration of animals in unmistakable terms, but the details of the
-wanderings and the conditions encountered in the newly established
-habitats are often left in doubt. To correlate the facts revealed at one
-locality with findings at other places and, if possible, to date all
-prehistoric events with a greater degree of accuracy are among the major
-tasks of current investigations.
-
-
-
-
-                             THE AGE OF MAN
-
-
-The Pleistocene or “Ice Age,” and the Recent period in which we are
-living at the present moment are not sharply separated by any event
-readily recognized or dated, and the two combined are of very short
-duration as compared with other periods more clearly established by the
-passing of centuries. Together they comprise the Age of Man as commonly
-recognized, with about a million years representing the Pleistocene
-period, some ten to twenty thousand years the Recent. When geologists of
-the nineteenth century suggested that the coming of man should be
-regarded as the beginning of a new era, the name Psychozoic was
-proposed, and to some extent this term has been applied to the present
-period. More in keeping with other period names is Holocene, meaning
-_entirely recent_. Common usage, however, applies the simple term Recent
-to this unfinished chapter which is also without a clear-cut beginning.
-
-Zoologically, man is merely one of the creatures that arrived in the
-course of time, along with other mammals. Just when he arrived and how
-he looked at the time of his coming cannot be determined from a study of
-fossils. Perhaps it is of no importance. There is nothing to indicate
-his existence before the Cenozoic, no completely satisfactory proof of
-existence before the Pleistocene period. As with other inhabitants of
-the earth, it is probable that he became prominent only after a great
-deal of competition with other creatures which kept his ancestors
-submerged for thousands of years. The Ice Age, with its check upon the
-progress of competing animals, undoubtedly gave him an advantage. His
-superior mentality enabled him to overcome adversity by methods not
-available to other mammals; his inventive and mechanical genius must
-have been greatly strengthened by his experience during this interval.
-
-At about this point, where prehistory begins to merge into history, the
-geologist and paleontologist must let other interpreters carry on.
-Archeologists and anthropologists take up the work, and through their
-efforts many details have been added to our knowledge of the human race.
-The study of biology, which is the science of life, has provided an
-instructive viewpoint that enables us to see ourselves against the vast
-background built up by investigations into the nature of the earth and
-its ancient inhabitants. This science deals with living creatures as
-_organisms_—plants and animals so organized as to be capable of
-existence only in an environment which provides exact life requirements.
-
-The Age of Man has been variously characterized as an age of soul, of
-higher intelligence, of culture, and finally, of civilization, freedom
-and democracy. The “crowning glory” of the organic world is pictured in
-history as a creature who has busied himself for thousands of years with
-the building up and tearing down of civilizations. Prehistory reveals
-this habit as something unique in the human character, for there is no
-other organism that has specialized so persistently in the creation of
-its own environment, no other that has had the combined power and talent
-to produce so much change.
-
-More than anything else, the prehistoric record is a lesson in
-adaptation, which in its broadest sense means fitness for life under
-particular conditions, and always subject to organic law. Man’s efforts
-to bring about an adjustment between himself and his civilization have
-centered largely on the method of forcing himself into the mold that
-happens to be present, one pattern today, another tomorrow. No creature
-of the past has had to adapt itself to anything so radically new or so
-thoroughly revolutionary. The vital problem now is whether this man-made
-environment will prove helpful or disastrous.
-
-Though one of its names is “culture,” it has grown sporadically and
-unevenly, with little evidence of the cultivation that is implied and
-required. Parts have been expanded to extraordinary proportions while
-others equally essential have been retarded in their growth. A more
-intelligent handling of this environment factor seems to be possible,
-and the present mania for “organization” may become tempered with an
-awakening consciousness of organic requirements where organism and
-environment are involved. Once we grasp the idea that “culture” results
-from man’s effort to improve his living, by putting into his environment
-something that was not there before—then, surely, this history of a
-billion years of living, and as many “ways of life,” should teach us
-something we ought to know as we go into an all-out endeavor to teach a
-whole world how to obtain a one-and-only way.
-
-We may stand at the beginning of an era for which an appropriate name
-has not yet been suggested. Civilization, on the other hand, may provide
-only a minor epoch to be added in some remote time to the story of
-fossils.
-
-
-
-
-                         SUPPLEMENTARY READING
-
-
-The literature pertaining to fossils is widely scattered and usually too
-technical for the layman. It is better to use the resources of the
-nearest library than to feel that a specified list of books is
-necessary.
-
-Any textbook on geology, zoology, or botany will provide helpful
-information. Most books of this type will be found interesting and
-readable if used to solve definite problems suggested by the student’s
-immediate curiosity. Very few can be read from beginning to end without
-a great deal of effort and discouragement.
-
-The following have been prominent among the books consulted by the
-author:
-
-_Textbook of Geology_; by Pirsson and Schuchert. This work has undergone
-several revisions and currently appears in two volumes: _Physical
-Geology_ by Longwell, Knopf, and Flint; _Historical Geology_ by C. O.
-Dunbar. Published by John Wiley & Sons. (Historical geology covers the
-entire range of prehistoric life—plant, invertebrate, and vertebrate.)
-
-_Historical Geology_ (The Geologic History of North America); by Russell
-C. Hussey. Published by McGraw-Hill. Concise, interesting, and
-informative.
-
-_Geology and Natural Resources of Colorado_; by R. D. George. Published
-by the University of Colorado. Contains an excellent summary of the
-historical geology and sedimentary formations of Colorado.
-
-_Vertebrate Paleontology_; by Alfred Sherwood Romer. Published by the
-University of Chicago Press. This is one of the most comprehensive and
-up-to-date treatments of the subject for students desiring to go beyond
-the elementary stage.
-
-_A History of Land Mammals in the Western Hemisphere_; by William
-Berryman Scott. Published by The Macmillan Company. This well-known
-account of living and extinct mammals is one of the favorites among
-students.
-
-_The Age of Mammals_; by Henry Fairfield Osborn. A classic in this field
-of literature, but for advanced reading. The book is now out of print.
-
-_The Dinosaur Book_; by Edwin H. Colbert. Published by the American
-Museum of Natural History, New York. An illustrated story of amphibian
-and reptilian evolution.
-
-_Down to Earth_; by Carey Croneis and William C. Krumbein. Published by
-the University of Chicago Press. An excellent popularization of the
-earth sciences—geology and paleontology.
-
-_Lexicon of Geologic Names of the United States_; compiled by M. Grace
-Wilmarth. Bulletin 896 (in two parts) of the United States Geological
-Survey. A rich source of information concerning the age, character, and
-distribution of geologic formations, with numerous references to
-fossil-bearing beds.
-
-_Bibliography of North American Geology_ (including paleontology);
-various bulletins of the United States Geological Survey. Where library
-facilities provide access to the technical literature of museums,
-universities, and scientific societies, this is a valuable aid in
-locating publications dealing with original work in paleontology.
-Bulletins 746 and 747 cover the years between 1785 and 1918; Bul. 823
-(1918-1928); Bul. 937 (1929-1939); Bul. 938 (1940-1941); Bul. 949
-(1942-1943); Bul. 952 (1944-1945); Bul. 958 (1940-1947); Bul. 968
-(1948); Bul 977 (1949). Preparation is a continuous process with recent
-bulletins appearing at one or two year intervals.
-
-_Ancient Man in North America and Prehistoric Indians of the Southwest_;
-by H. M. Wormington. Published by Denver Museum of Natural History, City
-Park, Denver 6, Colorado. Both volumes contain authentic and up-to-date
-accounts of early American cultures.
-
-
-MAPS
-
-_Geologic Maps._ United States Geological Survey: map of the United
-States (1932); map of Colorado (1935). Geologic maps of a few other
-states are available; information regarding these may be obtained from
-state universities or state geological surveys.
-
-  Note: Bulletins of the U.S.G.S. are purchasable from the
-  Superintendent of Documents, Washington, D. C. Maps are sold by the
-  Director of the Geological Survey, Washington, D. C.
-
-
-
-
-                          Transcriber’s Notes
-
-
-—Silently corrected a few typos
-
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-
-The Project Gutenberg EBook of Fossils: A Story of the Rocks and Their
-Record of Prehistoric Life, by Harvey C. Markman
-
-This eBook is for the use of anyone anywhere in the United States and most
-other parts of the world at no cost and with almost no restrictions
-whatsoever.  You may copy it, give it away or re-use it under the terms of
-the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org.  If you are not located in the United States, you'll have
-to check the laws of the country where you are located before using this ebook.
-
-Title: Fossils: A Story of the Rocks and Their Record of Prehistoric Life
-       Denver Museum of Natural History, Popular Series No. 3
-
-Author: Harvey C. Markman
-
-Illustrator: Mary Chilton Gray
-
-Release Date: July 22, 2017 [EBook #55168]
-
-Language: English
-
-Character set encoding: UTF-8
-
-*** START OF THIS PROJECT GUTENBERG EBOOK FOSSILS: A STORY OF THE ROCKS ***
-
-
-
-
-Produced by Stephen Hutcheson and the Online Distributed
-Proofreading Team at http://www.pgdp.net
-
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-
-</pre>
-
-<div id="cover" class="img">
-<img id="coverpage" src="images/cover.jpg" alt="A Story of the Rocks and Their Record of Prehistoric Life" width="500" height="725" />
-</div>
-<div class="box">
-<h1><span class="large">FOSSILS</span>
-<br /><span class="smaller">A Story of the Rocks
-<br />and
-<br />Their Record of Prehistoric Life</span></h1>
-<p class="center"><span class="large">By Harvey C. Markman</span>
-<br /><span class="small">Curator of Geology and Paleontology</span></p>
-<p class="center"><span class="small">Cover Design and Murals by</span>
-<br />Mary Chilton Gray</p>
-<p class="tbcenter"><span class="large">DENVER MUSEUM OF
-<br />NATURAL HISTORY</span></p>
-<p class="center small">Popular Series No. 3
-<br />Alfred M. Bailey, Editor</p>
-<p class="center small">Third Edition, Reprinted
-<br />October 1, 1954</p>
-</div>
-<div class="pb" id="Page_2">2</div>
-<div class="img" id="fig1">
-<img src="images/p01.jpg" alt="" width="800" height="532" />
-<p class="pcap"><span class="sc">At Work on a Dinosaur Skeleton</span></p>
-</div>
-<div class="pb" id="Page_3">3</div>
-<h2 class="center">CONTENTS</h2>
-<dl class="toc">
-<dt class="small">Page</dt>
-<dt><a href="#c1">Introduction</a> 5</dt>
-<dt><a href="#c2">The Prehistoric Record</a> 5</dt>
-<dd><a href="#c3">Varieties of Fossils</a> 8</dd>
-<dd><a href="#c4">Fossilization</a> 9</dd>
-<dd><a href="#c5">Floras and Faunas</a> 13</dd>
-<dd><a href="#c6">Formations</a> 16</dd>
-<dd><a href="#c7">Geological Time</a> 18</dd>
-<dd><a href="#c8">Explanation of the Time Chart</a> 23</dd>
-<dd><a href="#c9">The Geological Section</a> 25</dd>
-<dt><a href="#c10">Before the Age of Reptiles</a> 31</dt>
-<dd><a href="#c11">The Pre-Cambrian Complex</a> 31</dd>
-<dd><a href="#c12">Cambrian Life</a> 33</dd>
-<dd><a href="#c13">The Ordovician Record</a> 35</dd>
-<dd><a href="#c14">Silurian Events</a> 36</dd>
-<dd><a href="#c15">Devonian Progress</a> 37</dd>
-<dd><a href="#c16">Carboniferous Forests</a> 40</dd>
-<dd><a href="#c17">Permian Hardships</a> 43</dd>
-<dt><a href="#c18">The Age of Reptiles</a> 47</dt>
-<dd><a href="#c19">Dinosaurs</a> 48</dd>
-<dd><a href="#c20">Plant Life and Climate</a> 56</dd>
-<dd><a href="#c21">Coal and Fossil Footprints</a> 57</dd>
-<dd><a href="#c22">Mesozoic Invertebrates</a> 58</dd>
-<dd><a href="#c23">Extinct Birds</a> 60</dd>
-<dd><a href="#c24">Ancestors of the Mammals</a> 61</dd>
-<dt><a href="#c25">The Age of Mammals</a> 64</dt>
-<dd><a href="#c26">Prehistoric Horses</a> 70</dd>
-<dd><a href="#c27">Mastodons and Mammoths</a> 80</dd>
-<dd><a href="#c28">The Rancho La Brea Fossil Pits</a> 88</dd>
-<dt><a href="#c29">The Age of Man</a> 93</dt>
-<dt><a href="#c30">Supplementary Reading</a> 95</dt>
-</dl>
-<div class="pb" id="Page_4">4</div>
-<h1 title="">LIST OF ILLUSTRATIONS</h1>
-<dl class="toc">
-<dt class="small">Page</dt>
-<dt><a href="#fig1">At Work on a Dinosaur Skeleton</a> 2</dt>
-<dt><a href="#fig2">Fossil Bones in Rock Formation</a> 7</dt>
-<dt><a href="#fig3">Insect Fossils</a> 10</dt>
-<dt><a href="#fig4">Restoration of Rhinoceros</a> 12</dt>
-<dt><a href="#fig5">Dinosaur Tracks</a> 17</dt>
-<dt><a href="#fig6">Time Chart</a> 22</dt>
-<dt><a href="#fig7">Geological Section Showing Positions of Formations</a> 26</dt>
-<dt><a href="#fig8">Marine Beds of the Benton Formation</a> 28</dt>
-<dt><a href="#fig9">Plesiosaur Bones in Place</a> 28</dt>
-<dt><a href="#fig10">Invertebrate Fossils</a> 34</dt>
-<dt><a href="#fig11">Modernized Fishes</a> 38</dt>
-<dt><a href="#fig12">Prehistoric Plants</a> 41</dt>
-<dt><a href="#fig13">Marine Reptiles</a> 46</dt>
-<dt><a href="#fig14">Diplodocus</a> 49</dt>
-<dt><a href="#fig15">Trachodon</a> 51</dt>
-<dt><a href="#fig16">Stegosaur</a> 52</dt>
-<dt><a href="#fig17">Sea Turtle</a> 54</dt>
-<dt><a href="#fig18">Murals, Hall of Mammals</a> 63</dt>
-<dt><a href="#fig19">Uintatheres and Contemporary Life</a> 65</dt>
-<dt><a href="#fig20">Moropus</a> 67</dt>
-<dt><a href="#fig21">Titanotheres</a> 69</dt>
-<dt><a href="#fig22">Oligocene Mammals</a> 71</dt>
-<dt><a href="#fig23">Pleistocene Horse</a> 73</dt>
-<dt><a href="#fig24">Structure of Molar Teeth</a> 75</dt>
-<dt><a href="#fig25">Grazing Type of Molar Teeth</a> 77</dt>
-<dt><a href="#fig26">American Mastodon</a> 79</dt>
-<dt><a href="#fig27">Long-Jawed Mastodont</a> 81</dt>
-<dt><a href="#fig28">Molar Tooth of Mammoth</a> 83</dt>
-<dt><a href="#fig29">Nebraska Mammoth</a> 85</dt>
-<dt><a href="#fig30">Rancho La Brea Fossils</a> 87</dt>
-<dt><a href="#fig31">Folsom Bison</a> 90</dt>
-<dt><a href="#fig32">Man and Mammoth</a> 92</dt>
-</dl>
-<div class="pb" id="Page_5">5</div>
-<h1 title="">FOSSILS</h1>
-<h2 id="c1"><span class="small">INTRODUCTION</span></h2>
-<p>In the recent growth of knowledge there has been rapid progress
-in two directions. The commercial exploitation of natural resources,
-being fundamental to modern civilization, attracts a liberal share of the
-talents and energies of workers trained for the industrial professions.
-A second trend has specialized in the further development of the sciences
-which are characteristic of our time. Such activities, in the natural
-history field, deal largely with the refinements of exact definition, nomenclature
-and classification, all of which means little or nothing to the
-layman who is otherwise engaged.</p>
-<p>For the latter, however, there is a quality of interest which may be
-described as a wholesome curiosity about what has happened, how it
-happened, how we know it happened, and what it may signify to one
-who is neither industrialist nor scientist. This booklet is intended for the
-many who feel that there is more to be obtained from a natural history
-museum than an occasional glimpse of a bewildering &ldquo;marvel.&rdquo; In addition
-to being a guide to fossil exhibits it supplies parts of a great story
-which specimens alone can not relate.</p>
-<h2 id="c2"><span class="small">THE PREHISTORIC RECORD</span></h2>
-<p>All that is known of the extinct plants and animals which inhabited
-the earth before man began the practice of recording his observations
-has been obtained from a study of the rocks. The few possible exceptions
-to this rule, in which animal and plant remains have been preserved
-by freezing or drying, are so unusual as to be hardly worth
-mentioning.</p>
-<p>Explanation of this is that plant and animal tissues quickly decay
-under ordinary conditions when life ceases. Unless protected from destructive
-agencies which are especially active at the surface of the ground,
-even the heavier bones of animals and the large trunks of fallen trees will
-soon crumble into shapeless masses. The usual method employed by
-nature to prepare a fossil specimen is so closely related to the natural
-process of rock making that a little knowledge of that subject will be
-necessary in order to know what fossils are and how they are preserved
-for so long a time.</p>
-<p>It should be understood first that a fossil is some record, commonly
-preserved in rock, of a kind of plant or animal which no longer exists
-<span class="pb" id="Page_6">6</span>
-as a living type. This, at least, is the ordinary sense of the word and
-more elaborate definitions are of small service to anyone. It may be
-necessary to add, however, that all things which have lived at any time
-are regarded as either plants or animals.</p>
-<p>Nature&rsquo;s way of producing rocks and fossils remains a mystery to
-many of us because we are so wrapped up with the importance of
-finding names for things and materials that we frequently neglect the
-consideration of sources and histories. Everyone knows a rock when he
-sees it in a large mass, but when he looks at sand, mud, dust, or soil,
-he seldom thinks of it as related in any way to rocks. Although the
-difference is almost entirely a matter of size, our use of words makes
-it seem unreasonable to speak of the finer particles as rock.</p>
-<p>There can be no reality or meaning in the natural record for an
-individual who has failed to observe a few simple facts which involve
-changes going on in all parts of the world at the present time. With
-regard to rocks, it is supposed that what happens in our day also occurred
-under like circumstances ages ago. Anyone wishing to do so may see for
-himself that rock masses break down wherever they are exposed to the
-elements, that the larger pieces are reduced to smaller fragments, and that
-the final product is sand or dust.</p>
-<p>He may also note that this finely ground material is being moved
-and sorted, by rain, wind, and streams, transported to lower levels and
-accumulated in great quantities wherever it finds a resting place. Along
-with it go sticks and leaves, bugs, shells, bones and carcasses of animals,
-some of which in time may become fossils. In large lakes and seas there
-is a steady distribution of such materials over broad areas, yesterday&rsquo;s
-accumulation of sediments being buried by the contributions of today,
-the most recent of the settlings always resting upon older ones until
-something happens to disturb that arrangement.</p>
-<p>Not so readily observed are other parts of the process, such as the
-consolidation of sands and muds into the firm sandstones and shales which
-we again recognize as rocks. Much of this requires more than the few
-score years of a human lifetime for its accomplishment, but many of us
-have seen muds become so solidified, by merely drying, that they could
-hardly be distinguished from prehistoric shales. It is to be noted also that
-some ancient fossils come from sandstones which are scarcely more rock-like
-than the loose sands of an ocean beach. Thus we learn that firm
-consolidation of rock-making materials is not always a sign of great antiquity,
-and that hardness of rock is not always essential to the preservation
-of imbedded plants and animals.</p>
-<div class="pb" id="Page_7">7</div>
-<div class="img" id="fig2">
-<img src="images/p02.jpg" alt="" width="600" height="779" />
-<p class="pcap"><span class="sc">Rhinocerous Bones as Found in the Rock</span></p>
-<p class="pcapc">This exhibit was taken from the famous fossil quarry at Agate, Nebraska.
-The fossilized remains are still partially imbedded in the sandstone which preserved
-them for millions of years.</p>
-</div>
-<div class="pb" id="Page_8">8</div>
-<p>The rocks themselves must explain the many things which have
-happened during the course of millions of years, and this they do remarkably
-well when carefully studied, for many of the factors involved in
-their histories leave characteristic marks. Changing climates, the draining
-of seas, the uplifting of mountain ranges, all have ways of registering
-their occurrence which are as convincing and reliable as anything ever
-written by man. Piece by piece the story has been patched together
-through the efforts of thousands of investigators. Parts of the narrative
-remain buried at inaccessible depths, and whole chapters, no doubt, have
-been destroyed by the same forces that composed this tremendous record
-of prehistoric times.</p>
-<h3 id="c3">VARIETIES OF FOSSILS</h3>
-<p>It would be a serious mistake to regard nature as divided into a
-number of distinct and independent schools of fossil making, each refusing
-to use the methods and devices of another. There are, however,
-certain features which stand out so prominently that a little classification
-becomes helpful. While this procedure brings out differences it should
-be understood that processes actually work together, several of them
-usually being involved in the production of any individual specimen.</p>
-<p>(1) Impressions of animals and plants, or parts of these, are frequently
-left in soft sand or mud which later becomes converted into more
-durable rock. This type of fossils is represented by animal foot-prints
-and the imprints of leaves, flowers, insects, and like objects which may be
-mingled with the finely ground materials of the common sedimentary
-rocks.</p>
-<p>(2) Parts of plants and animals may be gradually replaced by mineral
-matter with little or no change from original form and texture. Fossils
-of this class are said to be petrified or turned to stone. They are also
-known as replacements. The fleshy parts of animals do not petrify.</p>
-<p>(3) Many animals among the invertebrates use mineral substances
-for protective or supporting structures. Small plants of various kinds follow
-a similar practice. These structures, being produced in stony materials,
-are readily converted into fossils. The shells of mollusks are the
-best known illustrations in this field, and all that is required for a shell to
-become a fossil is the extinction of the species of animal that produced it.
-Fossils of this type are extremely abundant.</p>
-<p>(4) Preservative substances other than those which produce common
-rocks may be mentioned among fossil-making possibilities. Bones
-are known to have been preserved in asphalt, and insects in resins, but
-such cases are few in comparison with the products of other methods.</p>
-<p>(5) In rare instances there has been preservation of extinct creatures
-by the process of drying or by refrigeration. Occasional mummies
-<span class="pb" id="Page_9">9</span>
-are found with shriveled flesh and skin still in place, but better preservation
-of all tissues occurs when the temperature is quickly reduced below
-freezing point and held there without interruption. This can happen
-only in the colder parts of the earth and is always subject to climatic
-change. The effect of drying also may be undone at any time by a slight
-increase in the amount of moisture.</p>
-<p>(6) Coal beds often produce fossils of an unusual sort. In the formation
-of coal, plant material gradually loses some of its more perishable
-substances but retains carbon which has better lasting qualities and slowly
-accumulates to produce the seams and beds that are mined. In the early
-stages of the process the original vegetation undergoes little change in
-appearance but eventually practically all of its character is lost. Many
-fossil leaves are found as thin layers of carbon, bedded in the clays which
-are commonly associated with coal deposits.</p>
-<p>(7) Concretions, which are hardened lumps of mineral substances
-occurring commonly in sandstones and shales, are often mistaken for fossils
-because of their peculiar shapes. However, there are localities in
-which the mineral solutions have been concentrated and deposited around
-shells, leaves, seeds, or similar objects, thus producing an abundance of
-fossils which may be obtained by opening the concretions. Fossils of this
-type are well known from Mazon Creek and other districts in Illinois,
-Kansas, Colorado, and elsewhere.</p>
-<h3 id="c4">FOSSILIZATION</h3>
-<p>Footprints need little explanation other than a consideration of the
-factors which make it possible for them to be preserved. The sand or
-mud must be neither too soft nor too hard to take the form of the foot
-and retain its shape when the foot is withdrawn. Then in some manner
-the impression must be protected while the rock-making process goes on.
-When such protection is obtained it is usually in the form of more mud
-and sand, deposited over the surface which received the impression. At
-a later time the covering may be separated from the lower part of the
-deposit, which serves as a mold, and if the separation be accomplished
-successfully a natural cast of the foot will be obtained as well as the
-mold in which it was produced. Since conditions for perfect work are
-not always present in a laboratory of this kind, it is not surprising that
-fossil footprints are very rare considering the number and variety of
-tracks left by wandering animals.</p>
-<p>Impressions of leaves are explained in much the same way except
-that the leaf remains under its protective covering until it decays. Similar
-impressions may be obtained from the bodies of delicate invertebrate animals
-<span class="pb" id="Page_10">10</span>
-but they are seldom preserved because of the softness of the tissues.
-The smaller fishes provide much better material for the production of
-fossils according to this method. While the fish is being flattened by the
-weight of surrounding sediments, scales, fins, and soft bones retain their
-positions and provide the necessary resistance to leave an impression of
-the body form when the flesh is gone.</p>
-<div class="img" id="fig3">
-<img src="images/p03.jpg" alt="" width="800" height="455" />
-<p class="pcap"><span class="sc">Insect Fossils</span> (enlarged)</p>
-<p class="pcapc">Fine specimens of this type are obtained from an old lake bed at
-Florissant, Colorado.</p>
-</div>
-<p>The larger and more spectacular fossils, such as skeletons, skulls,
-and detached bones are nearly always of the replacement type. Replacement
-of plant and animal substances by mineral matter is a slow process
-and in younger fossils the change is rarely completed, some of the original
-material being present in a partially altered condition or not modified at
-all. Since air does not often carry the necessary materials and provide
-other essential conditions, replacement may be regarded as something
-which happens underground or in water. It is perhaps best explained in
-connection with limestones, because calcite or &ldquo;lime&rdquo; is frequently the replacing
-substance although other minerals, especially quartz, may serve
-the purpose.</p>
-<p>Besides converting bony or woody objects into rock substance, mineral
-replacements may assist in the production and preservation of fossils
-in another manner. It often results in the filing of cavities with some
-rock-making substance which retards destruction through crushing or
-other injury. In many cases, so-called fossil shells are not shells at all;
-instead, they are merely a stony filling which was once surrounded by
-<span class="pb" id="Page_11">11</span>
-shell substance. In other instances the original shell remains as it was
-during the life of its former occupant, preservation of the shell being due
-largely to the substitution of a mineral filler for the soft animal tissues
-once present.</p>
-<p>Limestone comes into existence through a more elaborate process
-than that which produces sandstone and shales. It is one of the three
-types of common rocks, known collectively as the sedimentaries, in which
-fossils are found. It differs from sandstones and shales, however, in that
-much of its substance has been dissolved in water instead of being transported
-in the form of finely ground rock particles. Lime occurs in many
-varieties of rock which are exposed to the wear and tear of the elements
-throughout the world. Slowly but more or less continuously it is taken
-from this source by ground and surface waters coming in contact with it.
-Particularly active is carbonated water, moving underground through
-pores and crevices.</p>
-<p>This underground circulation of mineral matter in a dissolved condition
-explains the occurrence of fossils in land areas which have not
-necessarily been submerged during any great length of time, for it is well
-known that plant and animal remains are not invariably washed into
-lakes or seas, and that all sedimentary deposits have not been built up in
-large bodies of water. Here we are dealing with what is known as the
-continental type of sedimentation and such fossils as dinosaurs, mastodons,
-three-toed horses, and other former inhabitants of land areas.</p>
-<p>In order to become properly fossilized, certain conditions are absolutely
-necessary, and only a small percentage of the once-living multitude
-secures the required treatment. There must be present, soon after
-death, some protection from the activities of the carnivorous birds and
-beasts that would separate and scatter the parts of a carcass, also from
-the smaller gnawing animals that would continue the destruction, and
-finally from wind, sun, rain, frost, and bacterial and chemical activities
-which in the course of only a few years would remove everything but
-possibly a few scraps of tooth enamel, which is the hardest of animal
-tissues.</p>
-<p>A slight covering of earth substance in any form serves to check the
-disintegration, and this may be acquired in several ways. Animals that
-perish in bogs or quicksands are soon covered over; in many localities
-wind-blown dust and sand do the work; and flooded river valleys provide
-an abundance of mud for the necessary burial of others. Even underground,
-the decay of soft tissues is too rapid to permit of replacement by
-mineral substance in a manner that would reproduce form and texture.
-Skin and flesh are almost invariably lost, although in a few instances the
-thick scaly hides of dinosaurs are known to have produced natural molds
-and casts by the method explained in connection with footprints and
-other impressions.</p>
-<div class="pb" id="Page_12">12</div>
-<div class="img" id="fig4">
-<img src="images/p04.jpg" alt="" width="600" height="782" />
-<p class="pcapc">The skeleton of this prehistoric American rhinoceros is mounted in a
-&ldquo;half shell&rdquo; which was modeled over the bones to show the form of the living
-animal. The artist&rsquo;s reconstruction appears in the painting above the fossil exhibit.</p>
-</div>
-<div class="pb" id="Page_13">13</div>
-<p>With regard to the more durable tissues found in the teeth, bones,
-and shells of animals, or the woody parts of plants, the case is different.
-These parts become firmly imbedded in the ground, but moisture still
-has access, and it begins to work immediately; for all water moving underground
-finds soluble substances which it picks up and carries with it
-wherever it goes, and much of the load consists of mineral matter which
-may be unloaded again when the necessary conditions are found.</p>
-<p>Mineral-laden waters will drop one kind of substance to take up
-another which dissolves more readily, and this happens sooner or later
-when a buried bone or log is encountered. Complications of various
-sorts enter into the process, but the final outcome frequently is a complete
-change from one chemical composition to another which is more enduring,
-the transformation being brought about so gradually and thoroughly
-that in many fossils the inner structure of the original tissue is as accurately
-reproduced as the fine detail of surface features.</p>
-<p>Converted into stone, however, the result is still far from permanent.
-While yet underground the fossil is subjected to distortion and breakage
-due to earth movements which bend and dislocate the rock deposits.
-What causes these upheavals and depressions of the earth&rsquo;s surface remains
-the subject of much discussion, but that they have occurred on a
-large scale and continue to occur is clearly evident. At higher altitudes
-the surface rocks and fossils are exposed to a larger variety of destructive
-activities than at lower levels where protective coverings are more likely
-to be provided and retained. Once stripped of that protection there is
-little chance for a fossil to survive. Beyond a doubt there are many
-thousands of tons of prehistoric remains damaged or destroyed each year,
-by weather and stream erosion.</p>
-<h3 id="c5">FLORAS AND FAUNAS</h3>
-<p>As the various types of sediments continue to accumulate on land
-and in water they produce deposits of sandstones, claystones, and limestones
-which in time may acquire great thickness and cover wide areas
-of sea floor, or continental surface. Usually there is more or less mixing
-of sediments resulting in sandy limestones, limy clays, and other combinations.
-Quite commonly, however, the types remain fairly pure but
-become arranged in layers which alternate from one kind of material
-to another. At all times the character of the deposit will depend upon
-the nature of the rocks which supply the materials, and any fossils that
-may be produced will consist of such plants and animals as live and die
-during the time the rock is in the making.</p>
-<div class="pb" id="Page_14">14</div>
-<p>Some of the rock layers will be rich in plant and animal remains,
-others quite barren, the difference being due partly to conditions influencing
-the life of the region. In addition, the character and amount of rock-making
-materials at the time may be favorable or unfavorable to the
-preservation of fossils. Seas, lakes, and valleys may at any time be
-drained, or enlarged and deepened, by changes in the elevation of underlying
-rocks. The amount and variety of mineral substances dissolved in
-the waters of a region not only affect the character of rock deposits but
-also the plants and animals living in the water. Some of these chemical
-solutions provide cementing materials which bind together the grains of
-sands and mud; others have a detrimental effect upon cementing material
-previously deposited, and so construction and destruction go on continuously,
-more or less hand in hand, to produce complicated and often
-puzzling results.</p>
-<p>A little more salt, or a little less of it, may change completely the
-variety of life inhabiting a body of water. A slight change in the depth
-of the water often accomplishes the same thing, for plants and animals
-are so delicately adjusted to their environments that conditions fatal to
-one race of creatures may provide the exact life requirement of another.
-This is a matter of practical knowledge which is being used today in the
-cultivation of plants and animals for market purposes. It is being
-demonstrated continuously, also, upon living subjects in experimental
-laboratories throughout the world; and, in a bigger way, the facts are
-observable wherever life is considered in relation to habitat. That anything
-so obvious should be regarded as guesswork or theorizing, or
-opposed to truth, when applied to former inhabitants of the earth, is
-somewhat surprising. And, it may be added, the cultural worth of fossil
-study comes to a focus on this very point, for men and women are now
-meddling, consciously or unconsciously, wisely or unwisely, with an all-important
-environment about which they have learned very little&mdash;one
-called, among other things, &ldquo;civilization.&rdquo;</p>
-<p>For any portion of the world a complete-list of the different kinds
-of plant inhabitants comprises the <i>flora</i> of that region, and a like summary
-for the animal life is known as the <i>fauna</i> of the district. It is
-generally understood that different species of both plants and animals
-inhabit different regions of the earth, but outside of professional circles
-it is only beginning to be recognized that changes in floras and faunas
-occur from time to time, that slight differences may be noted in the
-course of observations extending over a period of only a few years, and
-that everything in a fauna or flora eventually may be displaced by new
-forms.</p>
-<div class="pb" id="Page_15">15</div>
-<p>It is, however, a convenient practice to use these terms in connection
-with time periods, rock beds, and types of environment, as well as
-geographical areas. Thus we have such phrases as a &ldquo;Cretaceous fauna&rdquo;
-(attaching the name of a geologic period), a &ldquo;Benton fauna&rdquo; (with
-reference to the fossils of a rock formation), a &ldquo;marine flora&rdquo; (using the
-name of an environment), an &ldquo;Arctic flora&rdquo; (which applies to a definite
-portion of the earth surface and its plant inhabitants).</p>
-<p>Faunas include animals which many persons do not recognize as
-such. Sponges, corals, insects, worms, crabs, oysters, and a host of other
-boneless creatures are grouped together as <i>invertebrate</i> animals, while
-another group includes the fishes, amphibians (toads, frogs, and salamanders
-of today), reptiles (crocodiles, lizards, snakes, and turtles being
-well known varieties), birds, and mammals. This second lot, provided
-with backbones and skeletons, comprise the great division of <i>vertebrate</i>
-animals.</p>
-<p>Floras also include types which are commonly seen but not popularly
-identified as plants. The algae are perhaps best known as seaweeds,
-water-silk, and pond scums; fungi as toadstools and moulds. Both groups
-are large and of important rank in the vegetable kingdom; only the algae,
-however, are recognized as important fossil producers. Better known
-types of plants are the mosses, ferns, evergreens, grasses, and the more
-conspicuous flower-bearing forms, from weed size to tree size.</p>
-<p>Many rocks owe their character to the work of large colonies of
-plants or animals, for the living organisms are frequently the active
-agency which takes dissolved mineral substance from the solvent liquid
-and gets it back into solid form. The liquid is, of course, the water in
-which the creatures live, while the mineral substance often becomes a
-commodity required by a plant or animal in its mode of living. Mollusks
-have a way of using lime in the production of shells, and many a bed of
-limestone consists almost entirely of this by-product of molluscan life.
-Tiny coral polyps build complicated and beautiful structures from the
-same mineral substance. Either intact or in broken condition, these structures
-contribute in a large way to the making of limestones. Algae, among
-the lowliest of plants, have done extensive work along similar lines, and
-numerous invertebrate animals could be named as important factors in
-the production of rocks. Many of the shells and other fabrications retain
-their peculiar patterns long after the extermination of their makers,
-and a highly informative part of the fossil record is provided in this
-manner. It is also by far the larger portion of the record, for the earlier
-ages of prehistoric time failed to produce a vertebrate animal of any kind,
-while the invertebrate record dates back to pre-Cambrian time.</p>
-<div class="pb" id="Page_16">16</div>
-<h3 id="c6">FORMATIONS</h3>
-<p>If in some part of North America there had been steady accumulation
-of sedimentary materials under constantly favorable conditions since
-the beginning of Cambrian time, the result would have been a deposit of
-sandstones, claystones, and limestones measuring nearly fifty miles from
-bottom to top. These figures are based on actual production in North
-America where extensive measurements have been made in many localities.
-When other parts of the world are as thoroughly investigated and
-older deposits included in the calculations, the total thickness of such
-beds will probably be more than one hundred miles.</p>
-<p>No single pile of rocks offering a complete cross section of the
-geological record has ever been produced, but portions of the section are
-exposed to view on all the continents. In order to carry on desirable investigations
-and make comparisons, it has been necessary to divide this
-great composite section into small units which may be named in some
-way and placed definitely with relation to lower and higher, or older
-and younger, layers. To serve this purpose there has been developed
-the idea of rock <i>formations</i>, and here we have a word which is not defined
-readily, even for the use of those who are familiar with it. Nevertheless
-it is used so commonly that some understanding of its meaning
-becomes desirable.</p>
-<p>A <i>formation</i> may be regarded as an extensive rock mass, variable,
-in thickness and other proportions, as well as in composition, but representing
-a period of time during which there was no great change in
-the character of plant and animal life, and no serious interruption in
-the depositing of the rock-making materials. Occasionally the lower and
-upper limits of a formation are well defined and readily located. Frequently,
-however, the transition is gradual, one formation merging into
-another with no apparent mark of separation. In such event the original
-description serves to establish more or less definitely the boundaries of
-a formation.</p>
-<p>Descriptions are published whenever a worker believes he has discovered
-a significant part of the great section which has not previously
-been named. The usual practice is to apply a name taken from the
-locality in which the beds were investigated, and in this manner the
-names of formations become associated with towns, rivers, counties,
-mountains, states and other geographical features. The locality which
-supplies the name is then regarded as the &ldquo;type locality&rdquo; for the formation,
-but wherever these same beds may be traced or otherwise identified
-the one formation name applies.</p>
-<div class="pb" id="Page_17">17</div>
-<div class="img" id="fig5">
-<img src="images/p05.jpg" alt="" width="800" height="526" />
-<p class="pcap"><span class="sc">Dinosaur Tracks</span></p>
-<p class="pcapc">An ancient trail in sandstone of the Dakota formation. East slope of the hogback, west of Denver.</p>
-</div>
-<div class="pb" id="Page_18">18</div>
-<p>The &ldquo;Dakota formation,&rdquo; to use a convenient illustration, is mentioned
-in scores of reports bearing on the geology of Colorado, Iowa,
-Kansas, Nebraska, New Mexico, Texas, Utah, and Wyoming, as well
-as the Dakotas. On the geological map of Colorado it appears on both
-sides of the Rockies, scattered in strips and patches from north to south
-boundary lines. The beds are easily located in the foothills district west
-of Denver because of their tendency to produce the prominent ridges
-known as hogbacks.</p>
-<p>Many formations are exposed over much less territory, some have
-even greater extent. Thickness may vary from a few inches to thousands
-of feet, and no two exposures will be exactly alike though some similarity
-necessarily prevails throughout. &ldquo;Exposures&rdquo; are simply portions of the
-beds which are not concealed by loose rock, soil and vegetation, or overlying
-formations. Canyon walls, steep cliffs and mountain slopes, gullies,
-and badlands provide a large variety of natural exposures. In such
-places rocks and fossils may be studied to best advantage.</p>
-<p>Since a formation may contain a variety of beds, including sandstones,
-shales, limestones, and all sorts of mixtures, there is sometimes
-need of subdividing it; but formations are the smallest units commonly
-shown on geological maps. They are actual rocks which fit into a historical
-scheme of things and may be regarded aptly as the pages of a book
-which nature has done in stone.</p>
-<h3 id="c7">GEOLOGICAL TIME</h3>
-<p>&ldquo;How old are they?&rdquo; &ldquo;How can you learn their names from the
-rocks?&rdquo; These are typical examples of questions most frequently asked
-concerning fossils. The second question follows the usual reply to the
-first, for prehistoric plants and animals are as old as the rocks in which
-they are found. The answer, as to age, must come from the rocks and
-what we have learned about them through many years of hard work,
-thoughtful observation, and careful study. Names, however, come from
-a different source. Nature, apparently, managed for a long time to
-carry on without the use of words. Since man began talking he has had
-no trouble inventing names for things which interest him.</p>
-<p>Early students of rocks and fossils likewise accomplished a great
-deal without being able to date events in terms of years although many
-of their efforts and interests centered on the problem of discovering a
-continuous sequence of events in the fragments of evidence that had
-been uncovered. This relatively simple problem has not been fully
-worked out, and some of the breaks in the record are recognized as
-&ldquo;time gaps&rdquo; which may never be converted into history.</p>
-<div class="pb" id="Page_19">19</div>
-<p>The question of time, expressed in years, has been a puzzle which
-attracted some attention even in the earliest days of investigation. Its
-solution was attempted by several methods long before there was sufficient
-information to make them work satisfactorily, which accounts in
-part for the extreme variation in results of the calculations. Even now it
-is to be expected that changes will have to be made as long as pertinent
-studies are continued. Two of the most promising methods of investigation
-in late years have been producing figures which are surprisingly
-large. More accuracy than ever before is probably present in modern
-estimates but, except for comparatively recent time, there is yet no way
-of knowing within a range of millions of years when a creature lived.</p>
-<p>Astronomy and physics were used in early calculations but, although
-taken seriously by some geologists, it was soon recognized by others that
-certain events revealed by earth history could not be explained with so
-short a time allowance as these methods indicated. One of the first estimates
-provided a total of only twenty-five millions of years and included
-a great stretch of time during which the earth, according to prevailing
-theory, was more sun-like than rock-like, a time when planets were
-being born and the earth could not have been in its present physical
-condition, which is the chief concern of the geologist. Since those earlier
-conditions could not have supported life as we know it, our knowledge
-of cosmic history renders small service in the study of fossils.</p>
-<p>Among the methods suggested by astronomy and the laws of physics
-is one which is based on the probable rate at which the earth cooled from
-its molten condition to present temperature. It is believed now that the
-heat of the earth is not necessarily due to an original molten state and
-that a steady rate of cooling cannot be ascertained. Any figures based on
-such procedure, therefore, are discredited today.</p>
-<p>The amount of salt in the oceans, and the time required for its concentration
-there by natural processes, offers another way of attacking the
-problem. It is a well known fact that salt is being added to the seas at
-a fairly constant rate; sea water, then, must become saltier from year to
-year. The salt comes from rocks exposed on land surfaces and is transported
-by the rivers which drain these areas. By analyzing the river
-waters it is possible to estimate the amount of salt annually dumped into
-the oceans and, also by chemical analysis, it is a comparatively simple matter
-to figure the total amount now present in the oceans. Some recent
-calculations indicate that thirty-five million tons of salt are being added
-each year, and this figure divided into the total amount for all the years
-places the age of the oceans at three hundred sixty millions of years.</p>
-<p>However, there are certain other factors which complicate the problem.
-For instance, it is known that land areas exposed to surface drainage
-<span class="pb" id="Page_20">20</span>
-have not always been of their present size, and the annual production
-of salt by the different types of rocks exposed at various times in the history
-of the earth has not always been as it is now. The rocks also must be
-older than the oceans, but how much older cannot be determined by
-means of figures obtained in this way.</p>
-<p>Until the beginning of this century there was little anticipation of
-a better measuring stick than one in use at the time which placed its
-reliance on the total thickness of the sedimentary deposits and the length
-of time required to produce this great accumulation of material which is
-known as the geological column. Since the total thickness, or height of
-the column, was not accurately known, and with recognized time gaps to
-bridge, there was little hope of working out a complete chronology by
-this device, but it has supplied highly desirable and reliable information
-concerning parts of the record.</p>
-<p>The system has been somewhat improved since its earliest use, and
-one of its latest applications gives us an age, for known sedimentary
-rocks, of nearly half a billion years, this being based on a total thickness
-of one hundred miles and an average rate of 880 years for the building
-up of one foot of sediments. Its greatest weakness is due to the absence
-of a reliable factor to take care of long stretches of time in which the
-sedimentary rocks are known to have been subjected to destructive processes.
-A yardstick of this character cannot be applied to rocks that have
-been destroyed, and there are excellent reasons for believing that these
-interruptions may account for several times the lapse of years indicated
-by the amount of rock remaining in the column which has been pieced
-together.</p>
-<p>Following the discovery of radium, however, the present century
-provided a new field of knowledge which has contributed greatly to the
-measurement of geologic time. The penetrating rays produced by radium
-and other radioactive substances are due to extremely slow but violent
-disintegration of the material. Uranium and thorium are radioactive elements
-which occur in the rocks of many parts of the world. There is
-little or no loss of material as the so-called disintegration proceeds; instead
-there is a complicated series of transformations in which other elements
-are produced, radium itself being one of these. Helium and lead
-eventually take the place of the less stable elements and the known rate
-at which these products accumulate provides the highly desired key to
-the age of the rocks.</p>
-<p>Part of the gas, helium, may escape, but except in rare instances
-where chemical alteration might occur, there probably is no loss of lead.
-Fortunately, when this metal is produced by radioactivity it differs slightly
-in atomic weight from ordinary lead; otherwise the presence of the latter
-<span class="pb" id="Page_21">21</span>
-would introduce a misleading factor. Since the speed at which the
-change goes on cannot be increased or decreased, it is assumed that
-throughout past ages it has never been faster or slower. The amount of
-such change that has been completed in any body of radioactive minerals
-may be measured by techniques employed in physics and chemistry. If it
-is found that the amount of helium or lead present requires a hundred
-million years for its production at the working speed of the parent elements,
-the mineral deposit must be at least that old.</p>
-<p>Certain conditions of course complicate the problem seriously: knowing
-the age of a piece of rock which happens to contain some radioactive
-element is of small service in historical studies unless the rock can be
-definitely associated with a flora or fauna, or some outstanding event
-disclosed by geological investigations. But there have been a few instances
-in which most of the necessary conditions were present, and more
-and better opportunities to apply this method will no doubt appear. Other
-elements, or their radioactive isotopes, are already being employed with
-good results. Some of these, such as carbon 14, are more sensitive indicators
-for the accurate dating of events in comparatively recent time.</p>
-<p>When it can be used, this type of measurement is far less subject
-to uncertainties than any other. It promises to eliminate all need for
-guessing, and comes close to a degree of accuracy which is satisfactory to
-the scientist, a person who thoroughly dislikes uncertainties of any kind.
-If suitable material can be found in just the right places it should accomplish
-what the preceding method cannot do&mdash;the accurate measurement
-of the great time breaks which interrupt the geological record in many
-places. Something along this line already has been accomplished, for radioactive
-material has been found in some of the oldest of the rocks. Regardless
-of the destruction going on in other localities, these rocks have
-continued to register the passing of time, and a tremendous antiquity for
-the earth and some of its first inhabitants has been indicated.</p>
-<p>Tests made on radioactive minerals from Gilpin County, Colorado,
-have established the age of late Cretaceous or early Cenozoic rocks at
-sixty million years, providing a convenient and reasonably accurate date
-for the beginning of the Age of Mammals. In Russia, one of the oldest
-mineral deposits yet studied in this way and regarded as early Pre-Cambrian,
-produced the astonishing figure of 1,850,000,000 years; what we
-commonly refer to as geological history may therefore be regarded as
-covering a range of approximately two billions of years. The earth, in
-some form or other, has in all probability passed through an earlier history
-of another billion years or more.</p>
-<p>Wherever we may roam, a portion of the prehistoric record is to
-be found in the rocks underfoot and not far from the surface. Formations
-as already mentioned may be regarded as the pages&mdash;often torn and
-badly scattered&mdash;of nature&rsquo;s own book, in which the geological periods
-are chapters. But instead of numbering these pages and chapters we have
-<i>named</i> them, in order to get the parts reassembled in orderly fashion and
-restored to a condition which makes the book legible. However, the names
-cannot render the service intended except in connection with a time chart
-and an outline of earth history.</p>
-<div class="pb" id="Page_22">22</div>
-<div class="img" id="fig6">
-<img src="images/p06.jpg" alt="" width="600" height="928" />
-<p class="pcap">GEOLOGICAL TIME
-<br />Figures to the left denote millions of years that have elapsed up to recent time</p>
-</div>
-<table class="center" summary="">
-<tr><td colspan="3" class="l">CENOZOIC</td></tr>
-<tr><td colspan="3" class="l"><span class="hst"><span class="sc">Age of Man</span></span></td></tr>
-<tr><td class="r"> </td><td class="l">RECENT </td><td class="l"><span class="sc">Man and his Culture</span></td></tr>
-<tr><td class="r">1 </td><td class="l">PLEISTOCENE </td><td class="l"><span class="sc">Last of Mammoths &amp; Mastodons</span></td></tr>
-<tr><td colspan="3" class="l"><span class="hst"><span class="sc">Age of Mammals</span></span></td></tr>
-<tr><td class="r">7 </td><td class="l">PLIOCENE </td><td class="l"><span class="sc">Horses modernized</span></td></tr>
-<tr><td class="r">20 </td><td class="l">MIOCENE </td><td class="l"><span class="sc">Grasses and Grazing Animals</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Three-toed Horses, Rhinos, Camels</span></td></tr>
-<tr><td class="r">35 </td><td class="l">OLIGOCENE </td><td class="l"><span class="sc">Specialization of Primitive Ancestors</span></td></tr>
-<tr><td class="r">60 </td><td class="l">EOCENE </td><td class="l"><span class="sc">Decline of archaic types</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Mammals flourishing</span></td></tr>
-<tr><td colspan="3" class="l">MESOZOIC</td></tr>
-<tr><td colspan="3" class="l"><span class="hst"><span class="sc">Age of Reptiles</span></span></td></tr>
-<tr><td class="r">125 </td><td class="l">CRETACEOUS </td><td class="l"><span class="sc">Last of Great Reptiles</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Specialization of Dinosaurs</span></td></tr>
-<tr><td class="r">160 </td><td class="l">JURASSIC </td><td class="l"><span class="sc">Bony Fishes thriving</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Flowering plants advance</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Cycads</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Birds and Flying Reptiles</span></td></tr>
-<tr><td class="r">200 </td><td class="l">TRIASSIC </td><td class="l"><span class="sc">Few small mammals of lower orders</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Dinosaurs become prominent</span></td></tr>
-<tr><td colspan="3" class="l">PALEOZOIC</td></tr>
-<tr><td colspan="3" class="l"><span class="hst"><span class="sc">Age of Amphibians</span></span></td></tr>
-<tr><td class="r">225 </td><td class="l">PERMIAN </td><td class="l"><span class="sc">Reptiles advancing</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Amphibians dominant insects</span></td></tr>
-<tr><td class="r">300 </td><td class="l">CARBONIFEROUS </td><td class="l"><span class="sc">Dense forests of spore-bearing plants</span></td></tr>
-<tr><td colspan="3" class="l"><span class="hst"><span class="sc">Age of Fishes</span></span></td></tr>
-<tr><td class="r">350 </td><td class="l">DEVONIAN </td><td class="l"><span class="sc">Shark-like Fishes</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Land floras established</span></td></tr>
-<tr><td class="r">375 </td><td class="l">SILURIAN </td><td class="l"><span class="sc">First land animals (scorpions)</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Armored Fishes prominent</span></td></tr>
-<tr><td colspan="3" class="l"><span class="hst"><span class="sc">Age of Invertebrates</span></span></td></tr>
-<tr><td class="r">425 </td><td class="l">ORDOVICIAN </td><td class="l"><span class="sc">Corals and Bryozoa</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Progress among Mollusks</span></td></tr>
-<tr><td class="r">500 </td><td class="l">CAMBRIAN </td><td class="l"><span class="sc">Brachiopods gaining</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Trilobites dominant</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Advance of shelled animals</span></td></tr>
-<tr><td colspan="3" class="l">PROTEROZOIC</td></tr>
-<tr><td colspan="3" class="l"><span class="hst">EARLIEST LIFE</span></td></tr>
-<tr><td class="r">1000 </td><td class="l">UPPER PRE-CAMBRIAN </td><td class="l"><span class="sc">Small marine invertebrates</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Lowest Forms of Plant and Animal Life</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">Few Fossils</span></td></tr>
-<tr><td colspan="3" class="l">ARCHEOZOIC</td></tr>
-<tr><td class="r">2000 </td><td class="l">LOWER PRE-CAMBRIAN </td><td class="l"><span class="sc">Some chemical evidence of life</span></td></tr>
-<tr><td class="r"> </td><td class="l"> </td><td class="l"><span class="sc">No fossils</span></td></tr>
-</table>
-<div class="pb" id="Page_23">23</div>
-<p>Such aids have been devised and revised from time to time. No
-figures have been offered as final or absolutely &ldquo;right&rdquo; since the beginning
-of scientific investigations. Time divisions have been proposed that
-are not yet in common use while others have been abandoned or modified.
-Sources of information are so numerous that appropriate credit cannot be
-given fairly for anything that is up-to-date. The combined chart and
-outline here provided is based on time calculations of recent date but with
-figures slightly rounded off for the sole purpose of making them easier
-to remember. In view of the still existent probability of error it is felt
-that the slight alteration of figures may justify itself. It need not be regarded
-as misleading if the present purpose be considered&mdash;the stimulation
-of a natural history interest which is not vitally concerned with the
-little difference between a thousand million years and nine hundred
-ninety-nine million years.</p>
-<h3 id="c8">EXPLANATION OF THE TIME CHART</h3>
-<p>The whole of geological time has been divided and subdivided according
-to varying practices. The development of life is perhaps the one
-outstanding feature of the time divisions, but for the most part the
-changes in floras and faunas have been gradual rather than abrupt, and
-this makes it very difficult to draw sharp lines or to visualize beginnings
-and endings of the various stages of development. Occasionally there
-is good excuse for drawing a line, where the record is broken and resumed
-again after a long lapse of time. The principal cause of such
-breaks is the elevation of great land masses, which brings on an interval
-of erosion and surface destruction for the areas uplifted.</p>
-<p>These movements of parts of the earth&rsquo;s crust have been exceptionally
-pronounced at certain times, often culminating in the production
-of mountain systems, and because of the extreme changes they introduce
-are known as revolutions. The major divisions of prehistoric
-time have been established, at least in part, by such <i>revolutions</i>; crustal,
-climatic, or other <i>disturbances</i>, on a smaller scale and recurring with
-greater frequency, may be regarded as establishing boundaries for the
-minor divisions. Hence we have five great Eras of geological history, and
-<span class="pb" id="Page_24">24</span>
-these are divided again into Periods. The time chart shows an arrangement
-commonly used in America. In the first column the names of the
-Eras are stated in technical form. Closely coinciding with these terms
-are the popular names of the Ages which appear in the second column.
-These names, describing the dominant life of each age, are very convenient.
-The more scientific terms used for the eras, while serving essentially
-the same purpose, are a little more systematic and generalized in that
-they refer to ancient life (Paleozoic), middle life (Mesozoic), and recent
-life (Cenozoic), without being specific as to any class of animals or plants
-for any one division of time.</p>
-<p>The period names, in the central column, have been derived from
-miscellaneous sources, some of them from geographical districts, some
-from descriptive references to prominent features of the rocks, others
-indicating a degree of approach to recent time. In paleontology (fossil
-study) it has long been a practice to cut the periods into lower, middle,
-and upper divisions, and in a few cases it has been found desirable to
-make two periods out of an old one. What was once known as the
-Lower Carboniferous is now commonly recognized as the Mississippian
-period while the upper portion has become the Pennsylvanian. The
-Lower Cretaceous is now the Comanchean of some authors.</p>
-<p>Both old and new practices are responsible for a little confusion at
-the present time. A former division into Primary, Secondary, Tertiary,
-and Quaternary eras has been partly abandoned, but the term &ldquo;Quaternary&rdquo;
-still applies to the Age of Man, while &ldquo;Tertiary time&rdquo; remains
-in good usage for the balance of the Cenozoic era. Among the newer
-introductions may be mentioned the use of a Paleocene period which
-precedes the Eocene. Geologists are not entirely in agreement as to the
-necessity for this addition and many would not give it equivalent rank
-with other periods. In the interest of simplicity these modern refinements
-have been omitted from the chart.</p>
-<p>The figures appearing in the third column, between the Ages and
-Periods, indicate the millions of years that have elapsed up to present
-time. They denote the age of the rocks at the beginning of each period.
-The age of a plant or animal which lived in Eocene time would be, according
-to this scale, somewhere between 35 million and 60 million
-years. In practice it is usually possible to determine whether a fossil was
-embedded in the rocks during an early or late portion of the period, and
-thus its age may be established within a shorter range, but it is
-impossible to be exact, even in terms of millions of years, with regard to
-anything as far back in prehistory as the Eocene period.</p>
-<p>The period in which we are living today is known as Recent. It
-began at the close of the Ice Age or Pleistocene period about ten thousand
-<span class="pb" id="Page_25">25</span>
-years ago and represents so little of earth history since the beginning of
-life that a chart many times the length of this page would be required to
-show the rest of the periods in proportion. The Cambrian period is an
-early chapter in which the story of prehistoric life suddenly becomes clear
-and richly varied. It is, however, much farther from the beginning of
-the record than it is from the present, and the Pre-Cambrian eras would
-require a great deal more space in order to show their relative lengths.
-The Archeozoic and Proterozoic eras have to some extent been divided
-into periods, but the great antiquity of the rocks has obscured much of
-their history, and divisions established for one locality have been of little
-service elsewhere. Consequently, the period names are in less general
-use and the common practice is to refer to all this great stretch of time
-as Pre-Cambrian.</p>
-<p>In the last column, at the right of the chart, some of the historical
-features are indicated. This column should be read from bottom to top
-in order to get the proper development of the story, and at best this
-sketchy outline of events can be no more than suggestive of the progress
-and decline through which the earth&rsquo;s inhabitants have passed.</p>
-<p>Rocks of every period except probably the Silurian are known to
-have been deposited somewhere in the Colorado area, although in most
-cases the record for each period is far from complete. Formations are
-too numerous and too varied locally to be shown on a chart of this type.</p>
-<h3 id="c9">THE GEOLOGICAL SECTION</h3>
-<p>In the study of fossils there are two important field aids usually
-available. For any locality there should be a geological map and a section
-showing the sequence and character of the strata. On a small-scale map
-many of the local details have to be omitted, but the position of the
-larger exposures is indicated and, with this information at hand, the
-fossil-bearing strata may be located with the help of a geological section.
-The latter is frequently obtained from technical reports published by
-State and National Geological Surveys. Frequently, however, it is possible
-to obtain only a general plan for a given locality, and a great deal
-of literature may have to be scanned in order to get that. Excellent
-geological maps of Colorado have been published by the Colorado Geological
-Survey and the United States Geological Survey.</p>
-<p>It often happens that a formation is not where we expect to find
-it, this being due to several possible factors. The sediments may not
-have been deposited there, or they may have been removed by erosion.
-Where the structure has been disturbed by folding and faulting, a multitude
-of complications is introduced. The expected sequence is sometimes
-inverted and repeated through a series of folds. Formations also
-may be moved miles out of place by faulting. Both thickness and character
-of sediments may vary considerably within a formation. In some
-regions the geology is very simple, in others extremely difficult to understand.</p>
-<div class="pb" id="Page_26">26</div>
-<div class="img" id="fig7">
-<img src="images/p07.jpg" alt="" width="986" height="600" />
-<p class="pcap"><span class="sc">FORMATIONS of the DENVER-FOOTHILLS REGION</span>
-<br /><span class="small">A GENERALIZED SECTION SHOWING SOME OF THE SURFACE FEATURES</span>
-<br /><span class="small">RED BEDS<span class="hst"> HOGBACKS</span><span class="hst"> TABLE MOUNTAIN</span><span class="hst"> DENVER DISTRICT</span></span></p>
-<p class="capc">REGION OF MOUNTAIN-MAKING UPLIFT</p>
-<p class="capc">Formations bordering the mountains have been bent into upright positions.</p>
-</div>
-<table class="center" summary="">
-<tr class="th"><th>PERIODS</th></tr>
-<tr><td class="l">RECENT</td></tr>
-<tr><td class="l">PLEISTOCENE</td></tr>
-<tr><td class="l">PLIOCENE</td></tr>
-<tr><td class="l">MIOCENE</td></tr>
-<tr><td class="l">OLIGOCENE</td></tr>
-<tr><td class="l">EOCENE</td></tr>
-<tr><td class="l">CRETACEOUS </td><td class="l"> </td><td class="r"><span class="small">THICKNESS</span></td></tr>
-<tr><td class="l">SOFT SANDSTONES GRITS &amp; CLAYS </td><td class="l">DENVER &amp; ARAPAHOE </td><td class="r">2000 ft.</td></tr>
-<tr><td class="l">SANDSTONES, SHALES &amp; LIGNITE </td><td class="l">LARAMIE </td><td class="r">1000 ft.</td></tr>
-<tr><td class="l">YELLOWISH SANDS &amp; SHALES </td><td class="l">FOX HILLS </td><td class="r">1000 ft.</td></tr>
-<tr><td class="l">SOFT DARK GRAY OR RUSTY SHALE </td><td class="l">PIERRE </td><td class="r">5000 ft.</td></tr>
-<tr><td class="l">LIMESTONES &amp; SHALES </td><td class="l">NIOBRARA </td><td class="r">500 ft.</td></tr>
-<tr><td class="l">DARK SHALES &amp; LIME </td><td class="l">BENTON </td><td class="r">400 ft.</td></tr>
-<tr><td class="l">GRAY OR BUFF SANDSTONES &amp; CLAYS </td><td class="l">DAKOTA </td><td class="r">300 ft.</td></tr>
-<tr><td class="l">SHALES, SANDSTONE &amp; LIME </td><td class="l">MORRISON </td><td class="r">200 ft.</td></tr>
-<tr><td class="l">JURASSIC</td></tr>
-<tr><td class="l">TRIASSIC</td></tr>
-<tr><td class="l">PERMIAN</td></tr>
-<tr><td class="l">DEEP-RED SANDY SHALES, LIME, GYPSUM </td><td class="l">LYKINS </td><td class="r">700 ft.</td></tr>
-<tr><td class="l">CARBONIFEROUS</td></tr>
-<tr><td class="l">MASSIVE PINK OR WHITE SANDSTONE </td><td class="l">LYONS </td><td class="r">200 ft.</td></tr>
-<tr><td class="l">RED OR BROWN SANDSTONE &amp; CONGLOMERATE </td><td class="l">FOUNTAIN </td><td class="r">1500 ft. </td><td> </td><td></td></tr>
-<tr><td class="l">DEVONIAN</td></tr>
-<tr><td class="l">SILURIAN</td></tr>
-<tr><td class="l">ORDOVICIAN</td></tr>
-<tr><td class="l">CAMBRIAN</td></tr>
-<tr><td class="l">PRE-CAMBRIAN</td></tr>
-<tr><td class="l">METAMORPHIC &amp; INTRUDED ROCKS SCHIST, GNEISS, QUARTZITE </td><td class="l">IDAHO SPRINGS (PART)</td></tr>
-<tr><td class="l"><span class="sc">BASEMENT ROCKS of IGNEOUS ORIGIN</span></td></tr>
-</table>
-<div class="pb" id="Page_27">27</div>
-<p>A generalized section for the western part of the Denver Basin is introduced
-here for the use of local students. The formations normally
-present in this region are shown in their usual position. They are briefly
-described on the chart, and their thickness is indicated by figures which
-may be regarded as near the maximum for the district. The section will
-apply to most of the foothills area between Morrison and Boulder though
-surface features and thickness of beds will vary considerably from place
-to place.</p>
-<p>Certain of the formations are known to be fossil bearing, others
-barren or nearly so. When fossils are present they are usually restricted
-to certain localities, and these may be widely scattered. The following
-remarks apply to the possibilities for finding fossils in the formations
-named.</p>
-<h4><i>Denver and Arapahoe.</i></h4>
-<p>Leaf impressions of palms, ferns, and numerous
-species of well-known trees and shrubs are common in many localities.
-Petrified wood is fairly abundant, and a few scattered bones of
-reptiles and mammals have been found. The two formations are treated
-as a unit because the Arapahoe is neither conspicuous nor sharply defined.
-Denver beds are well exposed on the slopes of Table Mountain at Golden;
-fossils, however, have been obtained from several localities nearer the city
-of Denver, notably from the hills just west of Overland Park.</p>
-<h4><i>Laramie.</i></h4>
-<p>Plant material is locally abundant, principally the leaves of
-familiar deciduous trees, palms, and ferns. Many of the clay pits being
-worked near Golden are in this formation. Oysters and a few other mollusks
-may be found in some places.</p>
-<h4><i>Fox Hills.</i></h4>
-<p>Better exposures of this formation are located to the
-north of Denver. Marine mollusks are most frequently found.</p>
-<h4><i>Pierre.</i></h4>
-<p>In addition to the characteristic dark shales, this formation
-includes some limy material and sandstone beds, both of which are fossiliferous
-in places. Two types of marine mollusks are characteristic: <i>Inoceramus</i>,
-generic name for several species of clam-like bivalves readily identified
-by concentric elevations which produce a rippled effect on the shell
-surfaces; and <i>Baculites</i>, cephalopods with straight, chambered shells which
-often break at the suture lines, where the fossil is weakened by the chamber
-walls. Small oyster shells are fairly common also. The formation is
-to be found some distance to the east of the prominent hogback where it
-weathers into smooth surfaces in the form of broad valleys and flats, with
-rounded contours on the few elevations that may be present. It forms a
-soft, flaky soil when dry, is a sticky &ldquo;gumbo&rdquo; when wet. The clay is
-generally of a rather dark grayish color when freshly exposed but it
-takes on a rusty appearance after weathering. At various levels there are
-numerous iron-cemented concretions, many of which contain fossil shells.</p>
-<div class="pb" id="Page_28">28</div>
-<div class="img" id="fig8">
-<img src="images/p08.jpg" alt="" width="789" height="548" />
-<p class="pcap"><span class="sc">Marine Beds of the Benton Formation, Northeastern Colorado</span></p>
-<p class="pcapc">A stream channel has cut deeply into the formation, uncovering and partly
-destroying a plesiosaur skeleton which was found at the level where the men
-are standing.</p>
-</div>
-<div class="img" id="fig9">
-<img src="images/p08a.jpg" alt="" width="793" height="556" />
-<p class="pcap"><span class="sc">Plesiosaur Bones in Place</span></p>
-<p class="pcapc">Benton formation. Surface rubble has been cleared away, and several vertebrae
-are partially uncovered in the area at the right of the hammer.</p>
-</div>
-<div class="pb" id="Page_29">29</div>
-<h4><i>Niobrara.</i></h4>
-<p>The formation contains fossils rather similar to those of
-the Pierre. Shark&rsquo;s teeth have been found in some of the lower beds.
-Limestone is a prominent feature, often forming a well defined ridge near
-the foot of the eastern slope of the main hogback. The limestones commonly
-have a chalky character.</p>
-<h4><i>Benton.</i></h4>
-<p>The formation is not especially productive in this region.
-Marine shells are numerous in some localities, and bones of marine reptiles
-have been found at various places. As usually seen, it is almost entirely
-composed of impure clay shales, very dark, brownish-gray to almost
-black, and commonly interbedded with thin patches of white bentonite,
-yellow ochre, gypsum, and limestone.</p>
-<h4><i>Dakota.</i></h4>
-<p>This formation produces the high hogback which is usually
-present some distance east of the Red Rocks. There are generally two or
-three layers of massive, light-colored sandstone separated by clays which
-are used extensively in the making of bricks and pottery. Leaf impressions
-and some fish scales are found in the clays and occasionally in the sandstone.
-The hogback is a good marker from which to locate other formations,
-because of its prominence in the foothills landscape.</p>
-<h4><i>Morrison.</i></h4>
-<p>Good dinosaur material has been taken from the Canon
-City and Morrison districts. The formation is to be found on the lower
-west slope of the Dakota hogback. It consists of continental deposits of
-the stream and lake types. There is considerable sandstone in this formation
-and a little limestone is to be found here and there, but the most
-characteristic feature is in the shales. When freshly exposed, the shales
-are delicately tinted with gray, green, and maroon, a bronze-green being
-rather prominent. This formation is highly variable in character, with
-much of the clay often buried under the valley floor. In addition to the
-bones of reptiles, there are plant fossils, usually of poor quality, and
-fresh-water gastropods more or less abundant in some localities.</p>
-<h4><i>Lykins.</i></h4>
-<p>Outcrops are not prominent, owing to the small amount of
-weather-resisting materials. The sandy clays are commonly of a deep red
-<span class="pb" id="Page_30">30</span>
-color mottled with spots of light gray. A white limestone is sometimes
-present near the middle of these deposits, and gypsum beds are included
-locally. The formation is often indicated only by red soil in the depressions
-between ridges. Few fossils have been reported.</p>
-<h4><i>Lyons.</i></h4>
-<p>This formation is usually prominent as the eastern wall of
-the uplifted Red Rocks series. In some localities it forms a ridge of pink
-or white sandstone distinctly separated from the older sediments to the
-west. Very few fossils are found.</p>
-<h4><i>Fountain.</i></h4>
-<p>Exposures usually are brown to red in color, though sometimes
-a dirty white. The prominent rocks are rather coarse sandstone,
-commonly with a gritty texture due to the angular character of the sand
-or gravel from which they were made. These are the westernmost of the
-Red Beds and the oldest of the uplifted sedimentary rocks bordering the
-foothills in most of our area. Fossils have been found in the formation,
-but it is practically barren for the territory here considered.</p>
-<p class="center"><span class="gs">* * * * * * * *</span></p><p>This geological section also illustrates a method of dating crustal
-movements and the birth of mountain ranges, for the folding of the strata
-along the flanks of the Rocky Mountains has a great deal of significance
-in this connection. The sedimentary layers were originally deposited over
-much of the present mountain area in a horizontal position, and only
-those formations in existence at the time could be distorted by the upheavals
-which produced the new elevations. Of the series generally involved
-in the movement the Laramie beds are the youngest. Since these
-beds had not been formed until near the close of the Cretaceous period it
-is to be assumed that the mountains must be of more recent date, younger
-than the topmost of the deformed beds and at least as old as the lowermost
-of the undisturbed formations overlying them.</p>
-<p>Some disturbance is evident also in the Arapahoe and Denver beds
-which overlie the Laramie, but this is believed to have occurred sometime
-after the occasion of the first great uplift. Volcanic materials in these
-beds lead to the belief that the sediments were deposited during a period
-of volcanic activity brought on by the crustal folding which terminated
-the Mesozoic era. Hence the conclusion arises that the age of the Denver
-and Arapahoe beds must coincide closely with some of the earlier stages
-in the history of the mountain system. This interval is often referred to
-as Post-Laramie time.</p>
-<div class="pb" id="Page_31">31</div>
-<h2 id="c10"><span class="small">BEFORE THE AGE OF REPTILES</span></h2>
-<h3 id="c11">THE PRE-CAMBRIAN COMPLEX</h3>
-<p>The rocks of Pre-Cambrian time have been buried deeply under the
-accumulation of younger sediments, and the resulting pressure in many
-places has been tremendous. In addition to the effects of pressure there
-also is recorded in these ancient formations the repeated movements of
-the materials since they were first deposited. Vertical and side adjustments
-of parts, with relation to other parts, have distorted the original arrangement
-of the rock particles to such an extent that ordinary fossils
-would eventually become unrecognizable. These crushing, grinding, and
-kneading forces working through millions of years alone would account
-for the absence of fossils from the older deposits. Frequently the rocks
-have become so changed in form that their original character can only be
-conjectured, and because of this change they are known as metamorphic
-rocks.</p>
-<p>A few beds of Archeozoic age remain in nearly their original condition,
-but they are either without fossils or they have produced very
-questionable and unsatisfactory specimens. The existence of life during
-these early stages of earth history is indicated largely by chemical rather
-than fossil evidence. Much of the ancient limestone has been converted
-into marble, but it is not unreasonable to believe that plants and animals
-were instrumental in the production of this type of rock as they are
-today. Certain varieties of iron ore deposits are now being built up by
-the aid of plants, and similar ores in the ancient rocks may have had
-a like origin. The presence of great quantities of carbon, in the form
-of graphite, may be regarded also as a sign of life, for this substance is
-accumulated on a large scale by living plants, and may be retained in a
-solid form after the partial decay of the plant tissues.</p>
-<p>So far as the direct evidence goes, there is no sign of any creature
-of large size or of such complicated structure as the common plants and
-animals of today. The chemistry of the mineral deposits is not entirely
-convincing as to the presence of life, but it is regarded as highly probable
-that microscopic, single-celled plants and animals, comparable to
-modern algae and protozoa, were in existence during Archean time.
-Throughout later eras there is unmistakable evidence of gradual development
-from simpler to more elaborate life-forms and the Archeozoic is
-commonly regarded as a time of preparation during which simple organisms
-of some kind were becoming adapted to early conditions which could
-<span class="pb" id="Page_32">32</span>
-not support life on a higher plane. The importance of the work done by
-such lowly creatures in the preparation of suitable environments for more
-advanced modes of living is overlooked almost entirely.</p>
-<p>During the next era, the Proterozoic, the record of life becomes
-somewhat clearer. Fossils are hardly to be regarded as abundant but there
-were several well-defined types of animals which left shells and other
-parts composed of mineral matter. Among these may be mentioned the
-Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced
-delicate, often lace-like shells of many patterns adorned with the radiating
-filaments or spines which have suggested the name for this group.
-Foraminifera produced minute shells, sometimes many chambered, and
-often bearing a confusing resemblance to the work of snails. Common
-chalk is composed almost entirely of such shells and fragments of them.</p>
-<p>Sponges and Bryozoa are animals of slightly higher organization.
-They are many-celled instead of one-celled and the cells have special
-work to perform, which is a most important step in the direction of the
-specialization which characterizes the structural and life pattern of later
-arrivals. The Bryozoa lived in moss-like colonies which have been important
-rock-makers; the fossil forms bear some resemblance to corals.
-Sponges are too well known to require description although the familiar
-article of commerce is merely the framework of once-living animals. They
-represent the earliest organization of true animal bodies even though in
-appearance they may have a resemblance to plants.</p>
-<p>Actual plants of this era were of the algae class, aquatic in habit as
-were their animal neighbors, the first to leave a record in the form
-of fossils. This record, obscure and distorted, has long been a source of
-perplexity to investigators. Without well-defined floras and faunas to
-guide them, and with rocks frequently in chaotic relationships, early
-geologists were content to regard it all as a &ldquo;Pre-Cambrian complex.&rdquo;
-Recent studies have contributed a great deal of information not available
-some years ago. It is quite possible that more advanced types of
-life were in abundance before the close of the second era, but material
-on which to base sound opinion is still scarce.</p>
-<p>Rocks of Pre-Cambrian age are plentiful in the foothills region west
-of Denver. The schists, gneisses, and quartzites exposed for some miles
-immediately beyond the red-beds are part of this great complex. The
-Idaho Springs formation is known to be one of the oldest in this district,
-although its exact age has not been determined. Other formations are
-recognized among the metamorphic rocks of the region but none has
-contributed to our knowledge of early life.</p>
-<div class="pb" id="Page_33">33</div>
-<h3 id="c12">CAMBRIAN LIFE</h3>
-<p>There can be no mistake as to the prolific development of life in
-Cambrian seas, for fossils of this age are to be found in many parts of
-the world, where ancient sea bottoms now form part of the land surface.
-Invertebrate animals appear to have made much progress, but plants
-were either scarce or too small and delicate to be productive of fossils.
-It is probable, however, that seaweeds and other algae were flourishing
-along with the invertebrates, because animal life is directly or indirectly
-dependent on the existence of plants. The latter sustain themselves by
-taking carbon and nitrogen from air, water, and soil, but animals must
-obtain their requirements by eating plants or eating each other. They
-cannot obtain what they need from the inorganic world without this
-help from the vegetable kingdom.</p>
-<p>One group of animals stands out prominently above all its contemporaries.
-Known as the trilobites they were by far the most distinguished
-and most characteristic of Cambrian invertebrates. Trilobites inhabited
-the warmer seas of this period and several later ones, but were extinct
-by the end of the Paleozoic era. Hundreds of species have been described,
-most of them under four inches in length. Well-known distant
-relatives now living are the shrimps, and other crustaceans. The name
-Trilobite has reference to the three lobes which are apparent in the
-form of the upper surface, the central lobe forming a broad ridge extending
-along the back. Beneath the outer lobes on each side there was, during
-life, a row of short, jointed legs used for swimming and walking, but
-these delicate appendages are seldom preserved in the fossils.</p>
-<p>Second in importance among the animals of the period were the
-brachiopods or lamp-shells, not true mollusks although they were provided
-with similar shells composed of calcium phosphate or calcium carbonate.
-Shells are of two parts (bivalved) as in the case of clams, but the
-valves are above and beneath the body instead of on the right and left
-sides, which is the arrangement among mollusks. Although abundant as
-individuals, there were only a few species during the earlier part of the
-period; the number of species increased, however, and the race became
-very persistent. About seven thousand species have been described, and
-the race is not yet extinct although the number of living species is relatively
-small.</p>
-<p>Cambrian life evidently included representatives of all the great divisions
-of invertebrates; sponges, jelly-fishes, worms, and primitive corals
-have been reported. At the end of the period there was an elaborate
-molluscan fauna. The closing of the period in North America was apparently
-a gentle elevation of continental areas and a consequent withdrawal
-of the sea.</p>
-<div class="pb" id="Page_34">34</div>
-<div class="img" id="fig10">
-<img src="images/p09.jpg" alt="" width="600" height="744" />
-<p class="pcap"><span class="sc">Invertebrate Fossils</span></p>
-<p class="pcapc">Only a few prominent types have been selected from thousands of invertebrates
-known to zoologists. The forms illustrated are of frequent occurrence as
-fossils.</p>
-</div>
-<dl class="undent pcap"><dt>CRINOIDS</dt>
-<dt>CEPHALOPODS</dt>
-<dd>Coiled types</dd>
-<dd>Ammonite</dd>
-<dd>Scaphite</dd>
-<dd>Straight-shell type</dd>
-<dd>Baculite</dd>
-<dt>TRILOBITE</dt>
-<dt>BRACHIOPODS</dt>
-<dt>BIVALVES</dt>
-<dd>Inoceramus</dd>
-<dd>Oyster</dd>
-<dt>GASTROPODS</dt>
-<dd>Snail-like Univalves</dd>
-<dt>PROTOZOA</dt>
-<dd>UNICELLULAR FORMS</dd>
-<dd class="t">Radiolaria (Microscopic)</dd>
-<dd class="t">Fusulina limestone</dd>
-<dd class="t">Foraminifera (Enlarged)</dd>
-<dd>MULTICELLULAR FORMS</dd>
-<dd class="t">Cup coral</dd>
-<dd class="t">Reef coral</dd>
-<dd class="t">Sponge</dd>
-<dd class="t">Bryozoa</dd></dl>
-<div class="pb" id="Page_35">35</div>
-<h3 id="c13">THE ORDOVICIAN RECORD</h3>
-<p>Extensive land areas must have subsided again early in the Ordovician
-period for marine sediments were laid down over a large part of
-the North American interior, and three epochs or subdivisions of the
-period have been based on as many invasions of the sea. In these ancient
-deposits the record of life continues to show new forms. Nothing of a
-very spectacular sort is recorded other than a great increase in the number
-of species among types that were established in earlier periods.</p>
-<p>Trilobites were at their best, brachiopods continued to flourish, and
-the mollusks made new progress, especially the cephalopods, a group
-which includes our cuttle fishes and squids. Some of these predatory
-creatures attained large size and were no doubt masters of the sea. Typical
-forms were provided with tapering chambered shells that occasionally
-reached a length of twelve or more feet. Most of the shells were straight
-and trumpet-like or but slightly curved. Some were closely coiled and in
-this respect more like the well-known nautilus now in existence.</p>
-<p>The bryozoans became very common in the later part of the period
-and corals made slight advances. Somewhat of a novelty at this time
-were the crinoids, commonly known as &ldquo;stone lilies&rdquo; although not plants
-at all. They have been described as starfishes with back turned downward
-and a thick stem attached beneath. Where they lived in great
-abundance the limestone deposits may consist almost entirely of their
-stems. Crinoids continued to produce a variety of forms throughout
-several of the succeeding periods.</p>
-<p>The brachiopods were commonest of all animals representing this
-period, however, and their wide distribution, together with their known
-preference for warm waters, is taken to be an indication of mild temperatures
-prevailing over a large portion of the earth. Land plants are indicated
-by spore-bearing forms related to the ferns and mosses. Impressions
-of such plants have been found in Europe but, since most of the known
-rocks of this age were formed in seas, the marine algae are more abundant
-as fossils.</p>
-<p>In the Colorado area, rocks of Ordovician time are exposed only in
-mountainous areas where they have been lifted high above their original
-levels. They are not especially rich in fossils although they have produced
-some fish remains which are of interest in that they suggest an age of
-vertebrates which is just ahead.</p>
-<div class="pb" id="Page_36">36</div>
-<h3 id="c14">SILURIAN EVENTS</h3>
-<p>Since land floras and faunas had not yet become conspicuous the
-fossil record for this period is limited to those areas which were invaded
-by the sea. Apparently there was no such invasion of the present Colorado
-region, for rocks of this age are not in evidence. If they exist at all
-they are restricted to localized districts which are deeply buried under
-sediments of later periods. There may have been no Silurian deposition
-in this area, or such rocks may have been produced only to be destroyed
-by elevation and consequent subjection to weathering and erosion during
-a long interval of time, in which they were completely removed. In the
-region of the Colorado Rockies there is no evidence of returning seas
-until late Devonian time.</p>
-<p>In other parts of the world, however, there was extensive deposition
-of rock-making sediments in seas which were inhabited by algae and
-invertebrates of the types previously described. Among the common
-animals of the time there were still numerous species of brachiopods,
-trilobites, corals, crinoids, and bryozoans. In addition to the primitive
-cone-shaped, cup corals there were several advanced types but the habit
-of building large reefs was not yet established.</p>
-<p>&ldquo;Sea scorpions,&rdquo; really large crustaceans, flourished during Silurian
-time, and late in the period there appeared a race of true scorpions which
-lived on dry land or between high and low tides along the seashore. These
-were smaller and much like modern descendants but probably they did
-not wander far from the ocean shores where an abundance of food was
-available. These little scorpions, the largest measuring only two and a
-half inches in length, are the oldest air-breathing land animals of the
-fossil record.</p>
-<p>It was not until the period was well advanced that fishes became
-numerous, and much of our knowledge of the beginning of an &ldquo;Age of
-Fishes&rdquo; has been obtained from European fossils. Although fishes are
-classed with the vertebrate or backboned animals there are large groups
-which do not have bony skeletons but are provided instead with a simple
-framework of cartilage. Among the earlier and more primitive types
-were the ostracoderms or bony-skinned fishes with no internal bones and
-only a small amount of bony substance in the armor-like plates and scales
-which covered the forward portion of the body.</p>
-<p>The ostracoderms comprise a small group of fishes about which very
-little is known. They appear to have been inhabitants of fresh-water
-streams as well as lagoons bordering the seas, and may have been related
-to the small sharks of the time. They lived during the Ordovician,
-Silurian, and Devonian periods, and left no descendants now recognized
-<span class="pb" id="Page_37">37</span>
-among living creatures. A much larger type of armored fishes is known
-as the arthrodires, a name which refers to a pointed neck and an arrangement
-of the armor plates to permit a movement of the head. These were
-the most ferocious fishes of the Silurian and Devonian seas, some of them
-reaching a length of twenty feet though most were much smaller. Their
-jaws were provided with formidable shearing and crushing plates instead
-of teeth.</p>
-<h3 id="c15">DEVONIAN PROGRESS</h3>
-<p>The Devonian is one of the most outstanding of all periods from the
-viewpoint of life development. Dominance of the fishes is its greatest
-achievement, the invertebrates remaining about as they were and the
-higher vertebrates barely in evidence, but life on a large scale was no
-longer confined to the seas. Fresh-water fishes became prominent and
-land plants well established. The first forests appeared, with fern-like
-plants predominating although woody trees of several types and considerable
-size were included. It is quite possible that extensive land areas
-had been well supplied with vegetation during earlier times, but the
-delicate tissues of plants are far less likely to be preserved than the limy
-parts of animals. The fossil record, therefore, cannot be expected to
-reveal more than a suggestion of the progress made at this level of living.
-The story of plant life becomes much clearer in the next period when
-conditions were more favorable for the production and preservation of
-plant fossils.</p>
-<p>Land animals of the time are almost unknown. A few snails and
-scorpions have been found, and some footprints made by early amphibians.
-Insects probably were in existence although the evidence is not quite
-clear on this point. The increasing number of fresh-water fishes, however,
-may be regarded as a sure indication that inland conditions were
-becoming more favorable for plant and animal inhabitants of all kinds.</p>
-<p>The extent of development among the fishes cannot be accurately
-indicated by naming a few types, for it is mainly in the number of species
-and genera within the larger groups that progress is seen. In general
-it may be stated that the fishes of the period had not yet acquired
-the bony skeleton and typical form of familiar modern species.
-Skeletons were of cartilage, partly hardened in some instances by lime.
-Armor plates were customary with certain races but were not present
-among all fishes. Neither were these armored forms exceptionally large,
-as compared with living sharks. Although occasional giants appeared,
-the majority were small. Many were sluggish creatures with poorly-developed
-jaws, living as scavengers on sea and stream bottoms. Tail
-fins were usually unbalanced as in the sharks, or pointed and rounded
-rather than evenly forked.</p>
-<div class="pb" id="Page_38">38</div>
-<div class="img" id="fig11">
-<img src="images/p10.jpg" alt="" width="800" height="569" />
-<p class="pcap"><span class="sc">Modernized Types of Fishes from Eocene Shales of Southwestern Wyoming</span></p>
-</div>
-<div class="pb" id="Page_39">39</div>
-<p>The great tribe of true bony fishes, such as the cod and perch,
-which includes more than ninety percent of the fishes living today, was
-not yet in existence. About one-third of the many kinds of fishes then
-living were related to the sharks, a group which is relatively insignificant
-in recent years. Nearly one-fourth of the total belonged to a tribe of
-enamel-scaled fishes, now represented only by a few sturgeon and gar-pike.</p>
-<p>Lung fishes have never been a large group but it is noteworthy
-that they have had existence since Middle Devonian time. Living members
-of the race, inhabitants of Africa and South America, make a
-practice of burrowing into the mud of stream channels during dry
-seasons and are provided with lungs which enable them to breathe air in
-the manner of higher vertebrates. They survive the complete drying-up
-of the streams and live for months without water. Other forms, with
-less development of lungs, frequent stagnant pools and come to the
-surface occasionally for a breath of air. All are provided with gills also,
-which enables them to obtain their oxygen as other fishes do. They
-are believed to be a connecting link between the fishes and the early
-amphibians. More accurately, perhaps, they should be regarded as holding
-an intermediate position without being directly ancestral to any
-higher type of vertebrate animal.</p>
-<p>Still dominant among the invertebrates were the brachiopods, on
-the whole averaging a little larger in size, and otherwise indicating congenial
-times for that type of organism. They reached the peak of their
-development during this period. Trilobites were declining although a few
-new and strangely ornamented varieties made a brief appearance. Crinoids
-apparently found living conditions less favorable during Devonian
-time, but in a later era they again became prominent. Corals were
-favored only at times and in certain localities. Along with the crinoids
-they appear to have suffered from the presence of an unusual amount of
-mud in the waters of their customary habitats. Both had a preference for
-clear water as indicated by the absence of fossils from limestones containing
-more than a very small percentage of muddy sediments. Crustaceans,
-similar to the sea-scorpions and better known as eurypterids,
-became prominent among fresh-water animals. Some were unusually
-large for creatures of this class, lengths of several feet being recorded from
-fragments. Gastropod mollusks came into prominence in localities where
-living conditions were favorable. Bivalves continued to thrive but the
-<span class="pb" id="Page_40">40</span>
-cephalopods had a rather meager development considering the heights
-they were to achieve in subsequent periods.</p>
-<p>In western North America the large expanse of territory known as
-the Great Plains was evidently well above sea level during this entire
-period, for no beds of this age are found in eastern Colorado. West of
-the Front Range, however, there was some deposition of marine sediments
-during late Devonian time. Formations of this age are exposed
-near Salida and Glenwood Springs, on the White River Plateau, and in
-the San Juan region.</p>
-<p>The Carboniferous period gets its name from the vast deposits of
-coal which were developed during that time in many parts of the northern
-hemisphere. Depressed land surfaces bordering the continents, and
-extending well into the interior of present boundaries, supported dense
-growths of vegetation and provided the swampy conditions most favorable
-to coal production. Varieties of plants which are now of small size
-and lowly position in the botanical world acquired the proportions of
-large trees.</p>
-<h3 id="c16">CARBONIFEROUS FORESTS</h3>
-<p>Best-known fossils of the period are carbonized portions of the
-larger trees, and impressions left in the muds and sands of ancient bogs.
-Forest trees of several kinds reached the height of a hundred feet, with
-a trunk diameter of two to six feet. This size often is exceeded in modern
-forests, but by trees of an entirely different type. Considering the amount
-of development among the plants of earlier periods, Carboniferous forests
-provide an outstanding spectacle of advancing life.</p>
-<p>Quite common among the larger trees were two varieties of club-mosses,
-also known as scale trees. They were cone-bearing evergreens with
-only slight resemblance to modern conifers. Instead of seeds they produced
-spores, a method of reproduction which is practiced among ferns.
-The trunks were marked from bottom to top with uniform patterns of
-cushions and scars indicating the points at which leaves were attached
-during the earlier stages of growth. In the Lepidodendrons the rows of
-scale-like cushions wind spirally upward while among the Sigillaria there
-is a vertical arrangement of leaf-scars which resemble the imprints of a
-seal, these impressions being in straight and parallel rows on a surface
-which may be either ribbed or smooth. The leaves of scale trees were
-stiff and slender, and arranged in grass-like tufts at the top.</p>
-<p>Calamites, related to our horsetail rushes, were somewhat smaller
-than the scale trees. Their trunks consisted of a thin, woody cylinder
-with a pithy interior, and were marked at intervals by nodes which gave
-them the &ldquo;jointed&rdquo; appearance of a bamboo stem. Leaves were arranged
-in circles around the nodes of main stem or branches. Spore-bearing
-cones appeared at the tips of the stems.</p>
-<div class="pb" id="Page_41">41</div>
-<div class="img" id="fig12">
-<img src="images/p11.jpg" alt="" width="600" height="754" />
-<p class="pcap"><span class="sc">Prehistoric Plants</span></p>
-<p class="pcapc">Some of the larger and better known plants of past ages are shown as reconstructed
-by artists. Finer details of the reconstructions often have to be neglected
-because of uncertainties due to the scattered and fragmentary character of the
-fossil record.</p>
-</div>
-<dl class="undent pcap"><dt>LEAF IMPRESSIONS</dt>
-<dd>Carboniferous Ferns</dd>
-<dd><span class="sc">Strap-leaf Conifer</span> (Cordaites)</dd>
-<dt>MODERNIZED TYPES</dt>
-<dd><span class="sc">Sequoia</span> Cone and foliage</dd>
-<dd><span class="sc">Miocene Fossils</span> (Florissant Shales)</dd>
-<dd class="t">Maple</dd>
-<dd class="t">Willow</dd>
-<dd><span class="sc">Eocene palm</span> (Denver Beds)</dd>
-<dt>HORSE TAIL RUSHES</dt>
-<dd><span class="sc">Restoration</span> (Calamites)</dd>
-<dd><span class="sc">Fossils</span> Leaves and Stem</dd>
-<dt>CYCADS</dt>
-<dd><span class="sc">Restoration</span></dd>
-<dd><span class="sc">Fossil Trunk</span></dd>
-<dt>CLUB MOSSES</dt>
-<dd><span class="sc">Restoration</span> (Sigillaria)</dd>
-<dd><span class="sc">Fossils</span> Trunk Impressions</dd>
-<dt>SCALE TREES</dt>
-<dd><span class="sc">Restoration</span> (Lepidodendron)</dd>
-<dd><span class="sc">Fossil</span> Leaf scars</dd></dl>
-<div class="pb" id="Page_42">42</div>
-<p>Also included among the larger trees were the Cordaites or large-leaved
-evergreens, tall and slender, seed-bearing but not true conifers
-as yet. Leaves were strap-shaped or grass-like, the larger ones having a
-length of six feet and a width of six inches. Trunks were woody, resembling
-pine, but with a central pith. The flowers were small and resembled
-catkins in form.</p>
-<p>Ferns and fernlike plants were so numerous that the period has
-been known as an age of ferns. Earlier knowledge of these forests was
-based on fossils of a fragmentary character from which an accurate association
-of parts could rarely be obtained. A general relationship with
-the ferns was apparent, but careful study of additional material has
-given us a rather different view of Carboniferous plant life and we note
-a highly diversified array of forms with many suggestions of modern
-tendencies. The flora as a whole may be regarded as highly specialized
-for the conditions which prevailed at the time and were not to continue
-through subsequent periods. Warm temperatures and abundant moisture
-were essential especially to spore-bearing types, and the cold, arid
-conditions of the next period put an end to many of the groups, or
-greatly reduced their prominence.</p>
-<p>This could be regarded equally well as an age of insects, for some
-of these invertebrates acquired the greatest size they have ever had, particularly
-the dragonflies with a wing-spread of more than two feet in
-one of the largest fossils so far discovered. Cockroaches numbering upwards
-of five hundred species have been named. Though large they are
-hardly to be regarded as giants, lengths of three or four inches being
-about the limit.</p>
-<p>Some of the insect types of today quite evidently existed among
-the inhabitants of Carboniferous forests, but it is apparent that there
-were also some antiquated forms which may have descended from
-the trilobites. Although some authorities regard this as the period in
-which insects originated, there are others who maintain that definite beginnings
-are not established so readily on present evidence. Spiders are
-believed to have made their appearance at this time.</p>
-<p>Four-footed vertebrates resembling salamanders were prominent animals
-of the Carboniferous swamps. At first adapted to a life in water
-and later to land conditions, they are known as amphibians, the name
-being based on the ability to live in two different kinds of environment.
-Common living representatives of this group are the toads and
-<span class="pb" id="Page_43">43</span>
-frogs, but these tailless forms are not known among fossils of the Paleozoic
-era and are almost unknown throughout the Mesozoic. The Age
-of Amphibians, as we apply that phrase, was definitely not an age of
-toads and frogs.</p>
-<p>These primitive land animals were of different types, ranging from
-much smaller sizes up to the length of a crocodile. Most of them had
-short legs, and feet which were suitable for locomotion upon land, but
-many of the creatures probably spent most of their lives in the water.
-Tails were usually high and flattened as if for swimming, sometimes
-long, at other times greatly shortened in proportion to the body. Heads
-were generally large, jaws long, and mouths wide.</p>
-<p>Before the close of the period true reptiles appear, but this race of
-animals is destined to make a more spectacular advance than the amphibians
-and will be discussed in connection with Mesozoic life. The
-amphibians, however, are regarded as being the ancestors of the reptiles
-as well as the higher quadrupeds which follow them. Although living
-reptiles are readily distinguished from living amphibians there is a different
-situation with regard to these primitive forms, for among the
-fossils it becomes increasingly difficult to separate the two groups as new
-material is investigated.</p>
-<p>Invertebrates had their ups and downs during the period. Trilobites
-became scarce, and brachiopods for a time were the most abundant
-of the shelled animals but later declined rapidly. In favorable localities
-the crinoids established a wonderful record for new species before the
-period had advanced very far. Hundreds of species of Carboniferous
-invertebrates are known, and in many of the rocks of the period they
-are the only fossils to be found, for the vertebrates were still unable to
-venture far from the swampy districts, and much inland territory was
-too well drained to support either the floras or faunas then existing.</p>
-<p>In the Colorado area there are both marine and continental formations
-but the great coal-making forests and their inhabitants were
-limited to other localities. As a consequence this region is not famous
-for Carboniferous fossils.</p>
-<h3 id="c17">PERMIAN HARDSHIPS</h3>
-<p>For a time there was no great change in North America following
-the opening of the Permian. Then began a series of mountain-making
-movements and continental uplifts which drained the swamps, lakes, and
-inland seas. With the passing of the vegetation which had established
-itself in and around these areas much of the animal life followed. It is
-<span class="pb" id="Page_44">44</span>
-probable that a considerable proportion of the marine life survived, much
-more than is indicated by the fossil record, but the receding seas carried
-the survivors into territory which is now inaccessible to fossil hunters.</p>
-<p>After Middle Permian time the climate everywhere seems to have
-been cold and dry. By the end of the period there had been accomplished
-more geographical change throughout the world than at any time since
-the beginning of the Paleozoic era. Traces of the crustal movements
-which produced new mountain ranges can be followed in Europe, Asia,
-and North America. The Appalachian region was raised to a great
-height, possibly in excess of three miles. A major disturbance of this
-character is known among geologists as a revolution, and to this particular
-one the name &ldquo;Appalachian Revolution&rdquo; has been given.</p>
-<p>The elevation of continents necessarily changes their coast lines.
-This, in turn, influences ocean currents which have an important bearing
-on climatic conditions. In addition to this, the elevation of mountain
-systems not only rearranges the distribution of hot and cold winds over
-the land areas but it may produce barriers to the migrations of floras
-and faunas, confining them to areas in which it is no longer possible
-to live. When the effect upon plants and animals is considered, it is
-easier to understand why a line is drawn across the geological time
-chart at such a point and an era of prehistory is regarded as closed.</p>
-<p>During the Permian period there was recorded in the rocks more
-widespread glacial action than ever before or since. With less inland
-water to provide the necessary evaporation there was a marked decrease
-in rainfall, and arid or semi-arid conditions replaced the hospitable climate
-that had been such an important factor in the prolific life of the Carboniferous.
-The struggle for existence became intense, but hardier types of
-plants and animals, with greater ability to adapt themselves to adverse
-conditions, established themselves here and there, as ancestral forms became
-extinct. Most of the large spore-bearing trees died out and seed-producing
-varieties began to acquire prominence, among them the coniferous
-evergreens. Ferns, however, proved their adaptability by producing
-some new forms which became prominent in Permian floras.</p>
-<p>The prehistoric amphibians have been divided into three orders, one
-of which includes all the larger forms. This group, known as the labyrinthodonts,
-continued on through Permian time but began to show
-backward tendencies, with dwindling limbs and a return to life in the
-water. Among the larger land varieties are typical fossils ranging from
-about fifteen inches to five feet in length. In outward appearance they
-differed from Carboniferous amphibians. One of the other orders,
-including a great diversity of smaller forms, became extinct during this
-<span class="pb" id="Page_45">45</span>
-period, leaving no known descendants. The third order is regarded as
-the oldest, and probably the ancestral group from which the modern
-newts and salamanders originated.</p>
-<p>The most successful of Permian land animals were the peculiar reptiles
-that learned to live in drier regions. Like the horned toad and Gila
-monster of our arid southwestern United States, the larger Permian
-reptiles were four-footed animals. In size and shape they were not
-greatly different from amphibians then living. An exception to this rule,
-among some of them, is the development of long, bony spines above the
-vertebrae of the back. A fairly common fossil of this type, found in
-Texas and known as <i>Dimetrodon</i>, had a total length of six feet, about
-half of this being in the tail. The tips of the spines adorning the back
-reached a height of three feet or more and there was probably a covering
-of skin over these bones, which would produce a sail-like structure or
-&ldquo;fin&rdquo; of large size. Its use has not been explained but it provides an
-easy name for these odd creatures&mdash;the &ldquo;fin-back lizards.&rdquo;</p>
-<p>Rock deposits produced in arid regions usually have characters which
-are not difficult to recognize. Gritty texture, irregular bedding, red color,
-and gypsum are common features. Formations of Permian age are to be
-found in Colorado but better fossil deposits have been discovered in
-Kansas and Texas.</p>
-<div class="pb" id="Page_46">46</div>
-<div class="img" id="fig13">
-<img src="images/p12.jpg" alt="" width="800" height="439" />
-<p class="pcap"><span class="sc">Marine Reptiles: Plesiosaur (Lower Skeleton) and Mosasaur</span></p>
-<p class="pcapc">The Mesozoic Era produced many types of reptiles besides the dinosaurs. Two of the marine forms are shown in this illustration,
-both from Cretaceous beds of the western United States.  Plesiosaurs were the giants of the seas in their time, lengths
-of forty to fifty feet being not uncommon. A long flat tail provided locomotion for the mosasaurs whereas the plesiosaurs resorted
-to the peculiar limb structures known as flippers or paddles.</p>
-</div>
-<div class="pb" id="Page_47">47</div>
-<h2 id="c18"><span class="small">THE AGE OF REPTILES</span></h2>
-<p>The Mesozoic, or era of middle life, was a long stretch of time
-during which there was marvelous development among the reptiles.
-Many strange types were produced and most of them became extinct
-before the end of the era. The reptilian stock branched out in many
-directions. Types emerged which differed from one another so widely
-that their mutual relationships have become obscure. Hideous and fantastic
-creatures suggesting sea serpents and dragons were worldwide in
-distribution. Reptiles of the air and seas acquired large size and weird
-forms, but greater advances were made upon land.</p>
-<p>The flying reptiles or pterosaurs flourished in Jurassic times with
-some of the larger varieties surviving until near the close of the Cretaceous.
-Although these winged lizards were the first of the vertebrates
-to fly they are not to be confused with birds. They were without feathers,
-and the earlier forms were provided with long tails bearing a flattened
-rudder-like tip. One of the best known of this type had a length
-of about eighteen inches. Its jaws were long and provided with sharp
-teeth. The wings were membranes attached to body and legs, stretched
-and manipulated by means of greatly elongated fingers. In later types
-there was a reduction in tooth equipment and length of tail. <i>Pteranodon</i>,
-found in Kansas, had a wing spread of twenty-five feet, a large toothless
-beak, a short body, and a mere stub of a tail. It was one of the last of
-these winged monsters.</p>
-<p>Several types of marine reptiles appeared during this era, among
-them the plesiosaurs which first appeared in Triassic seas. These peculiar
-animals were serpent-like with regard to the character of head, neck, and
-tail, but in other respects were quite different, the short barrel-shaped
-body being provided with four large paddles corresponding to the usual
-limbs of quadrupeds. Fossil remains of these animals are common in many
-Jurassic and Cretaceous deposits, some of the largest exceeding forty feet
-in length. Mosasaurs, also marine carnivores, inhabited shallow Cretaceous
-seas throughout the world and are especially abundant as fossils in
-the Kansas chalk beds. These were elongated forms with a resemblance
-to salamanders in some respects but provided with long pointed jaws and
-sharp teeth. Swimming was accomplished largely by the tail though
-probably aided to some extent by four webbed paddles or flippers. The
-ichthyosaurs were more fish-like in construction, as the name implies.
-The limbs were short and broad, and there was usually present a well-developed
-tail-fin as well as a large fin on the back. They were especially
-abundant in Jurassic time. Fossils are fairly common in marine deposits
-<span class="pb" id="Page_48">48</span>
-of western North America. Mosasaurs and ichthyosaurs were about half
-as long as the plesiosaurs.</p>
-<h3 id="c19">DINOSAURS</h3>
-<p>Most spectacular of the prehistoric reptiles were the dinosaurs, a
-large group of animals varying greatly as to size, form, and habits. They
-were adapted for a life on land though many of them probably spent
-much of their time partly submerged in the waters of lakes and streams.
-There is little that can be said of the group as a whole other than that
-all of them were reptiles. Further than that it is necessary to regard them
-as belonging to several different subdivisions of the Reptilia. Classification
-has been difficult and the names used for the various subdivisions are
-often misleading to the layman who tries to understand the terminology.</p>
-<p>Ancestral reptiles were five-toed and five-fingered but among the
-dinosaurs there were many departures from the standard formula. Three
-or four of the digits were commonly well developed, the others when
-present being shortened or reduced to mere rudiments. Early in the history
-of dinosaurs there was a division of the stock into two main branches,
-each of which includes a variety of types and sizes, and is again subdivided.
-The two main groups are best recognized by the construction
-of the bony framework which comprises the pelvic girdle or hip region
-of the skeleton. In order to avoid technical difficulties, however, the remaining
-discussion of these interesting reptiles will be confined to a few
-names and descriptions which serve to illustrate roughly the great amount
-of variation that developed from the comparatively simple ancestral pattern.
-The plan according to which the dinosaurs are usually classified is
-barely suggested by the types described.</p>
-<p>The meat-eaters were active creatures provided with powerful jaws
-and teeth. They were unarmored, moved about on their hind feet, and
-during their time were the most highly advanced of all animals. <i>Tyrannosaurus</i>
-with a length of forty-five feet or more, and <i>Deinodon</i>, nearly as
-large, were among the greatest of these. Both lived in the Cretaceous period.
-Their teeth were simple but strong, knife-like, curved, and finely
-serrated. Skulls were large and the forelimbs were reduced almost to a
-state of uselessness. Large carnivores lived also during Jurassic time and
-even as far back as late Triassic. Early Triassic forms were of smaller size.</p>
-<p>More primitive flesh-eating dinosaurs of the Triassic and Jurassic
-periods were delicately proportioned and lightly built bipeds bearing
-some resemblance to birds. <i>Struthiomimus</i>, which means ostrich-resembling,
-was about the size of the bird which provides the name. It was
-slender in the limbs, three-toed, long necked, long tailed. The skull was
-small, forelegs long for a biped. Unlike most dinosaurs it was toothless.
-All these bird-like carnivores were small as compared with other contemporary
-forms. Compsognathus, of Germany, and one of the smallest of
-all dinosaurs, had a length of less than three feet, including the long tail.</p>
-<div class="pb" id="Page_49">49</div>
-<div class="img" id="fig14">
-<img src="images/p13.jpg" alt="" width="800" height="217" />
-<p class="pcap"><span class="sc">One of the Large Jurassic Dinosaurs</span> (<i>Diplodocus longus</i>)</p>
-<p class="pcapc">This magnificent specimen, exhibited by the Denver Museum of Natural History, has a length of seventy-five feet six
-inches. Two years were required to complete the task of removing the bones from the matrix rock and preparing them for
-mounting. Diplodocus was one of many large reptiles which inhabited western North America a hundred and fifty million
-years ago. The skeleton was obtained from the Morrison beds of eastern Utah. The same formation is exposed in many Colorado
-localities, including the foothills west of Denver, where it acquired its name from the town of Morrison.</p>
-</div>
-<div class="pb" id="Page_50">50</div>
-<p>In Jurassic time there became prominent a group of large dinosaurs
-which were more equally developed as to fore and hind limbs. They
-were sluggish creatures, quadrupedal in their manner of locomotion,
-vegetarians in regard to their diet. Some of them reached enormous
-proportions and it is believed that they resorted to life in the water
-in order to get part of the weight off their feet. <i>Diplodocus</i> and <i>Brontosaurus</i>
-are the names of well-known giants in this group. They had long
-necks and tails, very small skulls, were the largest of all land animals
-and are known to have reached a length of eighty feet or more. Some
-estimates, based on measurements of incomplete skeletons, have exceeded
-one hundred feet, but these extremes are somewhat questionable. <i>Diplodocus</i>
-was the more elongated of the two, with much of its length in the
-whip-like tail. Our mounted skeleton has a length of seventy-five feet
-six inches, measured along the vertebrae. Its height at the pelvis is
-twelve feet six inches.</p>
-<p>The teeth of these large quadrupeds are of a slightly broadened and
-blunted form which has caused some speculation as to their possible use.
-It has even been suggested that the animals were fish-eaters but this seems
-impossible in view of the great size and general characteristics of the
-group. Although they differ extremely in some respects, they are regarded
-as being more closely related to the carnivores than to the herbivores
-of the second great branch of the tribe.</p>
-<p>The unquestioned herbivores, constituting this second branch of the
-dinosaurian race, also include both bipeds and quadrupeds. The better
-known plant-eaters were large animals but not such monsters as <i>Tyrannosaurus</i>
-or <i>Brontosaurus</i>. Of the bipeds, <i>Trachodon</i> is perhaps best known.
-It is one of the duck-billed dinosaurs which had an average length of
-about thirty feet. The duckbills were unarmored, active animals, good
-swimmers as well as runners. They were prominent and widely distributed
-during late Cretaceous time. Many skeletons have been found in
-western North America. Natural casts and impressions of mummified
-remains indicate that the hides were scaly and the feet provided with
-webs between the toes. The bill was broad, flat, and toothless, but the
-sides of the mouth were provided with a large number of simple teeth
-closely arranged in parallel rows. The fine skeleton exhibited in our hall
-is thirty feet six inches in length. Near relatives of <i>Trachodon</i>, such as
-<i>Corythosaurus</i> had hollow, bony crests, combs, or tubular structures on
-top of the head. These may have been of some service in connection
-with breathing while feeding under water.</p>
-<div class="pb" id="Page_51">51</div>
-<div class="img" id="fig15">
-<img src="images/p14.jpg" alt="" width="868" height="500" />
-<p class="pcap"><span class="sc">A Duck-billed Dinosaur of the Cretaceous Period</span> (<i>Trachodon mirabilis</i>)</p>
-</div>
-<div class="pb" id="Page_52">52</div>
-<div class="img" id="fig16">
-<img src="images/p15.jpg" alt="" width="800" height="510" />
-<p class="pcap"><span class="sc">Stegosaur</span> (<i>Stegosaurus stenops</i>)</p>
-</div>
-<div class="pb" id="Page_53">53</div>
-<p>Among the quadrupedal vegetarians an interesting family is represented
-by <i>Stegosaurus</i>, a late Jurassic dinosaur having a length of about
-twenty feet. These creatures had heavy limbs, all used in walking, an
-arched back, and almost no brain at all. A double row of large flattened
-plates standing upright and extending from the rear of the skull nearly to
-the tip of the tail provided some protection for the back of the animal,
-but otherwise there was no defensive armor. Several long spikes at the
-end of the tail probably served as weapons. The mounted skeleton in our
-collection was obtained from Garden Park, near Canon City, Colorado,
-a district which has long been famous for dinosaur remains.</p>
-<p>The ankylosaurs were more completely armored with closely set bony
-plates fitting neatly over the body. They were of about the same size as
-the stegosaurs but the body was broad and somewhat flattened. These
-armored quadrupeds apparently lived only during the Cretaceous period,
-after the disappearance of the stegosaurs. Their tooth equipment was
-very poor and in a few cases entirely lacking. <i>Ankylosaurus</i> and <i>Nodosaurus</i>
-are good examples of the type. They have been described as animated
-tanks and are sometimes referred to as having the appearance of
-enormous horned toads.</p>
-<p>Among the last of the dinosaurs to come and go were the horned
-quadrupeds known as the Ceratopsia. Their entire history appears to
-have been confined to the Upper Cretaceous and the closing stages of the
-reptilian era in America. <i>Triceratops</i> and <i>Monoclonius</i> are well-known
-representatives of the group. Besides the horns, which appeared above the
-eyes or near the center of the nose, there was a broad, flattened, backward
-extension of some of the skull bones which produced a great frill or collar
-reaching over the neck as far back as the shoulders. This frill, combined
-with the large skull, gave the animal the appearance of being nearly one-third
-head. <i>Triceratops</i> had three horns, <i>Monoclonius</i> only one. The
-average length of the animals was slightly under twenty feet.</p>
-<p>Although very little is known about the ancestry of the horned
-dinosaurs a valuable discovery in Mongolia may throw some light on the
-subject. A small dinosaur with a well-developed frill, but no horns, once
-inhabited the region of the present Gobi desert, and in recognition of
-the apparent relationship it has been named <i>Protoceratops</i>. In addition
-to numerous skeletons, several nests of eggs were found in association
-with the bones. Until this discovery was made, dinosaur eggs had been
-practically unknown. A reproduction of one of these nests is among
-our exhibits.</p>
-<div class="pb" id="Page_54">54</div>
-<div class="img" id="fig17">
-<img src="images/p16.jpg" alt="" width="630" height="700" />
-<p class="pcap"><span class="sc">A Sea Turtle of Cretaceous Time</span> (<i>Protostega gigas</i>)</p>
-<p class="pcapc">This marine animal belongs to a group which became extinct near the close
-of the great reptilian era, but a few related forms still survive. Their weight is
-greatly reduced by the peculiar construction of the shell, and the front feet are
-enlarged for use as oars, an excellent illustration of the manner in which a land
-type can become adapted to life in the sea.</p>
-</div>
-<div class="pb" id="Page_55">55</div>
-<p>With the possible exception of a very few short-lived survivals dinosaurs
-were extinct before the opening of the Age of Mammals, many of
-them for millions of years. Along with them went other types of ancient
-reptiles, and the cause of their extinction is a problem which may never
-be solved. Conditions remained favorable for the turtles, which made
-their first appearance during Triassic time, and for the crocodiles, which
-date back to the Jurassic period. Snakes were only at the beginning of
-their history as the era closed. The survival of these modern forms suggests
-that they were favored to a greater extent than the dinosaurs during
-a prolonged period of changing conditions the full details of which are
-unknown to us.</p>
-<p>In general it is to be expected that disaster would first overcome the
-highly specialized creatures, such as the dinosaurs, which had become
-more delicately adjusted to the particular environments in which they
-lived. It appears that some of them had been too progressive up to a
-certain point, but not sufficiently adaptable to get beyond that stage, or
-fortunate enough to make their advances in directions that could be
-followed, through fluctuations in the matter of food supply, predatory
-enemies, climate, and other factors which bear upon success and failure.</p>
-<p>The reptilian era closed with exceptional volcanic activities in many
-parts of the world, but these cannot account for the disappearance of the
-highly diversified and abundant reptilian life. The eruptions were merely
-incidental to movements and readjustments in large masses of rock comprising
-the earth&rsquo;s crust or surface. Such crustal folding and elevations
-always have been of serious consequences to both plants and animals because
-of their effect upon drainage and climate. There were disturbances
-of this kind in western North America in late Jurassic time, with folding
-and uplift in the region of the Sierras and probably extending from Mexico
-to southern Alaska. A great trough to the east of this elevated district
-was produced in the course of these movements and provided access to
-the sea from south to north. During the Cretaceous period there were
-repeated invasions and retreats of the sea by way of this great depression,
-consequent upon slight changes in the elevation of the floor. Hence there
-are numerous marine formations in Colorado and adjoining states, some
-of them rich in fossils.</p>
-<p>Before the close of the Cretaceous period the sea had made its final
-departure from this region, and the Mesozoic era was terminated by revolutionary
-disturbances which brought about the uplifting of a new mountain
-system. The Rocky Mountains may be regarded as part of this
-system and to have had their birth at this time. The Rockies, however,
-show unmistakable signs of repeated elevation, with intervals of erosion
-<span class="pb" id="Page_56">56</span>
-during which there was great reduction of their total height. What we
-see of them today is the result of more than fifty million years of continuous
-geological activity.</p>
-<h3 id="c20">PLANT LIFE AND CLIMATE</h3>
-<p>Some idea of the Mesozoic climate is obtained from the character and
-distribution of the plant life. Triassic floras are not large and there is
-very little fossil evidence for the earlier half of the period. It is quite
-possible that arid or desert conditions prevailed for a time in much of
-North America, as at the close of the Paleozoic era. Plant life was at
-first not abundant, and conditions were unfavorable for the production
-of fossils. In Upper Triassic rocks of Virginia, however, there are signs
-of swampy conditions, with rushes and ferns predominating. Adjoining
-forest areas were well timbered with large coniferous evergreens which
-show no annual growth rings, as similar trees do in regions where cold
-winters alternate with warm summers. This suggests, for that time and
-place at least, a uniformly warm climate, lacking seasonal variations.
-Warm temperature or subtropical climates are indicated again by some of
-the Jurassic and Cretaceous plants, but intervals of lower temperatures
-and variable climates are also apparent. Palms, figs, and other trees, very
-similar to modern types now living only in warmer regions, were widely
-distributed in late Cretaceous time, and their range was extended into
-regions which have since become too cold to support such growths.</p>
-<p>The trend toward modern forms in the plant world was gradual,
-but throughout the era there were occasional novelties that attract the
-attention of botanists. Ferns and horsetail rushes, reminiscent of the
-Paleozoic forests, soon began to lose their prominence as the seed-bearing
-trees gained the ascendency. Mesozoic time could well be called the age
-of cycads, because of the striking performance of this plant group. Different
-varieties flourished in the three periods, with the Jurassic standing out
-as the time of greatest abundance.</p>
-<p>To the uninitiated, the usual cycad fossils resemble &ldquo;petrified pineapples,&rdquo;
-but these are merely the scarred stems or trunks of small to
-medium-sized trees with a tufted arrangement of leaves at the top, and
-usually without branches. Foliage and habit of growth suggest something
-more like large ferns or low-growing palms, with short, thick
-trunks seldom more than fifteen feet tall and many of them under three
-feet. The leaves are rarely found entire or attached to the trunks, but
-occasional discoveries indicate a leaf-length of about ten feet. Although
-they are classed among the first and lowest of seed-bearing plants, and
-<span class="pb" id="Page_57">57</span>
-in this respect are related to the conifers, their appearance was quite
-unlike that of the modern cone-bearing evergreens.</p>
-<p>More nearly resembling the common conifers of today were the
-sequoias, of early Mesozoic origin and far more abundant during Cretaceous
-time than they are at present. The maidenhair trees, now represented
-by a single species of <i>Ginkgo</i> which is cultivated principally in
-China and Japan, were never very prominent but are of interest as an
-ancient family that persisted throughout the Mesozoic and down to our
-own time. Before the close of the Cretaceous period the flowering plants
-had greatly outnumbered the spore-bearing groups, such as the ferns and
-horsetails which were formerly so abundant. We know little of early
-flowers, however, except in connection with trees, the large gayly colored
-blossoms of the type now conspicuous in woodlands, meadows, and gardens
-being later arrivals and poor subjects for preservation as fossils.</p>
-<p>Cretaceous floras were surprisingly modern in character, far in advance
-of the animal life. Poplars, plane trees, magnolias, palms, figs, oaks,
-and buckthorns were abundant at the close of the Cretaceous, as indicated
-by fossils of the Laramie formation, which is the surface rock in many
-localities near Denver. Also abundant in various places at this time were
-walnut, hazelnut, laurel, tulip, maple, beech, birch, breadfruit, ivy, holly,
-and many other well-known trees and shrubs. Sedges and grasses, which
-became so important to the herbivorous mammals of the next era, made
-their first appearance in Cretaceous time but were then inconspicuous.</p>
-<h3 id="c21">COAL AND FOSSIL FOOTPRINTS</h3>
-<p>The abundance of plant life in the Colorado area during the Cretaceous
-period is indicated by the extent of coal deposits of this age.
-About one-fourth the area of the state is underlain by coal seams varying
-in thickness from a few inches to fifty feet or more, most of it being
-Cretaceous. In the northern Colorado district the coal-bearing formation
-is the Laramie. Near Denver there is some coal in the Arapahoe
-formation which overlies the Laramie and is of later age.</p>
-<p>Coal mines often produce excellent plant fossils, and occasionally
-other evidence of prehistoric life. In a mine near Canon City, Colorado,
-a series of natural casts of dinosaur feet was taken from the overlying
-rock after the coal had been removed. One of these, in the Denver
-Museum of Natural History, is seen to consist of sandstone inside a
-very thin layer of dark clay. Flattened against the lower surface is the
-carbonized stem of a Cretaceous plant which grew in the swamp where
-the coal deposit was formed.</p>
-<p>Since the shape of dinosaur feet is unmistakable we can only assume
-<span class="pb" id="Page_58">58</span>
-that a large reptile of this type walked over the surface of swampy
-ground in which a great thickness of decaying vegetation had accumulated.
-A layer of mud settled over the top and became sufficiently firm
-to retain the mold of the feet as the animal moved along. Any plant
-material either on the mud or included in it was pushed to the bottom
-of the impressions and flattened out by the weight of the huge creature.
-Then sand was washed into the footprints from some nearby source
-during a heavy rainstorm.</p>
-<p>Following these events there was probably a subsidence of the area,
-and a great thickness of rock-making sediments was built over the ancient
-swamp. The buried vegetation gradually became converted into
-coal, the sand consolidated into a firm sandstone, and the mud produced
-the shales forming the roof of the present mine, which is now at an
-elevation of a mile above sea level as a consequence of the general uplifting
-of the Rocky Mountain region during late Cretaceous and subsequent
-time.</p>
-<p>When the coal was removed, the hard sandstone casts separated
-readily from the softer shales surrounding them. A small amount of the
-shale adheres to the sandstone, and some of the flattened vegetation,
-now in the condition of coal, still remains attached.</p>
-<h3 id="c22">MESOZOIC INVERTEBRATES</h3>
-<p>As in other eras, the invertebrates fluctuate with the periods. Characteristic
-forms appear, become more or less prominent, then in many
-cases decline or disappear. Variations among the mollusks are particularly
-helpful in the identification of rocks which originated in the Cretaceous
-seas. Clam-like bivalves of the genus <i>Inoceramus</i>, the straight-shelled
-ammonids known as <i>Baculites</i>, and oysters, are locally common in
-some of the formations exposed a few miles west of Denver.</p>
-<p>The ammonids, or &ldquo;ammonites,&rdquo; were extremely abundant throughout
-the world during Mesozoic time. Their shells were chambered like
-those of the pearly nautilus, a related cephalopod mollusk inhabiting
-tropical seas at the present time. While only four species of the Nautilus
-tribe are living today, thousands of species of ammonids swarmed
-the prehistoric seas. Many new forms came into existence in Triassic
-time but few survived the period. A pronounced revival occurred in the
-Jurassic, only to be followed by a decline and eventual extinction at the
-close of the Cretaceous. Ammonites measuring three or four inches
-across the diameter of the coiled shell were about average size, but diameters
-up to three or four feet were not uncommon. Externally the
-shells were ornamented with ribs, knobs, and spines; inside was a pearly
-<span class="pb" id="Page_59">59</span>
-lining. The partitions were thin and composed of the same pearl-like
-substance as the lining. Each partition becomes wavy as it approaches
-the shell, and the line of union has a distinct pattern which is seen in
-specimens which have lost the outer shell layer. This wavy suture line becomes
-more complicated in the later members of the race, and the peculiar
-markings developed by the repeated partitions provide a convenient
-method of identification.</p>
-<p>The belemnites or ink-fishes, regarded as ancestors of the cuttlefishes
-now living, comprise another group of carnivorous mollusks. These,
-however, had lost the external shell, and the usual fossil is part of an
-internal shell or &ldquo;skeleton,&rdquo; known as the guard. This limy structure
-has the form of a pointed cigar, and is seldom over a foot long although
-the total body-length of the larger animals was commonly about six to
-eight feet. Several hundred species have been described, the majority
-being of Jurassic age. They declined rapidly toward the close of the era.</p>
-<p>The invertebrate life of the Mesozoic was strongly dominated by
-mollusks, with cephalopods in first place, the bivalve pelecypods and the
-single-valve gastropods or snail-like forms sharing subordinate positions.
-The dominating trilobites, sea-scorpions, and tetracorals of the Paleozoic
-had disappeared, while the brachiopods and crinoids were greatly modified
-and more like the forms which live today.</p>
-<p>Crinoids became moderately abundant at various times, but in many
-ways different from their relatives of the preceding era. Some of the
-largest known species, with stems estimated as fifty feet long, have
-been found in lower Jurassic rocks. A great abundance of microscopic
-life is indicated by the frequent occurrence of chalk in the Cretaceous
-formations. Corals of the modern reef-building type (hexacorals) were
-common in the warm seas of a large part of the world.</p>
-<p>The insects of the early Mesozoic are represented by few fossils
-although it is evident some new forms were becoming established. The
-warm climates prevailing throughout much of the world appear to have
-been a favorable factor in the progress of insect life. In addition to the
-older cockroach and dragon-fly types may be mentioned the arrival of
-grasshoppers, cicadas, caddis-flies, beetles, and ants.</p>
-<p>Several hundred species are found in Jurassic rocks, and by the
-end of the Cretaceous period most of the insect families now known to us
-were probably in existence. The record is seriously obscured by the
-erosion of rocks which so frequently marks the end of a period, also by
-the small size of the subjects, and by the exceptional conditions required
-for the production of such delicate fossils. Among the last of the familiar
-insects to appear were the bees and butterflies. These evidently
-<span class="pb" id="Page_60">60</span>
-came in with the more advanced types of flowering plants that produce
-the nectar on which many insects feed. It is probable, too, that without
-the arrival of these insects and their service in the pollination of flowers,
-the floras of today would be rather different from what they are.</p>
-<h3 id="c23">EXTINCT BIRDS</h3>
-<p>It is not surprising to find that birds made their first appearance
-in the Mesozoic era, for of all animals they are most like the reptiles as
-a class. Feathers are about the only dependable characteristic of the
-entire group, nearly every other feature being matched by some reptilian
-creature of great antiquity. The nesting habit, which includes care of
-the young as well as the eggs, is a matter of progress which relates to
-flight and to warmer body temperature. It appears to have been developed
-by forest-dwelling types living among trees and nesting there
-in comparative safety from enemies prowling on the ground.</p>
-<p>The oldest known prehistoric birds were found in lithographic
-stone of Upper Jurassic age. <i>Archaeopterix</i> was discovered in 1861 at
-Solenhofen, Germany. Sixteen years later a similar bird in a better state
-of preservation was found in Bavaria. The latter was named <i>Archaeornis</i>.
-These Jurassic fossils are regarded as true birds by some authorities,
-while others believe them to be more nearly related to the reptiles, the
-opinions being based on careful studies of the skulls and other skeletal
-features. Both birds had teeth of reptilian character, and it is evident
-that there was no beak, for the jaws were covered with scaly skin. The
-bony construction of the long tail would suggest lizards rather than
-birds, were it not for the presence of feathers which were attached at
-each side. Head, neck, and parts of the body were covered with scales.
-Wings were well provided with stout feathers but the skeletal framework
-indicates that the birds were gliders rather than true flyers. Claws
-on the wings served like fingers to aid in climbing among the branches
-of trees, a practice which is occasionally noted among the young of living
-birds. In adult birds of today, however, the claw-like appendages of the
-fore limbs are greatly reduced and of little service.</p>
-<p>The next fossil birds of importance have been found in Cretaceous
-rocks of Kansas, both of them fishers of the seas instead of forest inhabitants.
-<i>Ichthyornis</i> was a small bird, standing about eight inches in
-height, a powerful flyer with reptilian jaws and teeth. <i>Hesperornis</i> was
-built for diving and swimming, like the loon, but was somewhat larger
-and provided with teeth. Its wings were too poorly developed to be
-of use in flying.</p>
-<p>Toothed birds became extinct with the close of Cretaceous time,
-<span class="pb" id="Page_61">61</span>
-and the ancestors of modern types were in existence before the Age of
-Mammals, but fossil remains are few and poorly preserved. Large ostrich-like
-birds, however, are known to have lived in North America
-during the Eocene period. One of these, named <i>Diatryma</i>, stands nearly
-seven feet tall in the reconstructed skeleton. Its legs are heavy, wings
-greatly reduced, beak massive. In its relation to modern birds it is
-possibly nearer to the cranes than the ostriches.</p>
-<p>Flightless birds of large size are known from many parts of the
-world and seem to have been prominent throughout the Cenozoic era,
-as they are today in the southern hemisphere. <i>Aepyornis</i> lived in Madagascar
-during the Pleistocene period and may have become extinct quite
-recently. Its eggs are the largest known among fossils, several times the
-size of an ostrich egg. Also in this period the moas were living in New
-Zealand where their remains are still abundant. One of the largest,
-known as <i>Dinornis</i>, had about the same form as <i>Diatryma</i> but the neck
-was longer, head and beak smaller, legs better fitted for running, height
-about eleven feet.</p>
-<p>A much smaller flightless bird, the dodo, became extinct in modern
-time. This former inhabitant of Mauritius and other islands of the Indian
-Ocean was related to the doves and pigeons, and had lost its power
-of flight through disuse of the wings. It was a clumsy, defenseless bird
-weighing possibly as much as fifty pounds. Actual remains are few and
-incomplete, and descriptions published by the explorers who knew the
-bird two centuries ago are not entirely trustworthy. In the Pleistocene
-Rancho la Brea beds of California the largest of all prehistoric flying
-birds has been found, a vulture bearing the name of <i>Teratornis</i>. Re-assembled
-skeletons show them to be slightly bigger than existing condors.</p>
-<h3 id="c24">ANCESTORS OF THE MAMMALS</h3>
-<p>The monotremes or egg-laying types of mammals such as the duck-bill
-and spiny anteaters which now inhabit Australia are almost unknown
-as fossils. Marsupials, the next higher living group, which includes
-the opossum and kangaroo, appeared at the end of Cretaceous
-time along with the placentals or higher mammals which dominate the
-history of the Cenozoic era. Nevertheless, there are a few teeth and
-jaws from rocks of Triassic and Jurassic age to indicate that small
-mammals, from the size of mice to slightly larger than rats, existed
-throughout most of the Age of Reptiles. There is no complete skeleton
-of any of the earlier forms, and little is known of their relationships
-either with living orders of mammals or with probable ancestors among
-<span class="pb" id="Page_62">62</span>
-the reptiles. The record becomes somewhat clearer toward the end of
-the era but it is obscured again by the great disturbances which followed.</p>
-<p>Looking back among earlier land animals for the origin of the first
-mammalian stock it is necessary to go as far as Permian or even Carboniferous
-time. Reptiles then living had many structural features in common
-with mammals, and mammal-like forms continued to flourish until
-late in the Triassic. An interesting group of such animals, named
-therapsids, was one of the earliest reptilian stocks to appear, and is well
-known from fossils found in the Red Beds of Texas and New Mexico, in
-Europe, South Africa, and Asia. Quite a variety of types is included
-in this group, with many advances in dentition, and modifications of
-the skull, limbs and pelvic construction which strongly suggests a relationship
-to the mammals.</p>
-<div class="pb" id="Page_63">63</div>
-<div class="img" id="fig18">
-<img src="images/p17.jpg" alt="" width="851" height="600" />
-<p class="pcap"><span class="sc">Murals Over Fossil Exhibits, Hall of Mammals</span>
-<br />Top: Eocene; Protylopus, Tanyorhinus, Patriofelis, Uintatherium, Turtle, Crocodile, Eohippus.
-<br />Middle: Upper Oligocene; Mesohippus, Merycoidodon, Hoplophoneus, Metamynodon, Poebrotherium, Trigonias.
-<br />Bottom: Pliocene; Teleoceras, Turtle, Synthetoceras, Amebelodon, Teleoceras.</p>
-</div>
-<div class="pb" id="Page_64">64</div>
-<h2 id="c25"><span class="small">THE AGE OF MAMMALS</span></h2>
-<p>The striking feature of life development in the Cenozoic era is the
-great progress and expansion over the earth of the mammalian races.
-The division of the era into periods, however, was based largely on a
-study of fossil mollusks. In the Paris basin of France, it was noticed by
-the geologists of a century ago that the youngest of the sedimentary
-beds contained the greatest number of recent or still living species.
-Successively downward into the older beds the percentage of recent
-species decreased until there were practically no living species represented
-in the oldest rocks of the series. From the percentage of recent
-forms among prehistoric ones it was proposed that the following
-division be made: Eocene, meaning <i>dawn of the recent</i>; Miocene,
-meaning less recent; and Pliocene, meaning <i>more recent</i>. Sometime later
-it was suggested that another period be added, and to this was given the
-name Pleistocene, meaning <i>most recent</i>. In 1854, the older Miocene formations
-were segregated and referred to a newly provided Oligocene
-period, this name meaning <i>little of the recent</i>.</p>
-<p>Early geologists grouped the rocks in three great divisions, applying
-the names Primary, Secondary, and Tertiary. To these was added
-afterwards the name Quaternary, which applied to the youngest formations
-of the earth. Only two of these terms remain in common use at
-present: it is a frequent practice to refer to the combined Eocene, Oligocene,
-Miocene and Pliocene periods as the &ldquo;Tertiary&rdquo; division of
-Cenozoic time; to the Pleistocene and Recent periods as the &ldquo;Quaternary&rdquo;
-division. The geology of some remote future may be clearer with
-regard to the full significance of this subdivision of the Age of Mammals
-into two parts. It may be that a great era was concluded at the end of
-Pliocene time as others have been concluded, by the usual earth disturbances
-and climatic changes and by the decline of animals once prominent
-in the faunas of the world. Events of such character have registered
-their occurrence but may eventually prove to have been a series of minor
-events not comparable with the revolutionary changes that terminated
-other great time divisions. The favored practice of including ourselves
-and our times in the Cenozoic is based on a trend of opinion which
-holds that no great era has been ended since the Age of Reptiles was
-concluded.</p>
-<div class="pb" id="Page_65">65</div>
-<div class="img" id="fig19">
-<img src="images/p18.jpg" alt="" width="600" height="735" />
-<p class="pcapc">Skulls of the clumsy, six-horned uintathere and the early, hornless titanothere
-form part of this Eocene display. In the mural these animals and the little &ldquo;three-toed&rdquo;
-Eohippus, smallest of horses, are pictured with a contemporary turtle and
-crocodile.</p>
-</div>
-<div class="pb" id="Page_66">66</div>
-<p>The oldest of Eocene rocks show a great variety of mammals and
-a strange assortment of forms far in advance of the Mesozoic record.
-Ancestries and successive stages of development have been only partially
-worked out, though details have been better preserved for some of the
-groups than for others. Some oddly shaped creatures such as the uintatheres
-reached their full development in a relatively short time and
-passed out of the picture before the end of the period. These animals,
-represented in our collection by <i>Uintacolotherium</i>, acquired large bodies
-and many horns, but a peculiar tooth equipment fitted them for a special
-diet which apparently failed to be supplied in sufficient abundance at a
-critical time.</p>
-<p>On the other hand, we find in this period the ancestors of more successful
-groups, some of which continue on into modern times. Only a few
-of these histories can be traced in a brief sketch, but in a general way it
-may be stated that the successful races had modest beginnings and that
-they developed very slowly into what they are now, by a process of adjusting
-themselves, or by becoming better adapted to new or previously
-unused conditions in their respective environments. In their early stages
-the various types had much in common; they were generalized, rather
-than specialized for any particular kind of existence. What they were
-fitted for is best indicated by their teeth and feet, though other structural
-features frequently contribute valuable information. The rise of mentality
-is indicated by skull capacities and the increasing development of
-the upper lobes of the brain, as revealed by casts taken from the interior
-of skulls.</p>
-<p>The creodonts were the earliest and most primitive of the flesh-eating
-mammals or carnivores. Many of them were small of body and brain,
-and equipped with teeth that indicate a mixed or largely insectivorous
-diet, or possibly the habit of feeding on carrion. Although there was
-considerable variation among them, and some tendency toward specialization,
-there was little to suggest the coming of more progressive groups
-such as the cats and dogs, with teeth perfected for the tearing and cutting
-of flesh, and feet especially fitted for the life of hunters. The ancestry
-of the cats cannot be traced farther back than the Oligocene but it probably
-connects somewhere prior to that time with the creodonts.</p>
-<p><i>Cynodictis</i>, an Oligocene carnivore slightly under two feet in length
-is commonly regarded as a primitive dog, but its characters are so generalized
-that it probably differs but little from the ancestors of many other
-carnivores. The skeleton of this animal suggests a slender and flexible
-body like that of the weasel, with somewhat shortened limbs and a long
-tail. It lived in forested regions and was probably more or less of a tree
-dweller. The more advanced carnivores required longer legs, better
-adapted for running and overtaking the prey, which is the dog&rsquo;s way of
-hunting, or for stalking and springing upon the quarry, which is the
-method of the cat.</p>
-<div class="pb" id="Page_67">67</div>
-<div class="img" id="fig20">
-<img src="images/p19.jpg" alt="" width="600" height="740" />
-<p class="pcap"><span class="sc">Moropus</span> (<i>Moropus cooki</i>)</p>
-<p class="pcapc">Though its teeth clearly indicate a diet of plant material, this strange animal
-had claws on its toes, like the carnivores. It is probable that the claws were used
-in digging for roots, as indicated by the artist.</p>
-</div>
-<div class="pb" id="Page_68">68</div>
-<p>A prominent group of mammals today is that known as the ungulates,
-or hoofed animals, which includes the horses, cattle, deer, swine,
-rhinoceroses, tapirs, and other types both living and extinct. Their probable
-ancestors were the condylarths, primitive ungulates of the Eocene
-period. One of these, known as <i>Phenacodus</i>, serves well to illustrate the
-general character of the early hoofed mammals. It was about five and
-one-half feet long, rather large for its time, with long tail and short
-limbs, low elongated skull and small brain, very similar in many respects
-to the creodonts or ancestral carnivores. The teeth, however, were partially
-of the grinding type so essential to the welfare of plant feeders.</p>
-<p>The condylarths were five-toed animals and evidently provided with
-small hoofs, but the more progressive ungulates soon lost one or more of
-the toes, and a division of the group into odd-toed and even-toed branches
-became firmly established. Consequently, the families of ungulates having
-one, three, or five toes are classed together as being closely related to
-one another, and those having two or four toes are segregated in a second
-lot. The odd-toed clan, known as perissodactyls, included such animals as
-the horses, rhinoceroses, tapirs, and titanotheres, each of these types being
-placed in a separate family. The even-toed clan has been treated in a
-similar way and named the artiodactyls. In this division are such families
-as swine, cattle, deer, camels, oreodonts, and others. The odd-toed group
-dominated among the larger animals of North America for a time but
-has been completely replaced by the even-toed division which is still
-flourishing, although some of the older families have become extinct.</p>
-<p>Among the exhibits of the Denver Museum of Natural History
-may be seen complete skeletons of extinct horses, rhinoceroses, titanotheres,
-and chalicotheres representing the perissodactyls. <i>Moropus</i> was
-one of the chalicotheres, an exceptional family which never became very
-prominent although it had a prolonged history and persisted in Europe
-and Asia after its extinction in North America. The family is grouped
-with the ungulates because of many similarities found in the molar
-teeth, skulls, and other parts of the skeletons, but the toes were provided
-with claws instead of hoofs. The use of these claws is somewhat of a
-puzzle: possibly for defense against carnivorous enemies, for dragging
-down branches in order to obtain food, or for digging roots which may
-have been an important part of the diet.</p>
-<p>Titanotheres are represented in our collection by the skeletons of
-the large, horned type which was the last of the race and destined to extinction
-by the middle of the Oligocene period. Smaller hornless varieties
-of Eocene time are illustrated by skulls. This family of ungulates had an
-unprogressive dental equipment, and a small brain in a flattened skull.
-The molar teeth readily distinguish the group from other ungulates
-and enable us to trace the relationship between earlier and later varieties.
-These teeth were of a type which is soon destroyed by wear, and it is
-evident that the animals survived only so long as their environment provided
-them with an abundance of soft vegetation.</p>
-<div class="pb" id="Page_69">69</div>
-<div class="img" id="fig21">
-<img src="images/p20.jpg" alt="" width="600" height="720" />
-<p class="pcap"><span class="sc">Titanotheres of Oligocene Time</span></p>
-<p class="pcapc">The name of these animals refers to the large size though they were greatly
-exceeded in bulk by the mastodons and mammoths of later periods. Ancestral
-titanotheres, dating back to the Eocene, were hornless animals of much smaller
-size. These splendid specimens were obtained in Weld County, Colorado.</p>
-</div>
-<div class="pb" id="Page_70">70</div>
-<p>The large assortment of rhinoceros material provides an idea of the
-great abundance and variety of forms in this family which was once
-prominent in North America but no longer among the inhabitants of
-that continent. Some of the mounted skeletons have been restored on one
-side to show how these animals appeared in the flesh.</p>
-<p>Of the even-toed ungulates there are also several types illustrated by
-complete skeletons. <i>Merycochoerus</i>, the subject of one of our mounted
-groups, represents the oreodonts, a large family of mammals whose history
-begins with the Upper Eocene and ends in the Lower Pliocene.
-The oreodonts were small animals, rather pig-like in form and quite
-common in the western plains region shortly after the time of the titanotheres.
-Ancient swine are represented in our exhibits by two mounted
-skeletons which were obtained from northeastern Colorado, where the
-bones were found associated with rhinoceros and titanothere remains.
-Some of these animals were of very large proportions, and the entire
-family is commonly known as the &ldquo;giant pigs.&rdquo;</p>
-<p>Camels and closely related forms were quite abundant in North
-America from early Oligocene to comparatively recent time. Numerous
-types were developed during the course of their history, some small and
-delicately formed, others tall and clumsy and much like the giraffe in
-structure. Parts of many of these creatures have been found but the
-only completely prepared skeletons in our collection are of the little gazelle-camel,
-<i>Stenomylus</i>, from Lower Miocene deposits in northwestern
-Nebraska. Pleistocene bisons are represented by several complete skeletons
-and numerous skulls and horncores, some of the species showing an
-extreme development in the length of horns. With two of the bison
-skeletons are shown prehistoric weapon points, found with the bones and
-indicating that these animals were hunted by primitive men at some time
-near the close of the Ice Age. The artifacts first discovered near Folsom,
-New Mexico, by field workers of our Museum, have become known to
-archeologists as Folsom points.</p>
-<h3 id="c26">PREHISTORIC HORSES</h3>
-<p>The past history of horses is well known from an abundance of fossil
-material, ranging in age from the Eocene down to the present. Modern
-horses have only one toe in each foot, but there are remnants of two additional
-toes which may be seen only in the bony structure underlying
-the skin. Most of their ancestral relatives were three-toed as far back as
-the Oligocene period. During Eocene time, however, there was a stage
-which may be regarded as four-toed although it was evidently a temporary
-condition, linking known horses with more remote forms having five
-toes.</p>
-<div class="pb" id="Page_71">71</div>
-<div class="img" id="fig22">
-<img src="images/p21.jpg" alt="" width="800" height="416" />
-<p class="pcap"><span class="sc">Oligocene Mammals From Weld County, Colorado</span></p>
-<p class="pcapc">The giant pigs (<i>Archaeotherium mortoni</i>) at the left of the group, and the rhinoceros (<i>Trigonias osborni</i>) were common animals
-of the western plains region at one time.</p>
-</div>
-<div class="pb" id="Page_72">72</div>
-<p><i>Eohippus</i>, the &ldquo;dawn horse&rdquo; as it has been called, is one of the oldest
-and best known of the American horses. Its relation to existing
-members of the family can be traced by means of changes in tooth
-structure as well as in the gradual reduction in the number of toes that
-is seen among intermediate forms. Its ancestors some day may be positively
-identified in that group of generalized, primitive, five-toed, hoofed
-mammals which are known to have lived at the beginning of the mammalian
-era, but such identification has not yet been established. Even
-<i>Eohippus</i> bore little resemblance to the familiar horse of today. Its height
-was only eleven inches, and in body form it had much of the appearance
-of a modern dog. There were four toes on the front foot, one of them
-decidedly shorter than the others but complete in all its parts, and
-evidently capable of service in carrying a portion of the animal&rsquo;s weight.
-The hind foot had three complete toes and a tiny remnant of a fourth
-which could not have been apparent externally.</p>
-<p>As changes in the structure of the feet progressed, the central toe
-of the original five continued to increase in size while the adjacent digits
-became relatively shorter and eventually so reduced in length that they
-could touch the ground no longer. The smaller bones at the extremities,
-corresponding to the joints of our fingers and toes, eventually disappeared
-from the side toes. Then the longer bones of the outer digits lost the
-broadened supporting surface, where the missing toes had been attached,
-and became reduced to pointed remnants known as splints. Extreme
-shortening of the splint bones eventually leaves only a small knob which
-is often referred to as a rudimentary toe. In the skeleton of a large horse
-the splints are readily seen, but in some of the earlier species they are so
-small that they may easily be destroyed or overlooked by the collector
-who removes the fossilized material from the surrounding rocks. Even
-then, the bones of the wrist and ankle may indicate in an unmistakable
-manner that an additional toe once was present, for each bone is supported
-by another, and at the point of attachment there is a characteristic
-surface whose purpose is usually obvious.</p>
-<p>Throughout the Cenozoic era the changes continued. Among the
-horses of the North American Oligocene were <i>Mesohippus</i>, approximately
-the size of a collie dog, and <i>Miohippus</i> which was slightly larger.
-Both were three-toed, but the rudimentary splint of a fourth toe was still
-present in the front foot. <i>Parahippus</i> and <i>Merychippus</i> carried on during
-the Miocene period, the latter being characteristic of the time, and showing,
-in addition to other progress, a decided trend toward the modern
-structure of molar teeth. There was some increase in size but the largest
-horse of that period was hardly more than a small pony.</p>
-<div class="pb" id="Page_73">73</div>
-<div class="img" id="fig23">
-<img src="images/p22.jpg" alt="" width="800" height="642" />
-<p class="pcap"><span class="sc">A Pleistocene Horse of the Texas Plains</span> (<i>Equus scotti</i>)</p>
-</div>
-<div class="pb" id="Page_74">74</div>
-<p><i>Hipparion</i> and <i>Protohippus</i>, living during Upper Miocene and Pliocene
-time, represent later stages of the three-toed condition. The side
-toes were completely formed but greatly shortened, only the central toe
-touching the ground. In some of the species the outer toes had also become
-very slender, approaching the splint condition. By this time the molar
-teeth were longer and better adapted for feeding on grasses which
-were becoming sufficiently abundant to attract some of the forest dwellers
-into the open country.</p>
-<p>During the Pliocene period, in the genus <i>Pliohippus</i> and also in
-<i>Hipparion</i>, the feet were far advanced in structure, with most of the
-species single-toed, the side digits having reached the splint stage. Pleistocene
-horses of the genus <i>Equus</i>, like living species of that genus, were
-strictly one-toed animals, ranging over grassy areas and highly specialized
-for a life in that kind of environment.</p>
-<p>Specialization is to be noted partly in the foot and leg structures
-where the modifications have contributed to greater speed and travelling
-ability. This is of great service to an animal of the plains where food and
-water are often scarce, and great distances frequently have to be covered
-in order to obtain sustenance. The horse, as we know it, is built for
-speed, its limbs and feet being elongated to permit a greater stride, and
-also modified to decrease the weight without loss of strength. The ordinary
-ball-and-socket joint is replaced by a pulley-like construction which
-limits the direction of movement but provides an excellent mechanism
-for locomotion, especially over flat, open ground. Flexibility in other
-directions is sacrificed for greater strength, and the foot incidentally becomes
-less suited for other purposes.</p>
-<p>This is what is meant by &ldquo;specialization&rdquo;&mdash;a departure from &ldquo;generalization.&rdquo;
-The study of fossils provides numerous illustrations of
-specialized development which contributes greatly to an interest in prehistoric
-life. Any specialized structure or habit which increases fitness
-for a particular way of living is also known as an &ldquo;adaptation.&rdquo; Quite in
-line with the idea of specialization and adaptation is the change which
-occurred in the construction of the horses&rsquo; teeth, for the dental equipment
-of the modern grazing animals differs widely from that of the
-browsing creatures which lived on the soft leaves and other plant substances
-of the forests.</p>
-<div class="pb" id="Page_75">75</div>
-<div class="img" id="fig24">
-<img src="images/p23.jpg" alt="" width="610" height="700" />
-<p class="pcap"><span class="sc">The Structure of Molar Teeth</span></p>
-<p class="pcapc">The large lower molar of a long-jawed mastodon shows worn and unworn
-cusps, with the enamel layer forming a heavy border around the central dentine
-where the surface covering has been worn through. In the grinding teeth of
-rhinoceroses (illustrated at the right) the crown pattern is quite different, but
-both types are adapted for softer foods and are similar in having the protective
-enamel on the outside only. The central tooth shows the condition after the
-shallow surface depressions have been removed by wear.</p>
-</div>
-<div class="pb" id="Page_76">76</div>
-<p>The cheek teeth or grinding equipment of the horses underwent as
-complete a change as the feet. Modification resulted in a new type of
-tooth which enabled herbivorous animals to take advantage of a kind of
-vegetation which was late in arriving and has since become the principal
-diet of the ungulates. The grasses are coarse and harsh as compared with
-the leaves of forest shrubbery, requiring more thorough grinding to
-make them digestible. In addition they contain minute particles of silica,
-which is a highly abrasive mineral that quickly wears down the tooth
-substance, especially the softer materials found in tooth construction. An
-increase in the length of the tooth would offset the excessive wear but
-would not necessarily produce a better mechanism for grinding.</p>
-<p>The fulfillment of the new requirements is to be seen in the change
-from what is known as the low-crowned, browsing type of molar, to the
-high-crowned, grazing type. Details of the changes that may be traced
-through millions of years of gradual adjustment become apparent only
-from the examination of a great deal of fossil material. As compared
-with earlier types of construction, a modern molar tooth may appear extremely
-complicated, but the process which brought about the improved
-quality is very simple. A little discussion of tooth structure, however, is
-required to make this clear.</p>
-<p>A tooth, as everyone knows, is partly imbedded in the jaw, partly
-exposed outside the gum. In a short-crowned tooth the exposed portion
-is known as the crown, and the part imbedded in the jaw consists of one
-or more roots which are comparatively long. The crown is nearly always
-protected by a thin layer of hard enamel. In a grinding tooth, the working
-surface has a number of more or less prominent elevations known as
-cusps. The enamel layer completely covers this surface until wear begins.
-As the tooth goes into service the signs of use begin to appear; the enamel
-is soon worn from the tops of the cusps, and the underlying substance,
-called dentine, becomes exposed. This is far less resistant to wear, and
-as the enamel continues to be reduced the tooth becomes less efficient as
-a grinding device, partly because of the smoothing off of the surface,
-partly because of the relative softness of the inner material which is being
-exposed in increasing quantity. A very old molar tooth of the low-crowned
-type has a smooth surface from which almost the last trace of
-the enamel has been removed. In many prehistoric animals the enamel is
-of a darker color than the dentine or cement, this difference in color enabling
-one to see at a glance how the teeth are constructed.</p>
-<div class="pb" id="Page_77">77</div>
-<div class="img" id="fig25">
-<img src="images/p24.jpg" alt="" width="566" height="800" />
-<p class="pcap"><span class="sc">Grazing Type of Molar Teeth</span></p>
-<p class="pcapc">The side view of the bison&rsquo;s molar and premolar equipment illustrates the
-elongated construction which is common among grazing animals. In the pattern
-of the grinding surface may be seen a cross section of the enamel layers. One
-layer surrounds each tooth while two folded &ldquo;cylinders&rdquo; of the same material
-occupy the interior.</p>
-</div>
-<div class="pb" id="Page_78">78</div>
-<p>In a long-crowned tooth the roots are usually very short, for much
-of the crown itself is imbedded in jaw bone, and the longer roots are not
-required. Growth of the tooth is usually completed after a few years;
-then as it is gradually worn away it is continuously moved upward by the
-production of new bone under the roots, which slowly fills the bottom of
-the socket and continues to provide the necessary support. An equally
-important difference between the two types of teeth, however, is to be
-seen in the arrangement of the enamel, the long-crowned type being provided
-with this durable substance on the inside of the crown instead of
-having a mere protective cap on the outside.</p>
-<p>The more complicated structure was developed from the simpler
-form by the easy method of deepening certain depressions located between
-cusps at the top of the tooth. As the crown of the tooth increased
-its length these depressions remained tucked in, and eventually became
-deep pits roughly cylindrical in shape. In addition to the enamel and dentine,
-a third tooth substance, known as the cement, made its appearance
-at about this time, and we find that quantities of this new material were
-deposited outside the crown enamel and also inside the enamel walls of
-the pit, in this way producing a firmly consolidated structure otherwise
-weakened by deep channels and hollow pockets. The cement differs only
-slightly from the dentine but is deposited while the uncut tooth is in the
-gum tissues of the mouth, the enamel and dentine elements being formed
-earlier in the embryonic tooth before it emerges from the jaw bone.</p>
-<p>A tooth constructed by such a process, if cross-sectioned through the
-crown, will be found to consist of successive layers of hard and softer
-materials. In living animals the top of the tooth soon wears off and the
-enamel layers stand in higher relief because of their greater resistance to
-wear. A roughened surface of excellent grinding quality is thus provided,
-and as long as the wear continues there remains the same relative
-amount of enamel to retain the roughness, and resist abrasion.</p>
-<p>Among the various types of grazing animals there is a marked difference
-in the arrangement and form of the enamel layers. Within a
-species of genus, however, the complicated enamel patterns of the molar
-teeth are consistently similar. In the case of horses especially, these patterns
-provide a most helpful key to the identification of extinct forms.
-The general pattern, in any of the more modern horses, may be understood
-more readily if the wavy enamel layers be regarded as forming a
-set of cylinders with deeply crinkled walls. Near the outer border of the
-tooth, surrounded by a thin layer of cement, is the enclosing cylinder
-which represents the enamel cap of the old-fashioned, low-crowned tooth.
-Inside of this is the central mass of dentine which has been penetrated
-by two of the deep pits previously mentioned. The original enamel cap
-has been depressed into these pits, forming two inner cylinders which are
-filled with cement. Instead of being circular in outline, when the cap is
-worn through at the grinding surface these inner cylinder walls are seen
-to be wrinkled and folded so as to produce a most irregular pattern.
-However, if several teeth of the same kind of horse are compared, it will
-be found that the edges of these cylinders produce figures which are
-remarkably uniform and characteristic for that species.</p>
-<div class="pb" id="Page_79">79</div>
-<div class="img" id="fig26">
-<img src="images/p25.jpg" alt="" width="800" height="482" />
-<p class="pcap"><span class="sc">American Mastodon</span> (<i>Mastodon americanus</i>)
-<br />A true mastodon of the short-jawed type.</p>
-</div>
-<div class="pb" id="Page_80">80</div>
-<h3 id="c27">MASTODONS AND MAMMOTHS</h3>
-<p>Elephant-like mammals both living and extinct are classed together
-in a single order bearing the name Proboscidea. Living members of the
-group are the elephants, of which the large Indian and African species
-are best known. Among prehistoric representatives the most frequently
-mentioned in the popular literature of North American animals are the
-following:</p>
-<p>The American Mastodon, an immigrant from Siberia which ranged
-over nearly all of the United States and Canada. It was principally a
-forest dweller, rarely found in plains regions, was abundant during the
-Pleistocene period and may have been known to the early American
-Indians;</p>
-<p>The Woolly Mammoth, which was about nine feet tall. It ranged
-over British Columbia into the United States and across to the Atlantic,
-disappearing in late Pleistocene time;</p>
-<p>The Columbian Mammoth, about eleven feet tall, lived in the early
-half of the Pleistocene period, ranging over the warmer portions of
-North America, including practically all of the United States and much
-of Mexico;</p>
-<p>The Imperial Mammoth, reaching a height of more than thirteen
-feet, and becoming extinct in the Middle Pleistocene. It was a western
-form, remains being found from Nebraska to Mexico City.</p>
-<p>Originally placed in the genus <i>Elephas</i>, the mammoths are referred
-to commonly as elephants, though technically they should not be regarded
-as such. Recent explorations and researches have added greatly to
-our knowledge of these animals but have also caused much confusion
-with regard to scientific names, for many new subdivisions of the larger
-group are now recognized, and it has become necessary to change some
-of the older nomenclature.</p>
-<div class="pb" id="Page_81">81</div>
-<div class="img" id="fig27">
-<img src="images/p26.jpg" alt="" width="800" height="489" />
-<p class="pcap"><span class="sc">A Long-Jawed Mastodont</span> (<i>Trilophodon phippsi</i>).
-<br />One of the Early American Proboscideans</p>
-</div>
-<div class="pb" id="Page_82">82</div>
-<p>The large mammoth exhibited by the Museum bears the impressive
-name of <i>Archidiskodon meridionalis nebrascensis</i>. Fifty years ago it
-might have been identified simply as a specimen of the imperial elephant
-and in such case would have received the old name of that species, which
-was <i>Elephas imperator</i>. But late in the last century it was proposed that
-the mammoths be recognized by some other name to distinguish them
-more sharply from living elephants. The name suggested for the new
-genus thus established was <i>Archidiskodon</i>, in recognition of the more
-archaic or primitive construction of the enamel plates in the mammoths
-molar teeth. The specific name, <i>meridionalis</i>, had been given to a kind of
-mammoth which is well known from the southern part of Europe, and
-the Latin name, signifying &ldquo;southern,&rdquo; had been applied to differentiate
-this species from the northern or woolly mammoth.</p>
-<p>This mammoth, however, had disappeared from southern Europe
-and for many years its subsequent history remained a mystery. The late
-Dr. Henry Fairfield Osborn had been engaged in an extensive study of
-the subject, and when the nearly perfect skeleton from Angus, Nebraska,
-was brought to his attention he recognized it as being closely related to
-<i>meridionalis</i>, and considered it to be a record of the migration of that
-species into North America. Because of minor variations from the typical
-mammoth of southern Europe he regarded it as a variety or subspecies
-which had descended from the latter, and the subspecific name,
-<i>nebrascensis</i>, was added to take care of this situation, using a Latinized
-form of the name of the State in which the skeleton was found. With the
-knowledge we now have of these mammoths it becomes apparent that
-<i>Archidiskodon meridionalis nebrascensis</i> is an ancestor of the imperial
-mammoth, currently known as <i>Archidiskodon imperator</i>, and not identical
-with it.</p>
-<p>This instance is typical of the manner in which prehistoric animals
-obtain their names. Although given a Latin form, these technical names
-are derived from many languages, and the root words are applied with
-reference to anything that happens to appeal to the author as significant.
-Consequently there is seldom a name of this kind which may be translated
-directly into natural history or science. It is a mistake to believe
-that these strange phrases conceal important technical information which
-is available only to those who are familiar with dead and foreign languages.
-Actually they contain nothing of the sort, and the most enlightened
-of the Greeks and Romans could not find it there. When a
-name is needed there is none better than the one provided by the specialist
-who is skilled in the business of naming things. Some technical ability
-is required, to apply the name where it properly belongs, but technical
-knowledge is not obtained from such sources. Names, in any form, have
-another purpose to serve. There is no magic in them and there need
-be no mystery about them.</p>
-<div class="pb" id="Page_83">83</div>
-<div class="img" id="fig28">
-<img src="images/p27.jpg" alt="" width="800" height="366" />
-<p class="pcap"><span class="sc">Molar Tooth of Mammoth</span></p>
-<p class="pcapc">This type of tooth is constructed for long continued use and will withstand the wear of more abrasive foods. The position
-of the white enamel plates is seen in this view of the grinding surface. These plates extend all the way to the base of the tooth,
-which is of the long-crowned variety and not to be destroyed by the wearing away of a single outside layer of enamel.</p>
-</div>
-<div class="pb" id="Page_84">84</div>
-<p>Other specimens in the Museum collection are the long-jawed mastodonts,
-so named because of the elongated jaws and protruding chin
-which is often mistaken for a tusk. Early members of this group had
-more cheek teeth than later types of mastodons, and longer jaws were
-required for their accommodation. Some of them had flattened lower
-tusks which evidently were used for digging purposes. These are popularly
-known as &ldquo;shovel tuskers.&rdquo; The more modern American mastodon
-had shorter jaws and, like the mammoths and elephants, only one pair
-of tusks. Both the long-jawed and short-jawed types are represented by
-complete skeletons, and also by tusks, jaws, and teeth of many individuals.
-The American mastodons and mastodonts were of about the same
-size as the smaller mammoths.</p>
-<p>The difference between mastodons and mammoths is most readily
-recognized in the structure of the grinding teeth, the molars and pre-molars.
-In the mastodon these teeth are of the short-crowned type, while
-in the mammoths, as in the modern elephants, they are long-crowned.
-The difference between these two types of molars has been described with
-reference to horses, and the change from the older to the modern form
-may be regarded as coming about in the same general way, through a
-series of gradual modifications. In both horse and mammoth the final
-development shows internal enamel extending from the grinding surface
-nearly to the roots. Otherwise, however, there is almost no resemblance,
-for the mammoth tooth is made up of flattened enamel plates, the number
-of which is variable for different species. In the jaws of a very young
-individual these plates may be seen as separate parts. As the tooth continues
-to grow, the plates become cemented together, and when the
-ends of the plate are worn down it may be observed that each consists of
-a layer of enamel surrounding a flat central core of dentine. The type of
-construction is rather more obvious in the mammoth tooth than in that
-of a horse, partly because of the larger size, and partly because of the
-relative simplicity of construction.</p>
-<p>The earlier history of the Proboscidea is not recorded in the rocks
-of North America, for the group was of African origin and its migrations
-did not extend as far as the New World until middle Cenozoic
-times. The mastodons and mammoths were the largest of land animals
-since the Age of Reptiles, but their Old World ancestors were not conspicuous
-because of their bulk. Many of these ancient forms, even in the
-earliest stages, reveal some of the prominent characters that dominate
-the entire group. None of them, however, should be regarded as a miniature
-mammoth or mastodon, for these highly specialized types were perfected
-only at a comparatively recent date, and by a process that works
-very slowly. Among the earlier forms there were also some oddities
-which failed to survive or to produce a successful branch of the stock
-such as the elephants.</p>
-<div class="pb" id="Page_85">85</div>
-<div class="img" id="fig29">
-<img src="images/p28.jpg" alt="" width="800" height="503" />
-<p class="pcap"><span class="sc">Nebraska Mammoth</span>
-<br />(<i>Archidiskodon meridionalis nebrascensis</i>)</p>
-</div>
-<div class="pb" id="Page_86">86</div>
-<p>The earliest known member of the order was <i>Moeritherium</i>, an animal
-of the size of a tapir, living in Egypt during the late Eocene and
-early Oligocene time. At this stage the characteristic specializations leading
-to the mastodons and mammoths were apparent but not far advanced.
-The proboscis was probably much like the flexible snout of modern tapirs,
-for the need of a long trunk had not yet arrived. In upper and lower
-jaws the second pair of incisor teeth were becoming large and prominent.
-The enormous tusks of the mammoths later developed from the enlargement
-of the same pair of upper incisors, and in some of the long-jawed
-mastodonts the lower pair also produced large tusks, though frequently
-the lower tusks were not prominent.</p>
-<p><i>Dinotherium</i> had downward-growing tusks in the lower jaws, none
-in the upper. This genus was fairly common in the Miocene of Europe,
-Asia, and Africa. In the tropics it survived throughout the Pliocene and
-possibly into the Pleistocene. Some of the species acquired the size of
-elephants, but it is apparent that they were not ancestral to any of the
-more progressive types. They are to be regarded rather as an offshoot
-from the main line of descent.</p>
-<p>In 1859 only ten species of the elephant-like mammals were known,
-and all were referred to a single genus. At the present time eleven
-genera appear to be well founded, and the number of recognized species
-has reached a hundred, if it has not already passed that figure. New
-discoveries are expected to add to the existing total. With this mass of
-material before us we note certain definite trends among the more progressive
-types. The increasing weight was accompanied by the development
-of strong, upright limbs in which the bones have a columnar
-position instead of the angular assembly which prevails among most
-of the mammals. As the tusks increased in size there was a shortening of
-both skull and neck to bring the weight closer to the point of support.
-The front teeth disappeared except the second pair of upper incisors
-which remain as tusks in the modern elephant. The cheek teeth present
-in the shortened jaws of the mammoth were reduced to one pair at a time
-in the upper set and another pair below. From a simple, low-crowned
-origin these grinding teeth developed into the more successful high-crowned
-pattern with numerous plates of enamel inside. A prehensile
-upper lip acquired the length and usefulness of the elephants trunk.</p>
-<div class="pb" id="Page_87">87</div>
-<div class="img" id="fig30">
-<img src="images/p29.jpg" alt="" width="700" height="646" />
-<p class="pcap"><span class="sc">Rancho la Brea Fossils</span></p>
-<p class="pcapc">One of the most unusual of the many animals that have been taken from
-the tar pits is the large ground sloth, seen at the left in this group. Such sloths
-were very abundant during Pleistocene time, and some may have lived up to a
-few thousand years ago. Archaeologists have found indications that these creatures
-may have been hunted by cave-dwelling peoples of the American Southwest.</p>
-<p class="pcapc">Other skeletons include the saber-tooth tiger, characterized by the long curved
-upper canine teeth which undoubtedly were used for stabbing and slashing, and
-the dire wolf, the smaller of the two which are facing the sloth. The artist&rsquo;s
-reconstruction of this scene also shows the great vulture, Teratornis, which is the
-largest known bird of flight.</p>
-</div>
-<div class="pb" id="Page_88">88</div>
-<p>Over-specialization in the production of tusks appears to have been
-the principal factor in the downfall of the mammoths. The large size of
-the animals and the difficulties of finding sufficient food to sustain life
-must have been a serious handicap at times, but their ability and inclination
-to travel over long distances enabled some of them to find tolerable
-living conditions until the end of the Glacial Period. They are now extinct
-and the nearest living relatives are the elephants, somewhat reduced
-in size of tusks and body but otherwise very similar.</p>
-<p>There are many other tribes of mammals whose ancient history is
-partially known though broken by periods of time for which there is no
-fossil evidence. All have undergone changes in which various forms and
-degrees of specialization are featured; this general process is best revealed
-by the horses and elephant-like animals which have left a clearer
-record. For other groups the story would differ but little except as to
-names and specific details.</p>
-<h3 id="c28">THE RANCHO LA BREA FOSSIL PITS</h3>
-<p>The La Brea tar pits, as they are often called, provide a remarkable
-record of Pleistocene life in southwestern North America. Scattered
-over an area of about thirty acres just off Wilshire Boulevard in Los
-Angeles, these bone deposits were known, as far back as 1875, to contain
-the remains of prehistoric animals. It was not until 1905, however, that
-their value was recognized by paleontologists. In that year the University
-of California began an investigation, and excavations were carried on
-at intervals by various institutions during the next ten years. A great
-deal of material was acquired by the Los Angeles Museum of History,
-Science, and Art, where many skeletons, skulls, and other interesting
-specimens have been placed on exhibition.</p>
-<p>The pits have the form of small craters formed by the seeping of
-oil from the underlying rocks. The seeps appear to have been active during
-part of the Pleistocene period but apparently not at the beginning.
-The oil is rich in asphalt which has served as a preservative for the bones,
-and owing to its sticky properties has been an effective animal trap for
-thousands of years.</p>
-<p>The fossil beds at present are of oil-soaked earth and sand. In past
-times there must have been a greater percentage of oil, often concealed
-by a layer of dust or pools of water. The large number of carnivorous
-animals found in the deposits suggests that they were attracted by the
-cries and struggles of creatures wandering carelessly into the asphalt and
-serving as live bait to keep the traps in continuous operation.</p>
-<p>Animals found there include many species still living in the locality,
-some that have migrated to other territory, and a large number that have
-become extinct. Among the latter may be mentioned species that differ
-but slightly from living relatives, others that have left no descendants.
-<span class="pb" id="Page_89">89</span>
-Horses, bison, and wolves, though extinct species, were of relatively modern
-types. On the other hand the large sloths and saber-tooth cats seem
-rather out of place. True cats are represented by the mountain-lion,
-bob-cat, and a species of lion which is nearly one-fourth larger than any
-of the great cats of the Old World. A long-legged camel, with a height
-of approximately eight feet to the top of the head, was among the native
-animals of the district. Skunks, weasels, badgers, squirrels, rabbits, bear,
-deer, and antelope were more or less abundant.</p>
-<p>The La Brea group exhibited by the Denver Museum of Natural
-History includes the following species: horse (<i>Equus occidentalis</i>), bison
-(<i>Bison antiquus</i>), wolf (<i>Aenocyon dirus</i>), saber tooth (<i>Smilodon californicus</i>),
-sloth (<i>Mylodon harlani</i>). Horses had entirely disappeared
-from the North American continent by the time the first white man arrived.
-<i>Equus occidentalis</i> was one of the several species living during the
-Pleistocene period, this one apparently being restricted to California and
-perhaps adjacent states. <i>Bison antiquus</i> was slightly larger than the
-plains bison of recent times and had it horns set at a characteristic different
-angle. The species was first described from Kentucky and appears
-to have had a wide distribution.</p>
-<p>The wolves in this group are about the size of timber wolves, but
-have heavier skulls with less brain capacity, massive teeth especially
-adapted to biting and crushing large bones, and limbs of rather light
-construction. They probably assembled in packs where meat was abundant
-and, hunting in this fashion, were able to attack and overcome the
-larger ungulates and edentates. To most visitors the large ground sloth is
-the most interesting animal of the group. This edentate animal is shown
-at the edge of the pool with one foot stuck in the &ldquo;tar.&rdquo;</p>
-<p>The edentates are a group of primitive animals with very simple
-teeth, if any. Teeth are usually lacking in the front part of the mouth,
-sometimes entirely absent, as among anteaters. Better known living representatives
-of the group are the tree sloths, armadillos, and anteaters of
-South America. Ground-sloths were prominent among South American
-mammals during much of Cenozoic time. During Pliocene and Miocene
-time there was a marked tendency to large size, and it was principally
-during these two periods that they appeared in the United States area.</p>
-<p><i>Mylodon</i> was one of the larger North American ground-sloths. Its
-teeth, without the protective enamel which is present among higher
-mammals, are restricted to the cheek region, and have the form of simple
-pegs; instead of being specialized they stand close to the extreme of generalization.
-The construction of the entire skeleton is massive, suggesting
-great strength with slow movements. The hands are well developed,
-provided with stout claws, and must have served the creature well as
-protection against attacks by predatory neighbors. We have some idea
-as to what caused the extermination of the ground-sloths in this particular
-region, but the complete disappearance of such a large and widely distributed
-group at the close of the Pleistocene period is a mystery that may
-never be explained.</p>
-<div class="pb" id="Page_90">90</div>
-<div class="img" id="fig31">
-<img src="images/p30.jpg" alt="" width="734" height="600" />
-<p class="pcap"><span class="sc">The Folsom, New Mexico, Bison</span> (<i>Bison taylori</i>)</p>
-</div>
-<div class="pb" id="Page_91">91</div>
-<p>The saber-tooth cat, sometimes referred to as a tiger, was specialized
-as a meat eater though hardly as a hunting animal. In the La Brea
-region its principal food was probably the flesh of the sluggish ground-sloths.
-The size was equal to that of the African lion, with hind limbs
-slightly longer and the front legs more powerfully developed. The most
-remarkable characteristic is to be found in the development of the upper
-canine teeth and modifications of the skull which were necessary to enable
-the animal to use these teeth as weapons.</p>
-<p>In order to make the &ldquo;sabers&rdquo; effective it was necessary to get the
-lower jaws out of the way, and this was provided for in an unusual type
-of hinge which enabled the mouth to open wider than is possible in the
-case of the less specialized carnivores. Judging by all the structural features
-of the skeleton, <i>Smilodon</i> could not have lived well on small animals,
-for it was not equipped to capture that kind of prey. It is evident
-that large mammals were preferred, and that the method of attack was
-to spring upon the victim and cling there with the powerfully developed
-fore limbs until the kill was completed by stabbing into a vulnerable spot.
-That the position of the large sabers near the front of the mouth interfered
-with normal feeding, is a reasonable conclusion. There are also
-anatomical features which lead to the belief that this carnivore was a
-blood sucker, perhaps more than it was meat-eater.</p>
-<p>If most of these conclusions are correct we have here another case
-of over-specialization and a possible explanation of the extinction of two
-species. Such evidence as we have is far from conclusive, for there is no
-proof that Rancho La Brea was the last stand of either the saber-tooth
-or the ground-sloth. Both races were widely distributed and their living
-conditions could not have been exactly duplicated in other localities. It
-has been suggested, however, that <i>Smilodon</i> ate the last of <i>Mylodon</i>, and
-starved soon afterward because it had become unable to partake of other
-foods. The conjecture is offered for what it is worth, together with the
-facts on which the story has been based.</p>
-<p>The geological record for Pleistocene time is not as complete as one
-might imagine. Numerous localities have produced representative fossils
-but the yield is rarely large enough to solve many of the riddles which
-are constantly arising as investigation proceeds. Aside from those areas
-which bordered the retreating ice cap and where living conditions were
-far from favorable, the sedimentary deposits of this period are not continuous
-over large areas. Many Pleistocene fossils are found in stream
-channel beds which are always subject to removal by subsequent floods.</p>
-<div class="pb" id="Page_92">92</div>
-<div class="img" id="fig32">
-<img src="images/p31.jpg" alt="" width="600" height="757" />
-<p class="pcap"><span class="sc">Early Man in North America</span></p>
-<p class="pcapc">There is abundant evidence to indicate that the great elephants of Pleistocene
-time were hunted by primitive Americans whose only weapons were darts or
-spears tipped with points of stone. A skull and the lower jaws of several mammoths
-are shown here.</p>
-</div>
-<div class="pb" id="Page_93">93</div>
-<p>Isolated patches of fossil-bearing sediments frequently record the migration
-of animals in unmistakable terms, but the details of the wanderings
-and the conditions encountered in the newly established habitats
-are often left in doubt. To correlate the facts revealed at one locality
-with findings at other places and, if possible, to date all prehistoric
-events with a greater degree of accuracy are among the major tasks of
-current investigations.</p>
-<h2 id="c29"><span class="small">THE AGE OF MAN</span></h2>
-<p>The Pleistocene or &ldquo;Ice Age,&rdquo; and the Recent period in which we
-are living at the present moment are not sharply separated by any event
-readily recognized or dated, and the two combined are of very short
-duration as compared with other periods more clearly established by the
-passing of centuries. Together they comprise the Age of Man as commonly
-recognized, with about a million years representing the Pleistocene
-period, some ten to twenty thousand years the Recent. When geologists
-of the nineteenth century suggested that the coming of man should
-be regarded as the beginning of a new era, the name Psychozoic was
-proposed, and to some extent this term has been applied to the present
-period. More in keeping with other period names is Holocene, meaning
-<i>entirely recent</i>. Common usage, however, applies the simple term Recent
-to this unfinished chapter which is also without a clear-cut beginning.</p>
-<p>Zoologically, man is merely one of the creatures that arrived in the
-course of time, along with other mammals. Just when he arrived and
-how he looked at the time of his coming cannot be determined from a
-study of fossils. Perhaps it is of no importance. There is nothing to indicate
-his existence before the Cenozoic, no completely satisfactory proof
-of existence before the Pleistocene period. As with other inhabitants of
-the earth, it is probable that he became prominent only after a great deal
-of competition with other creatures which kept his ancestors submerged
-for thousands of years. The Ice Age, with its check upon the progress
-of competing animals, undoubtedly gave him an advantage. His superior
-mentality enabled him to overcome adversity by methods not available
-to other mammals; his inventive and mechanical genius must have been
-greatly strengthened by his experience during this interval.</p>
-<p>At about this point, where prehistory begins to merge into history,
-<span class="pb" id="Page_94">94</span>
-the geologist and paleontologist must let other interpreters carry on.
-Archeologists and anthropologists take up the work, and through their
-efforts many details have been added to our knowledge of the human
-race. The study of biology, which is the science of life, has provided an
-instructive viewpoint that enables us to see ourselves against the vast
-background built up by investigations into the nature of the earth and its
-ancient inhabitants. This science deals with living creatures as <i>organisms</i>&mdash;plants
-and animals so organized as to be capable of existence only in an
-environment which provides exact life requirements.</p>
-<p>The Age of Man has been variously characterized as an age of soul,
-of higher intelligence, of culture, and finally, of civilization, freedom and
-democracy. The &ldquo;crowning glory&rdquo; of the organic world is pictured in
-history as a creature who has busied himself for thousands of years with
-the building up and tearing down of civilizations. Prehistory reveals this
-habit as something unique in the human character, for there is no other
-organism that has specialized so persistently in the creation of its own environment,
-no other that has had the combined power and talent to produce
-so much change.</p>
-<p>More than anything else, the prehistoric record is a lesson in adaptation,
-which in its broadest sense means fitness for life under particular
-conditions, and always subject to organic law. Man&rsquo;s efforts to bring about
-an adjustment between himself and his civilization have centered largely
-on the method of forcing himself into the mold that happens to be present,
-one pattern today, another tomorrow. No creature of the past has
-had to adapt itself to anything so radically new or so thoroughly revolutionary.
-The vital problem now is whether this man-made environment
-will prove helpful or disastrous.</p>
-<p>Though one of its names is &ldquo;culture,&rdquo; it has grown sporadically and
-unevenly, with little evidence of the cultivation that is implied and required.
-Parts have been expanded to extraordinary proportions while
-others equally essential have been retarded in their growth. A more intelligent
-handling of this environment factor seems to be possible, and
-the present mania for &ldquo;organization&rdquo; may become tempered with an
-awakening consciousness of organic requirements where organism and
-environment are involved. Once we grasp the idea that &ldquo;culture&rdquo; results
-from man&rsquo;s effort to improve his living, by putting into his environment
-something that was not there before&mdash;then, surely, this history of a
-billion years of living, and as many &ldquo;ways of life,&rdquo; should teach us
-something we ought to know as we go into an all-out endeavor to teach
-a whole world how to obtain a one-and-only way.</p>
-<p>We may stand at the beginning of an era for which an appropriate
-<span class="pb" id="Page_95">95</span>
-name has not yet been suggested. Civilization, on the other hand, may
-provide only a minor epoch to be added in some remote time to the story
-of fossils.</p>
-<h2 id="c30"><span class="small">SUPPLEMENTARY READING</span></h2>
-<p>The literature pertaining to fossils is widely scattered and usually
-too technical for the layman. It is better to use the resources of the nearest
-library than to feel that a specified list of books is necessary.</p>
-<p>Any textbook on geology, zoology, or botany will provide helpful
-information. Most books of this type will be found interesting and readable
-if used to solve definite problems suggested by the student&rsquo;s immediate
-curiosity. Very few can be read from beginning to end without a
-great deal of effort and discouragement.</p>
-<p>The following have been prominent among the books consulted by
-the author:</p>
-<p><i>Textbook of Geology</i>; by Pirsson and Schuchert. This work has
-undergone several revisions and currently appears in two volumes:
-<i>Physical Geology</i> by Longwell, Knopf, and Flint; <i>Historical Geology</i> by
-C. O. Dunbar. Published by John Wiley &amp; Sons. (Historical geology
-covers the entire range of prehistoric life&mdash;plant, invertebrate, and vertebrate.)</p>
-<p><i>Historical Geology</i> (The Geologic History of North America); by
-Russell C. Hussey. Published by McGraw-Hill. Concise, interesting, and
-informative.</p>
-<p><i>Geology and Natural Resources of Colorado</i>; by R. D. George.
-Published by the University of Colorado. Contains an excellent summary
-of the historical geology and sedimentary formations of Colorado.</p>
-<p><i>Vertebrate Paleontology</i>; by Alfred Sherwood Romer. Published
-by the University of Chicago Press. This is one of the most comprehensive
-and up-to-date treatments of the subject for students desiring to go
-beyond the elementary stage.</p>
-<p><i>A History of Land Mammals in the Western Hemisphere</i>; by William
-Berryman Scott. Published by The Macmillan Company. This
-well-known account of living and extinct mammals is one of the favorites
-among students.</p>
-<p><i>The Age of Mammals</i>; by Henry Fairfield Osborn. A classic in this
-field of literature, but for advanced reading. The book is now out of
-print.</p>
-<div class="pb" id="Page_96">96</div>
-<p><i>The Dinosaur Book</i>; by Edwin H. Colbert. Published by the American
-Museum of Natural History, New York. An illustrated story of
-amphibian and reptilian evolution.</p>
-<p><i>Down to Earth</i>; by Carey Croneis and William C. Krumbein. Published
-by the University of Chicago Press. An excellent popularization
-of the earth sciences&mdash;geology and paleontology.</p>
-<p><i>Lexicon of Geologic Names of the United States</i>; compiled by
-M. Grace Wilmarth. Bulletin 896 (in two parts) of the United States
-Geological Survey. A rich source of information concerning the age,
-character, and distribution of geologic formations, with numerous references
-to fossil-bearing beds.</p>
-<p><i>Bibliography of North American Geology</i> (including paleontology);
-various bulletins of the United States Geological Survey. Where library
-facilities provide access to the technical literature of museums, universities,
-and scientific societies, this is a valuable aid in locating publications
-dealing with original work in paleontology. Bulletins 746 and 747
-cover the years between 1785 and 1918; Bul. 823 (1918-1928); Bul.
-937 (1929-1939); Bul. 938 (1940-1941); Bul. 949 (1942-1943); Bul.
-952 (1944-1945); Bul. 958 (1940-1947); Bul. 968 (1948); Bul 977
-(1949). Preparation is a continuous process with recent bulletins appearing
-at one or two year intervals.</p>
-<p><i>Ancient Man in North America and Prehistoric Indians of the
-Southwest</i>; by H. M. Wormington. Published by Denver Museum of
-Natural History, City Park, Denver 6, Colorado. Both volumes contain
-authentic and up-to-date accounts of early American cultures.</p>
-<h3 id="c31">MAPS</h3>
-<p><i>Geologic Maps.</i> United States Geological Survey: map of the
-United States (1932); map of Colorado (1935). Geologic maps of a few
-other states are available; information regarding these may be obtained
-from state universities or state geological surveys.</p>
-<blockquote>
-<p>Note: Bulletins of the U.S.G.S. are purchasable from the Superintendent
-of Documents, Washington, D. C. Maps are sold by the
-Director of the Geological Survey, Washington, D. C.</p>
-</blockquote>
-<h2>Transcriber&rsquo;s Notes</h2>
-<ul>
-<li>Silently corrected a few typos</li>
-<li>Restored one accidental omission in the Table of Illustrations</li>
-<li>Retained publication information from the printed edition: this eBook is public-domain in the country of publication.</li>
-<li>In the text versions only, text in italics is delimited by _underscores_.</li>
-</ul>
-
-
-
-
-
-
-
-<pre>
-
-
-
-
-
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-The Project Gutenberg EBook of Fossils: A Story of the Rocks and Their
-Record of Prehistoric Life, by Harvey C. Markman
-
-This eBook is for the use of anyone anywhere in the United States and most
-other parts of the world at no cost and with almost no restrictions
-whatsoever.  You may copy it, give it away or re-use it under the terms of
-the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org.  If you are not located in the United States, you'll have
-to check the laws of the country where you are located before using this ebook.
-
-Title: Fossils: A Story of the Rocks and Their Record of Prehistoric Life
-       Denver Museum of Natural History, Popular Series No. 3
-
-Author: Harvey C. Markman
-
-Illustrator: Mary Chilton Gray
-
-Release Date: July 22, 2017 [EBook #55168]
-
-Language: English
-
-Character set encoding: ASCII
-
-*** START OF THIS PROJECT GUTENBERG EBOOK FOSSILS: A STORY OF THE ROCKS ***
-
-
-
-
-Produced by Stephen Hutcheson and the Online Distributed
-Proofreading Team at http://www.pgdp.net
-
-
-
-
-
-
-
-
-
-                                FOSSILS
-                          A Story of the Rocks
-                                  and
-                    Their Record of Prehistoric Life
-
-
-                          By Harvey C. Markman
-                  Curator of Geology and Paleontology
-
-                       Cover Design and Murals by
-                           Mary Chilton Gray
-
-
-                            DENVER MUSEUM OF
-                            NATURAL HISTORY
-
-                          Popular Series No. 3
-                        Alfred M. Bailey, Editor
-
-                        Third Edition, Reprinted
-                            October 1, 1954
-
-    [Illustration: At Work on a Dinosaur Skeleton]
-
-
-
-
-                                CONTENTS
-
-
-                                                                     Page
-  Introduction                                                          5
-  The Prehistoric Record                                                5
-      Varieties of Fossils                                              8
-      Fossilization                                                     9
-      Floras and Faunas                                                13
-      Formations                                                       16
-      Geological Time                                                  18
-      Explanation of the Time Chart                                    23
-      The Geological Section                                           25
-  Before the Age of Reptiles                                           31
-      The Pre-Cambrian Complex                                         31
-      Cambrian Life                                                    33
-      The Ordovician Record                                            35
-      Silurian Events                                                  36
-      Devonian Progress                                                37
-      Carboniferous Forests                                            40
-      Permian Hardships                                                43
-  The Age of Reptiles                                                  47
-      Dinosaurs                                                        48
-      Plant Life and Climate                                           56
-      Coal and Fossil Footprints                                       57
-      Mesozoic Invertebrates                                           58
-      Extinct Birds                                                    60
-      Ancestors of the Mammals                                         61
-  The Age of Mammals                                                   64
-      Prehistoric Horses                                               70
-      Mastodons and Mammoths                                           80
-      The Rancho La Brea Fossil Pits                                   88
-  The Age of Man                                                       93
-  Supplementary Reading                                                95
-
-
-
-
-                         LIST OF ILLUSTRATIONS
-
-
-                                                                     Page
-  At Work on a Dinosaur Skeleton                                        2
-  Fossil Bones in Rock Formation                                        7
-  Insect Fossils                                                       10
-  Restoration of Rhinoceros                                            12
-  Dinosaur Tracks                                                      17
-  Time Chart                                                           22
-  Geological Section Showing Positions of Formations                   26
-  Marine Beds of the Benton Formation                                  28
-  Plesiosaur Bones in Place                                            28
-  Invertebrate Fossils                                                 34
-  Modernized Fishes                                                    38
-  Prehistoric Plants                                                   41
-  Marine Reptiles                                                      46
-  Diplodocus                                                           49
-  Trachodon                                                            51
-  Stegosaur                                                            52
-  Sea Turtle                                                           54
-  Murals, Hall of Mammals                                              63
-  Uintatheres and Contemporary Life                                    65
-  Moropus                                                              67
-  Titanotheres                                                         69
-  Oligocene Mammals                                                    71
-  Pleistocene Horse                                                    73
-  Structure of Molar Teeth                                             75
-  Grazing Type of Molar Teeth                                          77
-  American Mastodon                                                    79
-  Long-Jawed Mastodont                                                 81
-  Molar Tooth of Mammoth                                               83
-  Nebraska Mammoth                                                     85
-  Rancho La Brea Fossils                                               87
-  Folsom Bison                                                         90
-  Man and Mammoth                                                      92
-
-
-
-
-                                FOSSILS
-
-
-
-
-                              INTRODUCTION
-
-
-In the recent growth of knowledge there has been rapid progress in two
-directions. The commercial exploitation of natural resources, being
-fundamental to modern civilization, attracts a liberal share of the
-talents and energies of workers trained for the industrial professions.
-A second trend has specialized in the further development of the
-sciences which are characteristic of our time. Such activities, in the
-natural history field, deal largely with the refinements of exact
-definition, nomenclature and classification, all of which means little
-or nothing to the layman who is otherwise engaged.
-
-For the latter, however, there is a quality of interest which may be
-described as a wholesome curiosity about what has happened, how it
-happened, how we know it happened, and what it may signify to one who is
-neither industrialist nor scientist. This booklet is intended for the
-many who feel that there is more to be obtained from a natural history
-museum than an occasional glimpse of a bewildering "marvel." In addition
-to being a guide to fossil exhibits it supplies parts of a great story
-which specimens alone can not relate.
-
-
-
-
-                         THE PREHISTORIC RECORD
-
-
-All that is known of the extinct plants and animals which inhabited the
-earth before man began the practice of recording his observations has
-been obtained from a study of the rocks. The few possible exceptions to
-this rule, in which animal and plant remains have been preserved by
-freezing or drying, are so unusual as to be hardly worth mentioning.
-
-Explanation of this is that plant and animal tissues quickly decay under
-ordinary conditions when life ceases. Unless protected from destructive
-agencies which are especially active at the surface of the ground, even
-the heavier bones of animals and the large trunks of fallen trees will
-soon crumble into shapeless masses. The usual method employed by nature
-to prepare a fossil specimen is so closely related to the natural
-process of rock making that a little knowledge of that subject will be
-necessary in order to know what fossils are and how they are preserved
-for so long a time.
-
-It should be understood first that a fossil is some record, commonly
-preserved in rock, of a kind of plant or animal which no longer exists
-as a living type. This, at least, is the ordinary sense of the word and
-more elaborate definitions are of small service to anyone. It may be
-necessary to add, however, that all things which have lived at any time
-are regarded as either plants or animals.
-
-Nature's way of producing rocks and fossils remains a mystery to many of
-us because we are so wrapped up with the importance of finding names for
-things and materials that we frequently neglect the consideration of
-sources and histories. Everyone knows a rock when he sees it in a large
-mass, but when he looks at sand, mud, dust, or soil, he seldom thinks of
-it as related in any way to rocks. Although the difference is almost
-entirely a matter of size, our use of words makes it seem unreasonable
-to speak of the finer particles as rock.
-
-There can be no reality or meaning in the natural record for an
-individual who has failed to observe a few simple facts which involve
-changes going on in all parts of the world at the present time. With
-regard to rocks, it is supposed that what happens in our day also
-occurred under like circumstances ages ago. Anyone wishing to do so may
-see for himself that rock masses break down wherever they are exposed to
-the elements, that the larger pieces are reduced to smaller fragments,
-and that the final product is sand or dust.
-
-He may also note that this finely ground material is being moved and
-sorted, by rain, wind, and streams, transported to lower levels and
-accumulated in great quantities wherever it finds a resting place. Along
-with it go sticks and leaves, bugs, shells, bones and carcasses of
-animals, some of which in time may become fossils. In large lakes and
-seas there is a steady distribution of such materials over broad areas,
-yesterday's accumulation of sediments being buried by the contributions
-of today, the most recent of the settlings always resting upon older
-ones until something happens to disturb that arrangement.
-
-Not so readily observed are other parts of the process, such as the
-consolidation of sands and muds into the firm sandstones and shales
-which we again recognize as rocks. Much of this requires more than the
-few score years of a human lifetime for its accomplishment, but many of
-us have seen muds become so solidified, by merely drying, that they
-could hardly be distinguished from prehistoric shales. It is to be noted
-also that some ancient fossils come from sandstones which are scarcely
-more rock-like than the loose sands of an ocean beach. Thus we learn
-that firm consolidation of rock-making materials is not always a sign of
-great antiquity, and that hardness of rock is not always essential to
-the preservation of imbedded plants and animals.
-
-    [Illustration: Rhinocerous Bones as Found in the Rock
-
-    This exhibit was taken from the famous fossil quarry at Agate,
-    Nebraska. The fossilized remains are still partially imbedded in the
-    sandstone which preserved them for millions of years.]
-
-The rocks themselves must explain the many things which have happened
-during the course of millions of years, and this they do remarkably well
-when carefully studied, for many of the factors involved in their
-histories leave characteristic marks. Changing climates, the draining of
-seas, the uplifting of mountain ranges, all have ways of registering
-their occurrence which are as convincing and reliable as anything ever
-written by man. Piece by piece the story has been patched together
-through the efforts of thousands of investigators. Parts of the
-narrative remain buried at inaccessible depths, and whole chapters, no
-doubt, have been destroyed by the same forces that composed this
-tremendous record of prehistoric times.
-
-
-VARIETIES OF FOSSILS
-
-It would be a serious mistake to regard nature as divided into a number
-of distinct and independent schools of fossil making, each refusing to
-use the methods and devices of another. There are, however, certain
-features which stand out so prominently that a little classification
-becomes helpful. While this procedure brings out differences it should
-be understood that processes actually work together, several of them
-usually being involved in the production of any individual specimen.
-
-(1) Impressions of animals and plants, or parts of these, are frequently
-left in soft sand or mud which later becomes converted into more durable
-rock. This type of fossils is represented by animal foot-prints and the
-imprints of leaves, flowers, insects, and like objects which may be
-mingled with the finely ground materials of the common sedimentary
-rocks.
-
-(2) Parts of plants and animals may be gradually replaced by mineral
-matter with little or no change from original form and texture. Fossils
-of this class are said to be petrified or turned to stone. They are also
-known as replacements. The fleshy parts of animals do not petrify.
-
-(3) Many animals among the invertebrates use mineral substances for
-protective or supporting structures. Small plants of various kinds
-follow a similar practice. These structures, being produced in stony
-materials, are readily converted into fossils. The shells of mollusks
-are the best known illustrations in this field, and all that is required
-for a shell to become a fossil is the extinction of the species of
-animal that produced it. Fossils of this type are extremely abundant.
-
-(4) Preservative substances other than those which produce common rocks
-may be mentioned among fossil-making possibilities. Bones are known to
-have been preserved in asphalt, and insects in resins, but such cases
-are few in comparison with the products of other methods.
-
-(5) In rare instances there has been preservation of extinct creatures
-by the process of drying or by refrigeration. Occasional mummies are
-found with shriveled flesh and skin still in place, but better
-preservation of all tissues occurs when the temperature is quickly
-reduced below freezing point and held there without interruption. This
-can happen only in the colder parts of the earth and is always subject
-to climatic change. The effect of drying also may be undone at any time
-by a slight increase in the amount of moisture.
-
-(6) Coal beds often produce fossils of an unusual sort. In the formation
-of coal, plant material gradually loses some of its more perishable
-substances but retains carbon which has better lasting qualities and
-slowly accumulates to produce the seams and beds that are mined. In the
-early stages of the process the original vegetation undergoes little
-change in appearance but eventually practically all of its character is
-lost. Many fossil leaves are found as thin layers of carbon, bedded in
-the clays which are commonly associated with coal deposits.
-
-(7) Concretions, which are hardened lumps of mineral substances
-occurring commonly in sandstones and shales, are often mistaken for
-fossils because of their peculiar shapes. However, there are localities
-in which the mineral solutions have been concentrated and deposited
-around shells, leaves, seeds, or similar objects, thus producing an
-abundance of fossils which may be obtained by opening the concretions.
-Fossils of this type are well known from Mazon Creek and other districts
-in Illinois, Kansas, Colorado, and elsewhere.
-
-
-FOSSILIZATION
-
-Footprints need little explanation other than a consideration of the
-factors which make it possible for them to be preserved. The sand or mud
-must be neither too soft nor too hard to take the form of the foot and
-retain its shape when the foot is withdrawn. Then in some manner the
-impression must be protected while the rock-making process goes on. When
-such protection is obtained it is usually in the form of more mud and
-sand, deposited over the surface which received the impression. At a
-later time the covering may be separated from the lower part of the
-deposit, which serves as a mold, and if the separation be accomplished
-successfully a natural cast of the foot will be obtained as well as the
-mold in which it was produced. Since conditions for perfect work are not
-always present in a laboratory of this kind, it is not surprising that
-fossil footprints are very rare considering the number and variety of
-tracks left by wandering animals.
-
-Impressions of leaves are explained in much the same way except that the
-leaf remains under its protective covering until it decays. Similar
-impressions may be obtained from the bodies of delicate invertebrate
-animals but they are seldom preserved because of the softness of the
-tissues. The smaller fishes provide much better material for the
-production of fossils according to this method. While the fish is being
-flattened by the weight of surrounding sediments, scales, fins, and soft
-bones retain their positions and provide the necessary resistance to
-leave an impression of the body form when the flesh is gone.
-
-    [Illustration: Insect Fossils (enlarged)
-
-    Fine specimens of this type are obtained from an old lake bed at
-    Florissant, Colorado.]
-
-The larger and more spectacular fossils, such as skeletons, skulls, and
-detached bones are nearly always of the replacement type. Replacement of
-plant and animal substances by mineral matter is a slow process and in
-younger fossils the change is rarely completed, some of the original
-material being present in a partially altered condition or not modified
-at all. Since air does not often carry the necessary materials and
-provide other essential conditions, replacement may be regarded as
-something which happens underground or in water. It is perhaps best
-explained in connection with limestones, because calcite or "lime" is
-frequently the replacing substance although other minerals, especially
-quartz, may serve the purpose.
-
-Besides converting bony or woody objects into rock substance, mineral
-replacements may assist in the production and preservation of fossils in
-another manner. It often results in the filing of cavities with some
-rock-making substance which retards destruction through crushing or
-other injury. In many cases, so-called fossil shells are not shells at
-all; instead, they are merely a stony filling which was once surrounded
-by shell substance. In other instances the original shell remains as it
-was during the life of its former occupant, preservation of the shell
-being due largely to the substitution of a mineral filler for the soft
-animal tissues once present.
-
-Limestone comes into existence through a more elaborate process than
-that which produces sandstone and shales. It is one of the three types
-of common rocks, known collectively as the sedimentaries, in which
-fossils are found. It differs from sandstones and shales, however, in
-that much of its substance has been dissolved in water instead of being
-transported in the form of finely ground rock particles. Lime occurs in
-many varieties of rock which are exposed to the wear and tear of the
-elements throughout the world. Slowly but more or less continuously it
-is taken from this source by ground and surface waters coming in contact
-with it. Particularly active is carbonated water, moving underground
-through pores and crevices.
-
-This underground circulation of mineral matter in a dissolved condition
-explains the occurrence of fossils in land areas which have not
-necessarily been submerged during any great length of time, for it is
-well known that plant and animal remains are not invariably washed into
-lakes or seas, and that all sedimentary deposits have not been built up
-in large bodies of water. Here we are dealing with what is known as the
-continental type of sedimentation and such fossils as dinosaurs,
-mastodons, three-toed horses, and other former inhabitants of land
-areas.
-
-In order to become properly fossilized, certain conditions are
-absolutely necessary, and only a small percentage of the once-living
-multitude secures the required treatment. There must be present, soon
-after death, some protection from the activities of the carnivorous
-birds and beasts that would separate and scatter the parts of a carcass,
-also from the smaller gnawing animals that would continue the
-destruction, and finally from wind, sun, rain, frost, and bacterial and
-chemical activities which in the course of only a few years would remove
-everything but possibly a few scraps of tooth enamel, which is the
-hardest of animal tissues.
-
-A slight covering of earth substance in any form serves to check the
-disintegration, and this may be acquired in several ways. Animals that
-perish in bogs or quicksands are soon covered over; in many localities
-wind-blown dust and sand do the work; and flooded river valleys provide
-an abundance of mud for the necessary burial of others. Even
-underground, the decay of soft tissues is too rapid to permit of
-replacement by mineral substance in a manner that would reproduce form
-and texture. Skin and flesh are almost invariably lost, although in a
-few instances the thick scaly hides of dinosaurs are known to have
-produced natural molds and casts by the method explained in connection
-with footprints and other impressions.
-
-    [Illustration: The skeleton of this prehistoric American rhinoceros
-    is mounted in a "half shell" which was modeled over the bones to
-    show the form of the living animal. The artist's reconstruction
-    appears in the painting above the fossil exhibit.]
-
-With regard to the more durable tissues found in the teeth, bones, and
-shells of animals, or the woody parts of plants, the case is different.
-These parts become firmly imbedded in the ground, but moisture still has
-access, and it begins to work immediately; for all water moving
-underground finds soluble substances which it picks up and carries with
-it wherever it goes, and much of the load consists of mineral matter
-which may be unloaded again when the necessary conditions are found.
-
-Mineral-laden waters will drop one kind of substance to take up another
-which dissolves more readily, and this happens sooner or later when a
-buried bone or log is encountered. Complications of various sorts enter
-into the process, but the final outcome frequently is a complete change
-from one chemical composition to another which is more enduring, the
-transformation being brought about so gradually and thoroughly that in
-many fossils the inner structure of the original tissue is as accurately
-reproduced as the fine detail of surface features.
-
-Converted into stone, however, the result is still far from permanent.
-While yet underground the fossil is subjected to distortion and breakage
-due to earth movements which bend and dislocate the rock deposits. What
-causes these upheavals and depressions of the earth's surface remains
-the subject of much discussion, but that they have occurred on a large
-scale and continue to occur is clearly evident. At higher altitudes the
-surface rocks and fossils are exposed to a larger variety of destructive
-activities than at lower levels where protective coverings are more
-likely to be provided and retained. Once stripped of that protection
-there is little chance for a fossil to survive. Beyond a doubt there are
-many thousands of tons of prehistoric remains damaged or destroyed each
-year, by weather and stream erosion.
-
-
-FLORAS AND FAUNAS
-
-As the various types of sediments continue to accumulate on land and in
-water they produce deposits of sandstones, claystones, and limestones
-which in time may acquire great thickness and cover wide areas of sea
-floor, or continental surface. Usually there is more or less mixing of
-sediments resulting in sandy limestones, limy clays, and other
-combinations. Quite commonly, however, the types remain fairly pure but
-become arranged in layers which alternate from one kind of material to
-another. At all times the character of the deposit will depend upon the
-nature of the rocks which supply the materials, and any fossils that may
-be produced will consist of such plants and animals as live and die
-during the time the rock is in the making.
-
-Some of the rock layers will be rich in plant and animal remains, others
-quite barren, the difference being due partly to conditions influencing
-the life of the region. In addition, the character and amount of
-rock-making materials at the time may be favorable or unfavorable to the
-preservation of fossils. Seas, lakes, and valleys may at any time be
-drained, or enlarged and deepened, by changes in the elevation of
-underlying rocks. The amount and variety of mineral substances dissolved
-in the waters of a region not only affect the character of rock deposits
-but also the plants and animals living in the water. Some of these
-chemical solutions provide cementing materials which bind together the
-grains of sands and mud; others have a detrimental effect upon cementing
-material previously deposited, and so construction and destruction go on
-continuously, more or less hand in hand, to produce complicated and
-often puzzling results.
-
-A little more salt, or a little less of it, may change completely the
-variety of life inhabiting a body of water. A slight change in the depth
-of the water often accomplishes the same thing, for plants and animals
-are so delicately adjusted to their environments that conditions fatal
-to one race of creatures may provide the exact life requirement of
-another. This is a matter of practical knowledge which is being used
-today in the cultivation of plants and animals for market purposes. It
-is being demonstrated continuously, also, upon living subjects in
-experimental laboratories throughout the world; and, in a bigger way,
-the facts are observable wherever life is considered in relation to
-habitat. That anything so obvious should be regarded as guesswork or
-theorizing, or opposed to truth, when applied to former inhabitants of
-the earth, is somewhat surprising. And, it may be added, the cultural
-worth of fossil study comes to a focus on this very point, for men and
-women are now meddling, consciously or unconsciously, wisely or
-unwisely, with an all-important environment about which they have
-learned very little--one called, among other things, "civilization."
-
-For any portion of the world a complete-list of the different kinds of
-plant inhabitants comprises the _flora_ of that region, and a like
-summary for the animal life is known as the _fauna_ of the district. It
-is generally understood that different species of both plants and
-animals inhabit different regions of the earth, but outside of
-professional circles it is only beginning to be recognized that changes
-in floras and faunas occur from time to time, that slight differences
-may be noted in the course of observations extending over a period of
-only a few years, and that everything in a fauna or flora eventually may
-be displaced by new forms.
-
-It is, however, a convenient practice to use these terms in connection
-with time periods, rock beds, and types of environment, as well as
-geographical areas. Thus we have such phrases as a "Cretaceous fauna"
-(attaching the name of a geologic period), a "Benton fauna" (with
-reference to the fossils of a rock formation), a "marine flora" (using
-the name of an environment), an "Arctic flora" (which applies to a
-definite portion of the earth surface and its plant inhabitants).
-
-Faunas include animals which many persons do not recognize as such.
-Sponges, corals, insects, worms, crabs, oysters, and a host of other
-boneless creatures are grouped together as _invertebrate_ animals, while
-another group includes the fishes, amphibians (toads, frogs, and
-salamanders of today), reptiles (crocodiles, lizards, snakes, and
-turtles being well known varieties), birds, and mammals. This second
-lot, provided with backbones and skeletons, comprise the great division
-of _vertebrate_ animals.
-
-Floras also include types which are commonly seen but not popularly
-identified as plants. The algae are perhaps best known as seaweeds,
-water-silk, and pond scums; fungi as toadstools and moulds. Both groups
-are large and of important rank in the vegetable kingdom; only the
-algae, however, are recognized as important fossil producers. Better
-known types of plants are the mosses, ferns, evergreens, grasses, and
-the more conspicuous flower-bearing forms, from weed size to tree size.
-
-Many rocks owe their character to the work of large colonies of plants
-or animals, for the living organisms are frequently the active agency
-which takes dissolved mineral substance from the solvent liquid and gets
-it back into solid form. The liquid is, of course, the water in which
-the creatures live, while the mineral substance often becomes a
-commodity required by a plant or animal in its mode of living. Mollusks
-have a way of using lime in the production of shells, and many a bed of
-limestone consists almost entirely of this by-product of molluscan life.
-Tiny coral polyps build complicated and beautiful structures from the
-same mineral substance. Either intact or in broken condition, these
-structures contribute in a large way to the making of limestones. Algae,
-among the lowliest of plants, have done extensive work along similar
-lines, and numerous invertebrate animals could be named as important
-factors in the production of rocks. Many of the shells and other
-fabrications retain their peculiar patterns long after the extermination
-of their makers, and a highly informative part of the fossil record is
-provided in this manner. It is also by far the larger portion of the
-record, for the earlier ages of prehistoric time failed to produce a
-vertebrate animal of any kind, while the invertebrate record dates back
-to pre-Cambrian time.
-
-
-FORMATIONS
-
-If in some part of North America there had been steady accumulation of
-sedimentary materials under constantly favorable conditions since the
-beginning of Cambrian time, the result would have been a deposit of
-sandstones, claystones, and limestones measuring nearly fifty miles from
-bottom to top. These figures are based on actual production in North
-America where extensive measurements have been made in many localities.
-When other parts of the world are as thoroughly investigated and older
-deposits included in the calculations, the total thickness of such beds
-will probably be more than one hundred miles.
-
-No single pile of rocks offering a complete cross section of the
-geological record has ever been produced, but portions of the section
-are exposed to view on all the continents. In order to carry on
-desirable investigations and make comparisons, it has been necessary to
-divide this great composite section into small units which may be named
-in some way and placed definitely with relation to lower and higher, or
-older and younger, layers. To serve this purpose there has been
-developed the idea of rock _formations_, and here we have a word which
-is not defined readily, even for the use of those who are familiar with
-it. Nevertheless it is used so commonly that some understanding of its
-meaning becomes desirable.
-
-A _formation_ may be regarded as an extensive rock mass, variable, in
-thickness and other proportions, as well as in composition, but
-representing a period of time during which there was no great change in
-the character of plant and animal life, and no serious interruption in
-the depositing of the rock-making materials. Occasionally the lower and
-upper limits of a formation are well defined and readily located.
-Frequently, however, the transition is gradual, one formation merging
-into another with no apparent mark of separation. In such event the
-original description serves to establish more or less definitely the
-boundaries of a formation.
-
-Descriptions are published whenever a worker believes he has discovered
-a significant part of the great section which has not previously been
-named. The usual practice is to apply a name taken from the locality in
-which the beds were investigated, and in this manner the names of
-formations become associated with towns, rivers, counties, mountains,
-states and other geographical features. The locality which supplies the
-name is then regarded as the "type locality" for the formation, but
-wherever these same beds may be traced or otherwise identified the one
-formation name applies.
-
-    [Illustration: Dinosaur Tracks
-
-    An ancient trail in sandstone of the Dakota formation. East slope of
-    the hogback, west of Denver.]
-
-The "Dakota formation," to use a convenient illustration, is mentioned
-in scores of reports bearing on the geology of Colorado, Iowa, Kansas,
-Nebraska, New Mexico, Texas, Utah, and Wyoming, as well as the Dakotas.
-On the geological map of Colorado it appears on both sides of the
-Rockies, scattered in strips and patches from north to south boundary
-lines. The beds are easily located in the foothills district west of
-Denver because of their tendency to produce the prominent ridges known
-as hogbacks.
-
-Many formations are exposed over much less territory, some have even
-greater extent. Thickness may vary from a few inches to thousands of
-feet, and no two exposures will be exactly alike though some similarity
-necessarily prevails throughout. "Exposures" are simply portions of the
-beds which are not concealed by loose rock, soil and vegetation, or
-overlying formations. Canyon walls, steep cliffs and mountain slopes,
-gullies, and badlands provide a large variety of natural exposures. In
-such places rocks and fossils may be studied to best advantage.
-
-Since a formation may contain a variety of beds, including sandstones,
-shales, limestones, and all sorts of mixtures, there is sometimes need
-of subdividing it; but formations are the smallest units commonly shown
-on geological maps. They are actual rocks which fit into a historical
-scheme of things and may be regarded aptly as the pages of a book which
-nature has done in stone.
-
-
-GEOLOGICAL TIME
-
-"How old are they?" "How can you learn their names from the rocks?"
-These are typical examples of questions most frequently asked concerning
-fossils. The second question follows the usual reply to the first, for
-prehistoric plants and animals are as old as the rocks in which they are
-found. The answer, as to age, must come from the rocks and what we have
-learned about them through many years of hard work, thoughtful
-observation, and careful study. Names, however, come from a different
-source. Nature, apparently, managed for a long time to carry on without
-the use of words. Since man began talking he has had no trouble
-inventing names for things which interest him.
-
-Early students of rocks and fossils likewise accomplished a great deal
-without being able to date events in terms of years although many of
-their efforts and interests centered on the problem of discovering a
-continuous sequence of events in the fragments of evidence that had been
-uncovered. This relatively simple problem has not been fully worked out,
-and some of the breaks in the record are recognized as "time gaps" which
-may never be converted into history.
-
-The question of time, expressed in years, has been a puzzle which
-attracted some attention even in the earliest days of investigation. Its
-solution was attempted by several methods long before there was
-sufficient information to make them work satisfactorily, which accounts
-in part for the extreme variation in results of the calculations. Even
-now it is to be expected that changes will have to be made as long as
-pertinent studies are continued. Two of the most promising methods of
-investigation in late years have been producing figures which are
-surprisingly large. More accuracy than ever before is probably present
-in modern estimates but, except for comparatively recent time, there is
-yet no way of knowing within a range of millions of years when a
-creature lived.
-
-Astronomy and physics were used in early calculations but, although
-taken seriously by some geologists, it was soon recognized by others
-that certain events revealed by earth history could not be explained
-with so short a time allowance as these methods indicated. One of the
-first estimates provided a total of only twenty-five millions of years
-and included a great stretch of time during which the earth, according
-to prevailing theory, was more sun-like than rock-like, a time when
-planets were being born and the earth could not have been in its present
-physical condition, which is the chief concern of the geologist. Since
-those earlier conditions could not have supported life as we know it,
-our knowledge of cosmic history renders small service in the study of
-fossils.
-
-Among the methods suggested by astronomy and the laws of physics is one
-which is based on the probable rate at which the earth cooled from its
-molten condition to present temperature. It is believed now that the
-heat of the earth is not necessarily due to an original molten state and
-that a steady rate of cooling cannot be ascertained. Any figures based
-on such procedure, therefore, are discredited today.
-
-The amount of salt in the oceans, and the time required for its
-concentration there by natural processes, offers another way of
-attacking the problem. It is a well known fact that salt is being added
-to the seas at a fairly constant rate; sea water, then, must become
-saltier from year to year. The salt comes from rocks exposed on land
-surfaces and is transported by the rivers which drain these areas. By
-analyzing the river waters it is possible to estimate the amount of salt
-annually dumped into the oceans and, also by chemical analysis, it is a
-comparatively simple matter to figure the total amount now present in
-the oceans. Some recent calculations indicate that thirty-five million
-tons of salt are being added each year, and this figure divided into the
-total amount for all the years places the age of the oceans at three
-hundred sixty millions of years.
-
-However, there are certain other factors which complicate the problem.
-For instance, it is known that land areas exposed to surface drainage
-have not always been of their present size, and the annual production of
-salt by the different types of rocks exposed at various times in the
-history of the earth has not always been as it is now. The rocks also
-must be older than the oceans, but how much older cannot be determined
-by means of figures obtained in this way.
-
-Until the beginning of this century there was little anticipation of a
-better measuring stick than one in use at the time which placed its
-reliance on the total thickness of the sedimentary deposits and the
-length of time required to produce this great accumulation of material
-which is known as the geological column. Since the total thickness, or
-height of the column, was not accurately known, and with recognized time
-gaps to bridge, there was little hope of working out a complete
-chronology by this device, but it has supplied highly desirable and
-reliable information concerning parts of the record.
-
-The system has been somewhat improved since its earliest use, and one of
-its latest applications gives us an age, for known sedimentary rocks, of
-nearly half a billion years, this being based on a total thickness of
-one hundred miles and an average rate of 880 years for the building up
-of one foot of sediments. Its greatest weakness is due to the absence of
-a reliable factor to take care of long stretches of time in which the
-sedimentary rocks are known to have been subjected to destructive
-processes. A yardstick of this character cannot be applied to rocks that
-have been destroyed, and there are excellent reasons for believing that
-these interruptions may account for several times the lapse of years
-indicated by the amount of rock remaining in the column which has been
-pieced together.
-
-Following the discovery of radium, however, the present century provided
-a new field of knowledge which has contributed greatly to the
-measurement of geologic time. The penetrating rays produced by radium
-and other radioactive substances are due to extremely slow but violent
-disintegration of the material. Uranium and thorium are radioactive
-elements which occur in the rocks of many parts of the world. There is
-little or no loss of material as the so-called disintegration proceeds;
-instead there is a complicated series of transformations in which other
-elements are produced, radium itself being one of these. Helium and lead
-eventually take the place of the less stable elements and the known rate
-at which these products accumulate provides the highly desired key to
-the age of the rocks.
-
-Part of the gas, helium, may escape, but except in rare instances where
-chemical alteration might occur, there probably is no loss of lead.
-Fortunately, when this metal is produced by radioactivity it differs
-slightly in atomic weight from ordinary lead; otherwise the presence of
-the latter would introduce a misleading factor. Since the speed at which
-the change goes on cannot be increased or decreased, it is assumed that
-throughout past ages it has never been faster or slower. The amount of
-such change that has been completed in any body of radioactive minerals
-may be measured by techniques employed in physics and chemistry. If it
-is found that the amount of helium or lead present requires a hundred
-million years for its production at the working speed of the parent
-elements, the mineral deposit must be at least that old.
-
-Certain conditions of course complicate the problem seriously: knowing
-the age of a piece of rock which happens to contain some radioactive
-element is of small service in historical studies unless the rock can be
-definitely associated with a flora or fauna, or some outstanding event
-disclosed by geological investigations. But there have been a few
-instances in which most of the necessary conditions were present, and
-more and better opportunities to apply this method will no doubt appear.
-Other elements, or their radioactive isotopes, are already being
-employed with good results. Some of these, such as carbon 14, are more
-sensitive indicators for the accurate dating of events in comparatively
-recent time.
-
-When it can be used, this type of measurement is far less subject to
-uncertainties than any other. It promises to eliminate all need for
-guessing, and comes close to a degree of accuracy which is satisfactory
-to the scientist, a person who thoroughly dislikes uncertainties of any
-kind. If suitable material can be found in just the right places it
-should accomplish what the preceding method cannot do--the accurate
-measurement of the great time breaks which interrupt the geological
-record in many places. Something along this line already has been
-accomplished, for radioactive material has been found in some of the
-oldest of the rocks. Regardless of the destruction going on in other
-localities, these rocks have continued to register the passing of time,
-and a tremendous antiquity for the earth and some of its first
-inhabitants has been indicated.
-
-Tests made on radioactive minerals from Gilpin County, Colorado, have
-established the age of late Cretaceous or early Cenozoic rocks at sixty
-million years, providing a convenient and reasonably accurate date for
-the beginning of the Age of Mammals. In Russia, one of the oldest
-mineral deposits yet studied in this way and regarded as early
-Pre-Cambrian, produced the astonishing figure of 1,850,000,000 years;
-what we commonly refer to as geological history may therefore be
-regarded as covering a range of approximately two billions of years. The
-earth, in some form or other, has in all probability passed through an
-earlier history of another billion years or more.
-
-Wherever we may roam, a portion of the prehistoric record is to be found
-in the rocks underfoot and not far from the surface. Formations as
-already mentioned may be regarded as the pages--often torn and badly
-scattered--of nature's own book, in which the geological periods are
-chapters. But instead of numbering these pages and chapters we have
-_named_ them, in order to get the parts reassembled in orderly fashion
-and restored to a condition which makes the book legible. However, the
-names cannot render the service intended except in connection with a
-time chart and an outline of earth history.
-
-    [Illustration: GEOLOGICAL TIME
-    Figures to the left denote millions of years that have elapsed up to
-    recent time]
-
-  CENOZOIC
-     Age of Man
-             RECENT              Man and his Culture
-          1  PLEISTOCENE         Last of Mammoths & Mastodons
-     Age of Mammals
-          7  PLIOCENE            Horses modernized
-         20  MIOCENE             Grasses and Grazing Animals
-                                 Three-toed Horses, Rhinos, Camels
-         35  OLIGOCENE           Specialization of Primitive Ancestors
-         60  EOCENE              Decline of archaic types
-                                 Mammals flourishing
-  MESOZOIC
-     Age of Reptiles
-        125  CRETACEOUS          Last of Great Reptiles
-                                 Specialization of Dinosaurs
-        160  JURASSIC            Bony Fishes thriving
-                                 Flowering plants advance
-                                 Cycads
-                                 Birds and Flying Reptiles
-        200  TRIASSIC            Few small mammals of lower orders
-                                 Dinosaurs become prominent
-  PALEOZOIC
-     Age of Amphibians
-        225  PERMIAN             Reptiles advancing
-                                 Amphibians dominant insects
-        300  CARBONIFEROUS       Dense forests of spore-bearing plants
-     Age of Fishes
-        350  DEVONIAN            Shark-like Fishes
-                                 Land floras established
-        375  SILURIAN            First land animals (scorpions)
-                                 Armored Fishes prominent
-     Age of Invertebrates
-        425  ORDOVICIAN          Corals and Bryozoa
-                                 Progress among Mollusks
-        500  CAMBRIAN            Brachiopods gaining
-                                 Trilobites dominant
-                                 Advance of shelled animals
-  PROTEROZOIC
-     EARLIEST LIFE
-       1000  UPPER PRE-CAMBRIAN  Small marine invertebrates
-                                 Lowest Forms of Plant and Animal Life
-                                 Few Fossils
-  ARCHEOZOIC
-       2000  LOWER PRE-CAMBRIAN  Some chemical evidence of life
-                                 No fossils
-
-Such aids have been devised and revised from time to time. No figures
-have been offered as final or absolutely "right" since the beginning of
-scientific investigations. Time divisions have been proposed that are
-not yet in common use while others have been abandoned or modified.
-Sources of information are so numerous that appropriate credit cannot be
-given fairly for anything that is up-to-date. The combined chart and
-outline here provided is based on time calculations of recent date but
-with figures slightly rounded off for the sole purpose of making them
-easier to remember. In view of the still existent probability of error
-it is felt that the slight alteration of figures may justify itself. It
-need not be regarded as misleading if the present purpose be
-considered--the stimulation of a natural history interest which is not
-vitally concerned with the little difference between a thousand million
-years and nine hundred ninety-nine million years.
-
-
-EXPLANATION OF THE TIME CHART
-
-The whole of geological time has been divided and subdivided according
-to varying practices. The development of life is perhaps the one
-outstanding feature of the time divisions, but for the most part the
-changes in floras and faunas have been gradual rather than abrupt, and
-this makes it very difficult to draw sharp lines or to visualize
-beginnings and endings of the various stages of development.
-Occasionally there is good excuse for drawing a line, where the record
-is broken and resumed again after a long lapse of time. The principal
-cause of such breaks is the elevation of great land masses, which brings
-on an interval of erosion and surface destruction for the areas
-uplifted.
-
-These movements of parts of the earth's crust have been exceptionally
-pronounced at certain times, often culminating in the production of
-mountain systems, and because of the extreme changes they introduce are
-known as revolutions. The major divisions of prehistoric time have been
-established, at least in part, by such _revolutions_; crustal, climatic,
-or other _disturbances_, on a smaller scale and recurring with greater
-frequency, may be regarded as establishing boundaries for the minor
-divisions. Hence we have five great Eras of geological history, and
-these are divided again into Periods. The time chart shows an
-arrangement commonly used in America. In the first column the names of
-the Eras are stated in technical form. Closely coinciding with these
-terms are the popular names of the Ages which appear in the second
-column. These names, describing the dominant life of each age, are very
-convenient. The more scientific terms used for the eras, while serving
-essentially the same purpose, are a little more systematic and
-generalized in that they refer to ancient life (Paleozoic), middle life
-(Mesozoic), and recent life (Cenozoic), without being specific as to any
-class of animals or plants for any one division of time.
-
-The period names, in the central column, have been derived from
-miscellaneous sources, some of them from geographical districts, some
-from descriptive references to prominent features of the rocks, others
-indicating a degree of approach to recent time. In paleontology (fossil
-study) it has long been a practice to cut the periods into lower,
-middle, and upper divisions, and in a few cases it has been found
-desirable to make two periods out of an old one. What was once known as
-the Lower Carboniferous is now commonly recognized as the Mississippian
-period while the upper portion has become the Pennsylvanian. The Lower
-Cretaceous is now the Comanchean of some authors.
-
-Both old and new practices are responsible for a little confusion at the
-present time. A former division into Primary, Secondary, Tertiary, and
-Quaternary eras has been partly abandoned, but the term "Quaternary"
-still applies to the Age of Man, while "Tertiary time" remains in good
-usage for the balance of the Cenozoic era. Among the newer introductions
-may be mentioned the use of a Paleocene period which precedes the
-Eocene. Geologists are not entirely in agreement as to the necessity for
-this addition and many would not give it equivalent rank with other
-periods. In the interest of simplicity these modern refinements have
-been omitted from the chart.
-
-The figures appearing in the third column, between the Ages and Periods,
-indicate the millions of years that have elapsed up to present time.
-They denote the age of the rocks at the beginning of each period. The
-age of a plant or animal which lived in Eocene time would be, according
-to this scale, somewhere between 35 million and 60 million years. In
-practice it is usually possible to determine whether a fossil was
-embedded in the rocks during an early or late portion of the period, and
-thus its age may be established within a shorter range, but it is
-impossible to be exact, even in terms of millions of years, with regard
-to anything as far back in prehistory as the Eocene period.
-
-The period in which we are living today is known as Recent. It began at
-the close of the Ice Age or Pleistocene period about ten thousand years
-ago and represents so little of earth history since the beginning of
-life that a chart many times the length of this page would be required
-to show the rest of the periods in proportion. The Cambrian period is an
-early chapter in which the story of prehistoric life suddenly becomes
-clear and richly varied. It is, however, much farther from the beginning
-of the record than it is from the present, and the Pre-Cambrian eras
-would require a great deal more space in order to show their relative
-lengths. The Archeozoic and Proterozoic eras have to some extent been
-divided into periods, but the great antiquity of the rocks has obscured
-much of their history, and divisions established for one locality have
-been of little service elsewhere. Consequently, the period names are in
-less general use and the common practice is to refer to all this great
-stretch of time as Pre-Cambrian.
-
-In the last column, at the right of the chart, some of the historical
-features are indicated. This column should be read from bottom to top in
-order to get the proper development of the story, and at best this
-sketchy outline of events can be no more than suggestive of the progress
-and decline through which the earth's inhabitants have passed.
-
-Rocks of every period except probably the Silurian are known to have
-been deposited somewhere in the Colorado area, although in most cases
-the record for each period is far from complete. Formations are too
-numerous and too varied locally to be shown on a chart of this type.
-
-
-THE GEOLOGICAL SECTION
-
-In the study of fossils there are two important field aids usually
-available. For any locality there should be a geological map and a
-section showing the sequence and character of the strata. On a
-small-scale map many of the local details have to be omitted, but the
-position of the larger exposures is indicated and, with this information
-at hand, the fossil-bearing strata may be located with the help of a
-geological section. The latter is frequently obtained from technical
-reports published by State and National Geological Surveys. Frequently,
-however, it is possible to obtain only a general plan for a given
-locality, and a great deal of literature may have to be scanned in order
-to get that. Excellent geological maps of Colorado have been published
-by the Colorado Geological Survey and the United States Geological
-Survey.
-
-It often happens that a formation is not where we expect to find it,
-this being due to several possible factors. The sediments may not have
-been deposited there, or they may have been removed by erosion. Where
-the structure has been disturbed by folding and faulting, a multitude of
-complications is introduced. The expected sequence is sometimes inverted
-and repeated through a series of folds. Formations also may be moved
-miles out of place by faulting. Both thickness and character of
-sediments may vary considerably within a formation. In some regions the
-geology is very simple, in others extremely difficult to understand.
-
-    [Illustration: FORMATIONS of the DENVER-FOOTHILLS REGION
-    A GENERALIZED SECTION SHOWING SOME OF THE SURFACE FEATURES
-    RED BEDS    HOGBACKS    TABLE MOUNTAIN    DENVER DISTRICT
-
-REGION OF MOUNTAIN-MAKING UPLIFT
-
-Formations bordering the mountains have been bent into upright
-positions.]
-
-                PERIODS
-
-  RECENT
-  PLEISTOCENE
-  PLIOCENE
-  MIOCENE
-  OLIGOCENE
-  EOCENE
-  CRETACEOUS                                              THICKNESS
-  SOFT SANDSTONES GRITS & CLAYS        DENVER & ARAPAHOE   2000 ft.
-  SANDSTONES, SHALES & LIGNITE         LARAMIE             1000 ft.
-  YELLOWISH SANDS & SHALES             FOX HILLS           1000 ft.
-  SOFT DARK GRAY OR RUSTY SHALE        PIERRE              5000 ft.
-  LIMESTONES & SHALES                  NIOBRARA             500 ft.
-  DARK SHALES & LIME                   BENTON               400 ft.
-  GRAY OR BUFF SANDSTONES & CLAYS      DAKOTA               300 ft.
-  SHALES, SANDSTONE & LIME             MORRISON             200 ft.
-  JURASSIC
-  TRIASSIC
-  PERMIAN
-  DEEP-RED SANDY SHALES, LIME, GYPSUM  LYKINS               700 ft.
-  CARBONIFEROUS
-  MASSIVE PINK OR WHITE SANDSTONE      LYONS                200 ft.
-  RED OR BROWN SANDSTONE &             FOUNTAIN            1500 ft.
-  CONGLOMERATE
-  DEVONIAN
-  SILURIAN
-  ORDOVICIAN
-  CAMBRIAN
-  PRE-CAMBRIAN
-  METAMORPHIC & INTRUDED ROCKS         IDAHO SPRINGS
-  SCHIST, GNEISS, QUARTZITE            (PART)
-  BASEMENT ROCKS of IGNEOUS ORIGIN
-
-A generalized section for the western part of the Denver Basin is
-introduced here for the use of local students. The formations normally
-present in this region are shown in their usual position. They are
-briefly described on the chart, and their thickness is indicated by
-figures which may be regarded as near the maximum for the district. The
-section will apply to most of the foothills area between Morrison and
-Boulder though surface features and thickness of beds will vary
-considerably from place to place.
-
-Certain of the formations are known to be fossil bearing, others barren
-or nearly so. When fossils are present they are usually restricted to
-certain localities, and these may be widely scattered. The following
-remarks apply to the possibilities for finding fossils in the formations
-named.
-
-
-                         _Denver and Arapahoe._
-
-Leaf impressions of palms, ferns, and numerous species of well-known
-trees and shrubs are common in many localities. Petrified wood is fairly
-abundant, and a few scattered bones of reptiles and mammals have been
-found. The two formations are treated as a unit because the Arapahoe is
-neither conspicuous nor sharply defined. Denver beds are well exposed on
-the slopes of Table Mountain at Golden; fossils, however, have been
-obtained from several localities nearer the city of Denver, notably from
-the hills just west of Overland Park.
-
-
-                               _Laramie._
-
-Plant material is locally abundant, principally the leaves of familiar
-deciduous trees, palms, and ferns. Many of the clay pits being worked
-near Golden are in this formation. Oysters and a few other mollusks may
-be found in some places.
-
-
-                              _Fox Hills._
-
-Better exposures of this formation are located to the north of Denver.
-Marine mollusks are most frequently found.
-
-
-                               _Pierre._
-
-In addition to the characteristic dark shales, this formation includes
-some limy material and sandstone beds, both of which are fossiliferous
-in places. Two types of marine mollusks are characteristic:
-_Inoceramus_, generic name for several species of clam-like bivalves
-readily identified by concentric elevations which produce a rippled
-effect on the shell surfaces; and _Baculites_, cephalopods with
-straight, chambered shells which often break at the suture lines, where
-the fossil is weakened by the chamber walls. Small oyster shells are
-fairly common also. The formation is to be found some distance to the
-east of the prominent hogback where it weathers into smooth surfaces in
-the form of broad valleys and flats, with rounded contours on the few
-elevations that may be present. It forms a soft, flaky soil when dry, is
-a sticky "gumbo" when wet. The clay is generally of a rather dark
-grayish color when freshly exposed but it takes on a rusty appearance
-after weathering. At various levels there are numerous iron-cemented
-concretions, many of which contain fossil shells.
-
-    [Illustration: Marine Beds of the Benton Formation, Northeastern
-    Colorado
-
-    A stream channel has cut deeply into the formation, uncovering and
-    partly destroying a plesiosaur skeleton which was found at the level
-    where the men are standing.]
-
-    [Illustration: Plesiosaur Bones in Place
-
-    Benton formation. Surface rubble has been cleared away, and several
-    vertebrae are partially uncovered in the area at the right of the
-    hammer.]
-
-
-                              _Niobrara._
-
-The formation contains fossils rather similar to those of the Pierre.
-Shark's teeth have been found in some of the lower beds. Limestone is a
-prominent feature, often forming a well defined ridge near the foot of
-the eastern slope of the main hogback. The limestones commonly have a
-chalky character.
-
-
-                               _Benton._
-
-The formation is not especially productive in this region. Marine shells
-are numerous in some localities, and bones of marine reptiles have been
-found at various places. As usually seen, it is almost entirely composed
-of impure clay shales, very dark, brownish-gray to almost black, and
-commonly interbedded with thin patches of white bentonite, yellow ochre,
-gypsum, and limestone.
-
-
-                               _Dakota._
-
-This formation produces the high hogback which is usually present some
-distance east of the Red Rocks. There are generally two or three layers
-of massive, light-colored sandstone separated by clays which are used
-extensively in the making of bricks and pottery. Leaf impressions and
-some fish scales are found in the clays and occasionally in the
-sandstone. The hogback is a good marker from which to locate other
-formations, because of its prominence in the foothills landscape.
-
-
-                              _Morrison._
-
-Good dinosaur material has been taken from the Canon City and Morrison
-districts. The formation is to be found on the lower west slope of the
-Dakota hogback. It consists of continental deposits of the stream and
-lake types. There is considerable sandstone in this formation and a
-little limestone is to be found here and there, but the most
-characteristic feature is in the shales. When freshly exposed, the
-shales are delicately tinted with gray, green, and maroon, a
-bronze-green being rather prominent. This formation is highly variable
-in character, with much of the clay often buried under the valley floor.
-In addition to the bones of reptiles, there are plant fossils, usually
-of poor quality, and fresh-water gastropods more or less abundant in
-some localities.
-
-
-                               _Lykins._
-
-Outcrops are not prominent, owing to the small amount of
-weather-resisting materials. The sandy clays are commonly of a deep red
-color mottled with spots of light gray. A white limestone is sometimes
-present near the middle of these deposits, and gypsum beds are included
-locally. The formation is often indicated only by red soil in the
-depressions between ridges. Few fossils have been reported.
-
-
-                                _Lyons._
-
-This formation is usually prominent as the eastern wall of the uplifted
-Red Rocks series. In some localities it forms a ridge of pink or white
-sandstone distinctly separated from the older sediments to the west.
-Very few fossils are found.
-
-
-                              _Fountain._
-
-Exposures usually are brown to red in color, though sometimes a dirty
-white. The prominent rocks are rather coarse sandstone, commonly with a
-gritty texture due to the angular character of the sand or gravel from
-which they were made. These are the westernmost of the Red Beds and the
-oldest of the uplifted sedimentary rocks bordering the foothills in most
-of our area. Fossils have been found in the formation, but it is
-practically barren for the territory here considered.
-
-                            * * * * * * * *
-
-This geological section also illustrates a method of dating crustal
-movements and the birth of mountain ranges, for the folding of the
-strata along the flanks of the Rocky Mountains has a great deal of
-significance in this connection. The sedimentary layers were originally
-deposited over much of the present mountain area in a horizontal
-position, and only those formations in existence at the time could be
-distorted by the upheavals which produced the new elevations. Of the
-series generally involved in the movement the Laramie beds are the
-youngest. Since these beds had not been formed until near the close of
-the Cretaceous period it is to be assumed that the mountains must be of
-more recent date, younger than the topmost of the deformed beds and at
-least as old as the lowermost of the undisturbed formations overlying
-them.
-
-Some disturbance is evident also in the Arapahoe and Denver beds which
-overlie the Laramie, but this is believed to have occurred sometime
-after the occasion of the first great uplift. Volcanic materials in
-these beds lead to the belief that the sediments were deposited during a
-period of volcanic activity brought on by the crustal folding which
-terminated the Mesozoic era. Hence the conclusion arises that the age of
-the Denver and Arapahoe beds must coincide closely with some of the
-earlier stages in the history of the mountain system. This interval is
-often referred to as Post-Laramie time.
-
-
-
-
-                       BEFORE THE AGE OF REPTILES
-
-
-THE PRE-CAMBRIAN COMPLEX
-
-The rocks of Pre-Cambrian time have been buried deeply under the
-accumulation of younger sediments, and the resulting pressure in many
-places has been tremendous. In addition to the effects of pressure there
-also is recorded in these ancient formations the repeated movements of
-the materials since they were first deposited. Vertical and side
-adjustments of parts, with relation to other parts, have distorted the
-original arrangement of the rock particles to such an extent that
-ordinary fossils would eventually become unrecognizable. These crushing,
-grinding, and kneading forces working through millions of years alone
-would account for the absence of fossils from the older deposits.
-Frequently the rocks have become so changed in form that their original
-character can only be conjectured, and because of this change they are
-known as metamorphic rocks.
-
-A few beds of Archeozoic age remain in nearly their original condition,
-but they are either without fossils or they have produced very
-questionable and unsatisfactory specimens. The existence of life during
-these early stages of earth history is indicated largely by chemical
-rather than fossil evidence. Much of the ancient limestone has been
-converted into marble, but it is not unreasonable to believe that plants
-and animals were instrumental in the production of this type of rock as
-they are today. Certain varieties of iron ore deposits are now being
-built up by the aid of plants, and similar ores in the ancient rocks may
-have had a like origin. The presence of great quantities of carbon, in
-the form of graphite, may be regarded also as a sign of life, for this
-substance is accumulated on a large scale by living plants, and may be
-retained in a solid form after the partial decay of the plant tissues.
-
-So far as the direct evidence goes, there is no sign of any creature of
-large size or of such complicated structure as the common plants and
-animals of today. The chemistry of the mineral deposits is not entirely
-convincing as to the presence of life, but it is regarded as highly
-probable that microscopic, single-celled plants and animals, comparable
-to modern algae and protozoa, were in existence during Archean time.
-Throughout later eras there is unmistakable evidence of gradual
-development from simpler to more elaborate life-forms and the Archeozoic
-is commonly regarded as a time of preparation during which simple
-organisms of some kind were becoming adapted to early conditions which
-could not support life on a higher plane. The importance of the work
-done by such lowly creatures in the preparation of suitable environments
-for more advanced modes of living is overlooked almost entirely.
-
-During the next era, the Proterozoic, the record of life becomes
-somewhat clearer. Fossils are hardly to be regarded as abundant but
-there were several well-defined types of animals which left shells and
-other parts composed of mineral matter. Among these may be mentioned the
-Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced
-delicate, often lace-like shells of many patterns adorned with the
-radiating filaments or spines which have suggested the name for this
-group. Foraminifera produced minute shells, sometimes many chambered,
-and often bearing a confusing resemblance to the work of snails. Common
-chalk is composed almost entirely of such shells and fragments of them.
-
-Sponges and Bryozoa are animals of slightly higher organization. They
-are many-celled instead of one-celled and the cells have special work to
-perform, which is a most important step in the direction of the
-specialization which characterizes the structural and life pattern of
-later arrivals. The Bryozoa lived in moss-like colonies which have been
-important rock-makers; the fossil forms bear some resemblance to corals.
-Sponges are too well known to require description although the familiar
-article of commerce is merely the framework of once-living animals. They
-represent the earliest organization of true animal bodies even though in
-appearance they may have a resemblance to plants.
-
-Actual plants of this era were of the algae class, aquatic in habit as
-were their animal neighbors, the first to leave a record in the form of
-fossils. This record, obscure and distorted, has long been a source of
-perplexity to investigators. Without well-defined floras and faunas to
-guide them, and with rocks frequently in chaotic relationships, early
-geologists were content to regard it all as a "Pre-Cambrian complex."
-Recent studies have contributed a great deal of information not
-available some years ago. It is quite possible that more advanced types
-of life were in abundance before the close of the second era, but
-material on which to base sound opinion is still scarce.
-
-Rocks of Pre-Cambrian age are plentiful in the foothills region west of
-Denver. The schists, gneisses, and quartzites exposed for some miles
-immediately beyond the red-beds are part of this great complex. The
-Idaho Springs formation is known to be one of the oldest in this
-district, although its exact age has not been determined. Other
-formations are recognized among the metamorphic rocks of the region but
-none has contributed to our knowledge of early life.
-
-
-CAMBRIAN LIFE
-
-There can be no mistake as to the prolific development of life in
-Cambrian seas, for fossils of this age are to be found in many parts of
-the world, where ancient sea bottoms now form part of the land surface.
-Invertebrate animals appear to have made much progress, but plants were
-either scarce or too small and delicate to be productive of fossils. It
-is probable, however, that seaweeds and other algae were flourishing
-along with the invertebrates, because animal life is directly or
-indirectly dependent on the existence of plants. The latter sustain
-themselves by taking carbon and nitrogen from air, water, and soil, but
-animals must obtain their requirements by eating plants or eating each
-other. They cannot obtain what they need from the inorganic world
-without this help from the vegetable kingdom.
-
-One group of animals stands out prominently above all its
-contemporaries. Known as the trilobites they were by far the most
-distinguished and most characteristic of Cambrian invertebrates.
-Trilobites inhabited the warmer seas of this period and several later
-ones, but were extinct by the end of the Paleozoic era. Hundreds of
-species have been described, most of them under four inches in length.
-Well-known distant relatives now living are the shrimps, and other
-crustaceans. The name Trilobite has reference to the three lobes which
-are apparent in the form of the upper surface, the central lobe forming
-a broad ridge extending along the back. Beneath the outer lobes on each
-side there was, during life, a row of short, jointed legs used for
-swimming and walking, but these delicate appendages are seldom preserved
-in the fossils.
-
-Second in importance among the animals of the period were the
-brachiopods or lamp-shells, not true mollusks although they were
-provided with similar shells composed of calcium phosphate or calcium
-carbonate. Shells are of two parts (bivalved) as in the case of clams,
-but the valves are above and beneath the body instead of on the right
-and left sides, which is the arrangement among mollusks. Although
-abundant as individuals, there were only a few species during the
-earlier part of the period; the number of species increased, however,
-and the race became very persistent. About seven thousand species have
-been described, and the race is not yet extinct although the number of
-living species is relatively small.
-
-Cambrian life evidently included representatives of all the great
-divisions of invertebrates; sponges, jelly-fishes, worms, and primitive
-corals have been reported. At the end of the period there was an
-elaborate molluscan fauna. The closing of the period in North America
-was apparently a gentle elevation of continental areas and a consequent
-withdrawal of the sea.
-
-    [Illustration: Invertebrate Fossils
-
-    Only a few prominent types have been selected from thousands of
-    invertebrates known to zoologists. The forms illustrated are of
-    frequent occurrence as fossils.]
-
-  CRINOIDS
-  CEPHALOPODS
-    Coiled types
-    Ammonite
-    Scaphite
-    Straight-shell type
-    Baculite
-  TRILOBITE
-  BRACHIOPODS
-  BIVALVES
-    Inoceramus
-    Oyster
-  GASTROPODS
-    Snail-like Univalves
-  PROTOZOA
-    UNICELLULAR FORMS
-      Radiolaria (Microscopic)
-      Fusulina limestone
-      Foraminifera (Enlarged)
-    MULTICELLULAR FORMS
-      Cup coral
-      Reef coral
-      Sponge
-      Bryozoa
-
-
-THE ORDOVICIAN RECORD
-
-Extensive land areas must have subsided again early in the Ordovician
-period for marine sediments were laid down over a large part of the
-North American interior, and three epochs or subdivisions of the period
-have been based on as many invasions of the sea. In these ancient
-deposits the record of life continues to show new forms. Nothing of a
-very spectacular sort is recorded other than a great increase in the
-number of species among types that were established in earlier periods.
-
-Trilobites were at their best, brachiopods continued to flourish, and
-the mollusks made new progress, especially the cephalopods, a group
-which includes our cuttle fishes and squids. Some of these predatory
-creatures attained large size and were no doubt masters of the sea.
-Typical forms were provided with tapering chambered shells that
-occasionally reached a length of twelve or more feet. Most of the shells
-were straight and trumpet-like or but slightly curved. Some were closely
-coiled and in this respect more like the well-known nautilus now in
-existence.
-
-The bryozoans became very common in the later part of the period and
-corals made slight advances. Somewhat of a novelty at this time were the
-crinoids, commonly known as "stone lilies" although not plants at all.
-They have been described as starfishes with back turned downward and a
-thick stem attached beneath. Where they lived in great abundance the
-limestone deposits may consist almost entirely of their stems. Crinoids
-continued to produce a variety of forms throughout several of the
-succeeding periods.
-
-The brachiopods were commonest of all animals representing this period,
-however, and their wide distribution, together with their known
-preference for warm waters, is taken to be an indication of mild
-temperatures prevailing over a large portion of the earth. Land plants
-are indicated by spore-bearing forms related to the ferns and mosses.
-Impressions of such plants have been found in Europe but, since most of
-the known rocks of this age were formed in seas, the marine algae are
-more abundant as fossils.
-
-In the Colorado area, rocks of Ordovician time are exposed only in
-mountainous areas where they have been lifted high above their original
-levels. They are not especially rich in fossils although they have
-produced some fish remains which are of interest in that they suggest an
-age of vertebrates which is just ahead.
-
-
-SILURIAN EVENTS
-
-Since land floras and faunas had not yet become conspicuous the fossil
-record for this period is limited to those areas which were invaded by
-the sea. Apparently there was no such invasion of the present Colorado
-region, for rocks of this age are not in evidence. If they exist at all
-they are restricted to localized districts which are deeply buried under
-sediments of later periods. There may have been no Silurian deposition
-in this area, or such rocks may have been produced only to be destroyed
-by elevation and consequent subjection to weathering and erosion during
-a long interval of time, in which they were completely removed. In the
-region of the Colorado Rockies there is no evidence of returning seas
-until late Devonian time.
-
-In other parts of the world, however, there was extensive deposition of
-rock-making sediments in seas which were inhabited by algae and
-invertebrates of the types previously described. Among the common
-animals of the time there were still numerous species of brachiopods,
-trilobites, corals, crinoids, and bryozoans. In addition to the
-primitive cone-shaped, cup corals there were several advanced types but
-the habit of building large reefs was not yet established.
-
-"Sea scorpions," really large crustaceans, flourished during Silurian
-time, and late in the period there appeared a race of true scorpions
-which lived on dry land or between high and low tides along the
-seashore. These were smaller and much like modern descendants but
-probably they did not wander far from the ocean shores where an
-abundance of food was available. These little scorpions, the largest
-measuring only two and a half inches in length, are the oldest
-air-breathing land animals of the fossil record.
-
-It was not until the period was well advanced that fishes became
-numerous, and much of our knowledge of the beginning of an "Age of
-Fishes" has been obtained from European fossils. Although fishes are
-classed with the vertebrate or backboned animals there are large groups
-which do not have bony skeletons but are provided instead with a simple
-framework of cartilage. Among the earlier and more primitive types were
-the ostracoderms or bony-skinned fishes with no internal bones and only
-a small amount of bony substance in the armor-like plates and scales
-which covered the forward portion of the body.
-
-The ostracoderms comprise a small group of fishes about which very
-little is known. They appear to have been inhabitants of fresh-water
-streams as well as lagoons bordering the seas, and may have been related
-to the small sharks of the time. They lived during the Ordovician,
-Silurian, and Devonian periods, and left no descendants now recognized
-among living creatures. A much larger type of armored fishes is known as
-the arthrodires, a name which refers to a pointed neck and an
-arrangement of the armor plates to permit a movement of the head. These
-were the most ferocious fishes of the Silurian and Devonian seas, some
-of them reaching a length of twenty feet though most were much smaller.
-Their jaws were provided with formidable shearing and crushing plates
-instead of teeth.
-
-
-DEVONIAN PROGRESS
-
-The Devonian is one of the most outstanding of all periods from the
-viewpoint of life development. Dominance of the fishes is its greatest
-achievement, the invertebrates remaining about as they were and the
-higher vertebrates barely in evidence, but life on a large scale was no
-longer confined to the seas. Fresh-water fishes became prominent and
-land plants well established. The first forests appeared, with fern-like
-plants predominating although woody trees of several types and
-considerable size were included. It is quite possible that extensive
-land areas had been well supplied with vegetation during earlier times,
-but the delicate tissues of plants are far less likely to be preserved
-than the limy parts of animals. The fossil record, therefore, cannot be
-expected to reveal more than a suggestion of the progress made at this
-level of living. The story of plant life becomes much clearer in the
-next period when conditions were more favorable for the production and
-preservation of plant fossils.
-
-Land animals of the time are almost unknown. A few snails and scorpions
-have been found, and some footprints made by early amphibians. Insects
-probably were in existence although the evidence is not quite clear on
-this point. The increasing number of fresh-water fishes, however, may be
-regarded as a sure indication that inland conditions were becoming more
-favorable for plant and animal inhabitants of all kinds.
-
-The extent of development among the fishes cannot be accurately
-indicated by naming a few types, for it is mainly in the number of
-species and genera within the larger groups that progress is seen. In
-general it may be stated that the fishes of the period had not yet
-acquired the bony skeleton and typical form of familiar modern species.
-Skeletons were of cartilage, partly hardened in some instances by lime.
-Armor plates were customary with certain races but were not present
-among all fishes. Neither were these armored forms exceptionally large,
-as compared with living sharks. Although occasional giants appeared, the
-majority were small. Many were sluggish creatures with poorly-developed
-jaws, living as scavengers on sea and stream bottoms. Tail fins were
-usually unbalanced as in the sharks, or pointed and rounded rather than
-evenly forked.
-
-    [Illustration: Modernized Types of Fishes from Eocene Shales of
-    Southwestern Wyoming]
-
-The great tribe of true bony fishes, such as the cod and perch, which
-includes more than ninety percent of the fishes living today, was not
-yet in existence. About one-third of the many kinds of fishes then
-living were related to the sharks, a group which is relatively
-insignificant in recent years. Nearly one-fourth of the total belonged
-to a tribe of enamel-scaled fishes, now represented only by a few
-sturgeon and gar-pike.
-
-Lung fishes have never been a large group but it is noteworthy that they
-have had existence since Middle Devonian time. Living members of the
-race, inhabitants of Africa and South America, make a practice of
-burrowing into the mud of stream channels during dry seasons and are
-provided with lungs which enable them to breathe air in the manner of
-higher vertebrates. They survive the complete drying-up of the streams
-and live for months without water. Other forms, with less development of
-lungs, frequent stagnant pools and come to the surface occasionally for
-a breath of air. All are provided with gills also, which enables them to
-obtain their oxygen as other fishes do. They are believed to be a
-connecting link between the fishes and the early amphibians. More
-accurately, perhaps, they should be regarded as holding an intermediate
-position without being directly ancestral to any higher type of
-vertebrate animal.
-
-Still dominant among the invertebrates were the brachiopods, on the
-whole averaging a little larger in size, and otherwise indicating
-congenial times for that type of organism. They reached the peak of
-their development during this period. Trilobites were declining although
-a few new and strangely ornamented varieties made a brief appearance.
-Crinoids apparently found living conditions less favorable during
-Devonian time, but in a later era they again became prominent. Corals
-were favored only at times and in certain localities. Along with the
-crinoids they appear to have suffered from the presence of an unusual
-amount of mud in the waters of their customary habitats. Both had a
-preference for clear water as indicated by the absence of fossils from
-limestones containing more than a very small percentage of muddy
-sediments. Crustaceans, similar to the sea-scorpions and better known as
-eurypterids, became prominent among fresh-water animals. Some were
-unusually large for creatures of this class, lengths of several feet
-being recorded from fragments. Gastropod mollusks came into prominence
-in localities where living conditions were favorable. Bivalves continued
-to thrive but the cephalopods had a rather meager development
-considering the heights they were to achieve in subsequent periods.
-
-In western North America the large expanse of territory known as the
-Great Plains was evidently well above sea level during this entire
-period, for no beds of this age are found in eastern Colorado. West of
-the Front Range, however, there was some deposition of marine sediments
-during late Devonian time. Formations of this age are exposed near
-Salida and Glenwood Springs, on the White River Plateau, and in the San
-Juan region.
-
-The Carboniferous period gets its name from the vast deposits of coal
-which were developed during that time in many parts of the northern
-hemisphere. Depressed land surfaces bordering the continents, and
-extending well into the interior of present boundaries, supported dense
-growths of vegetation and provided the swampy conditions most favorable
-to coal production. Varieties of plants which are now of small size and
-lowly position in the botanical world acquired the proportions of large
-trees.
-
-
-CARBONIFEROUS FORESTS
-
-Best-known fossils of the period are carbonized portions of the larger
-trees, and impressions left in the muds and sands of ancient bogs.
-Forest trees of several kinds reached the height of a hundred feet, with
-a trunk diameter of two to six feet. This size often is exceeded in
-modern forests, but by trees of an entirely different type. Considering
-the amount of development among the plants of earlier periods,
-Carboniferous forests provide an outstanding spectacle of advancing
-life.
-
-Quite common among the larger trees were two varieties of club-mosses,
-also known as scale trees. They were cone-bearing evergreens with only
-slight resemblance to modern conifers. Instead of seeds they produced
-spores, a method of reproduction which is practiced among ferns. The
-trunks were marked from bottom to top with uniform patterns of cushions
-and scars indicating the points at which leaves were attached during the
-earlier stages of growth. In the Lepidodendrons the rows of scale-like
-cushions wind spirally upward while among the Sigillaria there is a
-vertical arrangement of leaf-scars which resemble the imprints of a
-seal, these impressions being in straight and parallel rows on a surface
-which may be either ribbed or smooth. The leaves of scale trees were
-stiff and slender, and arranged in grass-like tufts at the top.
-
-Calamites, related to our horsetail rushes, were somewhat smaller than
-the scale trees. Their trunks consisted of a thin, woody cylinder with a
-pithy interior, and were marked at intervals by nodes which gave them
-the "jointed" appearance of a bamboo stem. Leaves were arranged in
-circles around the nodes of main stem or branches. Spore-bearing cones
-appeared at the tips of the stems.
-
-    [Illustration: Prehistoric Plants
-
-    Some of the larger and better known plants of past ages are shown as
-    reconstructed by artists. Finer details of the reconstructions often
-    have to be neglected because of uncertainties due to the scattered
-    and fragmentary character of the fossil record.]
-
-  LEAF IMPRESSIONS
-    Carboniferous Ferns
-    Strap-leaf Conifer (Cordaites)
-  MODERNIZED TYPES
-    Sequoia Cone and foliage
-    Miocene Fossils (Florissant Shales)
-      Maple
-      Willow
-    Eocene palm (Denver Beds)
-  HORSE TAIL RUSHES
-    Restoration (Calamites)
-    Fossils Leaves and Stem
-  CYCADS
-    Restoration
-    Fossil Trunk
-  CLUB MOSSES
-    Restoration (Sigillaria)
-    Fossils Trunk Impressions
-  SCALE TREES
-    Restoration (Lepidodendron)
-    Fossil Leaf scars
-
-Also included among the larger trees were the Cordaites or large-leaved
-evergreens, tall and slender, seed-bearing but not true conifers as yet.
-Leaves were strap-shaped or grass-like, the larger ones having a length
-of six feet and a width of six inches. Trunks were woody, resembling
-pine, but with a central pith. The flowers were small and resembled
-catkins in form.
-
-Ferns and fernlike plants were so numerous that the period has been
-known as an age of ferns. Earlier knowledge of these forests was based
-on fossils of a fragmentary character from which an accurate association
-of parts could rarely be obtained. A general relationship with the ferns
-was apparent, but careful study of additional material has given us a
-rather different view of Carboniferous plant life and we note a highly
-diversified array of forms with many suggestions of modern tendencies.
-The flora as a whole may be regarded as highly specialized for the
-conditions which prevailed at the time and were not to continue through
-subsequent periods. Warm temperatures and abundant moisture were
-essential especially to spore-bearing types, and the cold, arid
-conditions of the next period put an end to many of the groups, or
-greatly reduced their prominence.
-
-This could be regarded equally well as an age of insects, for some of
-these invertebrates acquired the greatest size they have ever had,
-particularly the dragonflies with a wing-spread of more than two feet in
-one of the largest fossils so far discovered. Cockroaches numbering
-upwards of five hundred species have been named. Though large they are
-hardly to be regarded as giants, lengths of three or four inches being
-about the limit.
-
-Some of the insect types of today quite evidently existed among the
-inhabitants of Carboniferous forests, but it is apparent that there were
-also some antiquated forms which may have descended from the trilobites.
-Although some authorities regard this as the period in which insects
-originated, there are others who maintain that definite beginnings are
-not established so readily on present evidence. Spiders are believed to
-have made their appearance at this time.
-
-Four-footed vertebrates resembling salamanders were prominent animals of
-the Carboniferous swamps. At first adapted to a life in water and later
-to land conditions, they are known as amphibians, the name being based
-on the ability to live in two different kinds of environment. Common
-living representatives of this group are the toads and frogs, but these
-tailless forms are not known among fossils of the Paleozoic era and are
-almost unknown throughout the Mesozoic. The Age of Amphibians, as we
-apply that phrase, was definitely not an age of toads and frogs.
-
-These primitive land animals were of different types, ranging from much
-smaller sizes up to the length of a crocodile. Most of them had short
-legs, and feet which were suitable for locomotion upon land, but many of
-the creatures probably spent most of their lives in the water. Tails
-were usually high and flattened as if for swimming, sometimes long, at
-other times greatly shortened in proportion to the body. Heads were
-generally large, jaws long, and mouths wide.
-
-Before the close of the period true reptiles appear, but this race of
-animals is destined to make a more spectacular advance than the
-amphibians and will be discussed in connection with Mesozoic life. The
-amphibians, however, are regarded as being the ancestors of the reptiles
-as well as the higher quadrupeds which follow them. Although living
-reptiles are readily distinguished from living amphibians there is a
-different situation with regard to these primitive forms, for among the
-fossils it becomes increasingly difficult to separate the two groups as
-new material is investigated.
-
-Invertebrates had their ups and downs during the period. Trilobites
-became scarce, and brachiopods for a time were the most abundant of the
-shelled animals but later declined rapidly. In favorable localities the
-crinoids established a wonderful record for new species before the
-period had advanced very far. Hundreds of species of Carboniferous
-invertebrates are known, and in many of the rocks of the period they are
-the only fossils to be found, for the vertebrates were still unable to
-venture far from the swampy districts, and much inland territory was too
-well drained to support either the floras or faunas then existing.
-
-In the Colorado area there are both marine and continental formations
-but the great coal-making forests and their inhabitants were limited to
-other localities. As a consequence this region is not famous for
-Carboniferous fossils.
-
-
-PERMIAN HARDSHIPS
-
-For a time there was no great change in North America following the
-opening of the Permian. Then began a series of mountain-making movements
-and continental uplifts which drained the swamps, lakes, and inland
-seas. With the passing of the vegetation which had established itself in
-and around these areas much of the animal life followed. It is probable
-that a considerable proportion of the marine life survived, much more
-than is indicated by the fossil record, but the receding seas carried
-the survivors into territory which is now inaccessible to fossil
-hunters.
-
-After Middle Permian time the climate everywhere seems to have been cold
-and dry. By the end of the period there had been accomplished more
-geographical change throughout the world than at any time since the
-beginning of the Paleozoic era. Traces of the crustal movements which
-produced new mountain ranges can be followed in Europe, Asia, and North
-America. The Appalachian region was raised to a great height, possibly
-in excess of three miles. A major disturbance of this character is known
-among geologists as a revolution, and to this particular one the name
-"Appalachian Revolution" has been given.
-
-The elevation of continents necessarily changes their coast lines. This,
-in turn, influences ocean currents which have an important bearing on
-climatic conditions. In addition to this, the elevation of mountain
-systems not only rearranges the distribution of hot and cold winds over
-the land areas but it may produce barriers to the migrations of floras
-and faunas, confining them to areas in which it is no longer possible to
-live. When the effect upon plants and animals is considered, it is
-easier to understand why a line is drawn across the geological time
-chart at such a point and an era of prehistory is regarded as closed.
-
-During the Permian period there was recorded in the rocks more
-widespread glacial action than ever before or since. With less inland
-water to provide the necessary evaporation there was a marked decrease
-in rainfall, and arid or semi-arid conditions replaced the hospitable
-climate that had been such an important factor in the prolific life of
-the Carboniferous. The struggle for existence became intense, but
-hardier types of plants and animals, with greater ability to adapt
-themselves to adverse conditions, established themselves here and there,
-as ancestral forms became extinct. Most of the large spore-bearing trees
-died out and seed-producing varieties began to acquire prominence, among
-them the coniferous evergreens. Ferns, however, proved their
-adaptability by producing some new forms which became prominent in
-Permian floras.
-
-The prehistoric amphibians have been divided into three orders, one of
-which includes all the larger forms. This group, known as the
-labyrinthodonts, continued on through Permian time but began to show
-backward tendencies, with dwindling limbs and a return to life in the
-water. Among the larger land varieties are typical fossils ranging from
-about fifteen inches to five feet in length. In outward appearance they
-differed from Carboniferous amphibians. One of the other orders,
-including a great diversity of smaller forms, became extinct during this
-period, leaving no known descendants. The third order is regarded as the
-oldest, and probably the ancestral group from which the modern newts and
-salamanders originated.
-
-The most successful of Permian land animals were the peculiar reptiles
-that learned to live in drier regions. Like the horned toad and Gila
-monster of our arid southwestern United States, the larger Permian
-reptiles were four-footed animals. In size and shape they were not
-greatly different from amphibians then living. An exception to this
-rule, among some of them, is the development of long, bony spines above
-the vertebrae of the back. A fairly common fossil of this type, found in
-Texas and known as _Dimetrodon_, had a total length of six feet, about
-half of this being in the tail. The tips of the spines adorning the back
-reached a height of three feet or more and there was probably a covering
-of skin over these bones, which would produce a sail-like structure or
-"fin" of large size. Its use has not been explained but it provides an
-easy name for these odd creatures--the "fin-back lizards."
-
-Rock deposits produced in arid regions usually have characters which are
-not difficult to recognize. Gritty texture, irregular bedding, red
-color, and gypsum are common features. Formations of Permian age are to
-be found in Colorado but better fossil deposits have been discovered in
-Kansas and Texas.
-
-    [Illustration: Marine Reptiles: Plesiosaur (Lower Skeleton) and
-    Mosasaur
-
-    The Mesozoic Era produced many types of reptiles besides the
-    dinosaurs. Two of the marine forms are shown in this illustration,
-    both from Cretaceous beds of the western United States.  Plesiosaurs
-    were the giants of the seas in their time, lengths of forty to fifty
-    feet being not uncommon. A long flat tail provided locomotion for
-    the mosasaurs whereas the plesiosaurs resorted to the peculiar limb
-    structures known as flippers or paddles.]
-
-
-
-
-                          THE AGE OF REPTILES
-
-
-The Mesozoic, or era of middle life, was a long stretch of time during
-which there was marvelous development among the reptiles. Many strange
-types were produced and most of them became extinct before the end of
-the era. The reptilian stock branched out in many directions. Types
-emerged which differed from one another so widely that their mutual
-relationships have become obscure. Hideous and fantastic creatures
-suggesting sea serpents and dragons were worldwide in distribution.
-Reptiles of the air and seas acquired large size and weird forms, but
-greater advances were made upon land.
-
-The flying reptiles or pterosaurs flourished in Jurassic times with some
-of the larger varieties surviving until near the close of the
-Cretaceous. Although these winged lizards were the first of the
-vertebrates to fly they are not to be confused with birds. They were
-without feathers, and the earlier forms were provided with long tails
-bearing a flattened rudder-like tip. One of the best known of this type
-had a length of about eighteen inches. Its jaws were long and provided
-with sharp teeth. The wings were membranes attached to body and legs,
-stretched and manipulated by means of greatly elongated fingers. In
-later types there was a reduction in tooth equipment and length of tail.
-_Pteranodon_, found in Kansas, had a wing spread of twenty-five feet, a
-large toothless beak, a short body, and a mere stub of a tail. It was
-one of the last of these winged monsters.
-
-Several types of marine reptiles appeared during this era, among them
-the plesiosaurs which first appeared in Triassic seas. These peculiar
-animals were serpent-like with regard to the character of head, neck,
-and tail, but in other respects were quite different, the short
-barrel-shaped body being provided with four large paddles corresponding
-to the usual limbs of quadrupeds. Fossil remains of these animals are
-common in many Jurassic and Cretaceous deposits, some of the largest
-exceeding forty feet in length. Mosasaurs, also marine carnivores,
-inhabited shallow Cretaceous seas throughout the world and are
-especially abundant as fossils in the Kansas chalk beds. These were
-elongated forms with a resemblance to salamanders in some respects but
-provided with long pointed jaws and sharp teeth. Swimming was
-accomplished largely by the tail though probably aided to some extent by
-four webbed paddles or flippers. The ichthyosaurs were more fish-like in
-construction, as the name implies. The limbs were short and broad, and
-there was usually present a well-developed tail-fin as well as a large
-fin on the back. They were especially abundant in Jurassic time. Fossils
-are fairly common in marine deposits of western North America. Mosasaurs
-and ichthyosaurs were about half as long as the plesiosaurs.
-
-
-DINOSAURS
-
-Most spectacular of the prehistoric reptiles were the dinosaurs, a large
-group of animals varying greatly as to size, form, and habits. They were
-adapted for a life on land though many of them probably spent much of
-their time partly submerged in the waters of lakes and streams. There is
-little that can be said of the group as a whole other than that all of
-them were reptiles. Further than that it is necessary to regard them as
-belonging to several different subdivisions of the Reptilia.
-Classification has been difficult and the names used for the various
-subdivisions are often misleading to the layman who tries to understand
-the terminology.
-
-Ancestral reptiles were five-toed and five-fingered but among the
-dinosaurs there were many departures from the standard formula. Three or
-four of the digits were commonly well developed, the others when present
-being shortened or reduced to mere rudiments. Early in the history of
-dinosaurs there was a division of the stock into two main branches, each
-of which includes a variety of types and sizes, and is again subdivided.
-The two main groups are best recognized by the construction of the bony
-framework which comprises the pelvic girdle or hip region of the
-skeleton. In order to avoid technical difficulties, however, the
-remaining discussion of these interesting reptiles will be confined to a
-few names and descriptions which serve to illustrate roughly the great
-amount of variation that developed from the comparatively simple
-ancestral pattern. The plan according to which the dinosaurs are usually
-classified is barely suggested by the types described.
-
-The meat-eaters were active creatures provided with powerful jaws and
-teeth. They were unarmored, moved about on their hind feet, and during
-their time were the most highly advanced of all animals. _Tyrannosaurus_
-with a length of forty-five feet or more, and _Deinodon_, nearly as
-large, were among the greatest of these. Both lived in the Cretaceous
-period. Their teeth were simple but strong, knife-like, curved, and
-finely serrated. Skulls were large and the forelimbs were reduced almost
-to a state of uselessness. Large carnivores lived also during Jurassic
-time and even as far back as late Triassic. Early Triassic forms were of
-smaller size.
-
-More primitive flesh-eating dinosaurs of the Triassic and Jurassic
-periods were delicately proportioned and lightly built bipeds bearing
-some resemblance to birds. _Struthiomimus_, which means
-ostrich-resembling, was about the size of the bird which provides the
-name. It was slender in the limbs, three-toed, long necked, long tailed.
-The skull was small, forelegs long for a biped. Unlike most dinosaurs it
-was toothless. All these bird-like carnivores were small as compared
-with other contemporary forms. Compsognathus, of Germany, and one of the
-smallest of all dinosaurs, had a length of less than three feet,
-including the long tail.
-
-    [Illustration: One of the Large Jurassic Dinosaurs (_Diplodocus
-    longus_)
-
-    This magnificent specimen, exhibited by the Denver Museum of Natural
-    History, has a length of seventy-five feet six inches. Two years
-    were required to complete the task of removing the bones from the
-    matrix rock and preparing them for mounting. Diplodocus was one of
-    many large reptiles which inhabited western North America a hundred
-    and fifty million years ago. The skeleton was obtained from the
-    Morrison beds of eastern Utah. The same formation is exposed in many
-    Colorado localities, including the foothills west of Denver, where
-    it acquired its name from the town of Morrison.]
-
-In Jurassic time there became prominent a group of large dinosaurs which
-were more equally developed as to fore and hind limbs. They were
-sluggish creatures, quadrupedal in their manner of locomotion,
-vegetarians in regard to their diet. Some of them reached enormous
-proportions and it is believed that they resorted to life in the water
-in order to get part of the weight off their feet. _Diplodocus_ and
-_Brontosaurus_ are the names of well-known giants in this group. They
-had long necks and tails, very small skulls, were the largest of all
-land animals and are known to have reached a length of eighty feet or
-more. Some estimates, based on measurements of incomplete skeletons,
-have exceeded one hundred feet, but these extremes are somewhat
-questionable. _Diplodocus_ was the more elongated of the two, with much
-of its length in the whip-like tail. Our mounted skeleton has a length
-of seventy-five feet six inches, measured along the vertebrae. Its
-height at the pelvis is twelve feet six inches.
-
-The teeth of these large quadrupeds are of a slightly broadened and
-blunted form which has caused some speculation as to their possible use.
-It has even been suggested that the animals were fish-eaters but this
-seems impossible in view of the great size and general characteristics
-of the group. Although they differ extremely in some respects, they are
-regarded as being more closely related to the carnivores than to the
-herbivores of the second great branch of the tribe.
-
-The unquestioned herbivores, constituting this second branch of the
-dinosaurian race, also include both bipeds and quadrupeds. The better
-known plant-eaters were large animals but not such monsters as
-_Tyrannosaurus_ or _Brontosaurus_. Of the bipeds, _Trachodon_ is perhaps
-best known. It is one of the duck-billed dinosaurs which had an average
-length of about thirty feet. The duckbills were unarmored, active
-animals, good swimmers as well as runners. They were prominent and
-widely distributed during late Cretaceous time. Many skeletons have been
-found in western North America. Natural casts and impressions of
-mummified remains indicate that the hides were scaly and the feet
-provided with webs between the toes. The bill was broad, flat, and
-toothless, but the sides of the mouth were provided with a large number
-of simple teeth closely arranged in parallel rows. The fine skeleton
-exhibited in our hall is thirty feet six inches in length. Near
-relatives of _Trachodon_, such as _Corythosaurus_ had hollow, bony
-crests, combs, or tubular structures on top of the head. These may have
-been of some service in connection with breathing while feeding under
-water.
-
-    [Illustration: A Duck-billed Dinosaur of the Cretaceous Period
-    (_Trachodon mirabilis_)]
-
-    [Illustration: Stegosaur (_Stegosaurus stenops_)]
-
-Among the quadrupedal vegetarians an interesting family is represented
-by _Stegosaurus_, a late Jurassic dinosaur having a length of about
-twenty feet. These creatures had heavy limbs, all used in walking, an
-arched back, and almost no brain at all. A double row of large flattened
-plates standing upright and extending from the rear of the skull nearly
-to the tip of the tail provided some protection for the back of the
-animal, but otherwise there was no defensive armor. Several long spikes
-at the end of the tail probably served as weapons. The mounted skeleton
-in our collection was obtained from Garden Park, near Canon City,
-Colorado, a district which has long been famous for dinosaur remains.
-
-The ankylosaurs were more completely armored with closely set bony
-plates fitting neatly over the body. They were of about the same size as
-the stegosaurs but the body was broad and somewhat flattened. These
-armored quadrupeds apparently lived only during the Cretaceous period,
-after the disappearance of the stegosaurs. Their tooth equipment was
-very poor and in a few cases entirely lacking. _Ankylosaurus_ and
-_Nodosaurus_ are good examples of the type. They have been described as
-animated tanks and are sometimes referred to as having the appearance of
-enormous horned toads.
-
-Among the last of the dinosaurs to come and go were the horned
-quadrupeds known as the Ceratopsia. Their entire history appears to have
-been confined to the Upper Cretaceous and the closing stages of the
-reptilian era in America. _Triceratops_ and _Monoclonius_ are well-known
-representatives of the group. Besides the horns, which appeared above
-the eyes or near the center of the nose, there was a broad, flattened,
-backward extension of some of the skull bones which produced a great
-frill or collar reaching over the neck as far back as the shoulders.
-This frill, combined with the large skull, gave the animal the
-appearance of being nearly one-third head. _Triceratops_ had three
-horns, _Monoclonius_ only one. The average length of the animals was
-slightly under twenty feet.
-
-Although very little is known about the ancestry of the horned dinosaurs
-a valuable discovery in Mongolia may throw some light on the subject. A
-small dinosaur with a well-developed frill, but no horns, once inhabited
-the region of the present Gobi desert, and in recognition of the
-apparent relationship it has been named _Protoceratops_. In addition to
-numerous skeletons, several nests of eggs were found in association with
-the bones. Until this discovery was made, dinosaur eggs had been
-practically unknown. A reproduction of one of these nests is among our
-exhibits.
-
-    [Illustration: A Sea Turtle of Cretaceous Time (_Protostega gigas_)
-
-    This marine animal belongs to a group which became extinct near the
-    close of the great reptilian era, but a few related forms still
-    survive. Their weight is greatly reduced by the peculiar
-    construction of the shell, and the front feet are enlarged for use
-    as oars, an excellent illustration of the manner in which a land
-    type can become adapted to life in the sea.]
-
-With the possible exception of a very few short-lived survivals
-dinosaurs were extinct before the opening of the Age of Mammals, many of
-them for millions of years. Along with them went other types of ancient
-reptiles, and the cause of their extinction is a problem which may never
-be solved. Conditions remained favorable for the turtles, which made
-their first appearance during Triassic time, and for the crocodiles,
-which date back to the Jurassic period. Snakes were only at the
-beginning of their history as the era closed. The survival of these
-modern forms suggests that they were favored to a greater extent than
-the dinosaurs during a prolonged period of changing conditions the full
-details of which are unknown to us.
-
-In general it is to be expected that disaster would first overcome the
-highly specialized creatures, such as the dinosaurs, which had become
-more delicately adjusted to the particular environments in which they
-lived. It appears that some of them had been too progressive up to a
-certain point, but not sufficiently adaptable to get beyond that stage,
-or fortunate enough to make their advances in directions that could be
-followed, through fluctuations in the matter of food supply, predatory
-enemies, climate, and other factors which bear upon success and failure.
-
-The reptilian era closed with exceptional volcanic activities in many
-parts of the world, but these cannot account for the disappearance of
-the highly diversified and abundant reptilian life. The eruptions were
-merely incidental to movements and readjustments in large masses of rock
-comprising the earth's crust or surface. Such crustal folding and
-elevations always have been of serious consequences to both plants and
-animals because of their effect upon drainage and climate. There were
-disturbances of this kind in western North America in late Jurassic
-time, with folding and uplift in the region of the Sierras and probably
-extending from Mexico to southern Alaska. A great trough to the east of
-this elevated district was produced in the course of these movements and
-provided access to the sea from south to north. During the Cretaceous
-period there were repeated invasions and retreats of the sea by way of
-this great depression, consequent upon slight changes in the elevation
-of the floor. Hence there are numerous marine formations in Colorado and
-adjoining states, some of them rich in fossils.
-
-Before the close of the Cretaceous period the sea had made its final
-departure from this region, and the Mesozoic era was terminated by
-revolutionary disturbances which brought about the uplifting of a new
-mountain system. The Rocky Mountains may be regarded as part of this
-system and to have had their birth at this time. The Rockies, however,
-show unmistakable signs of repeated elevation, with intervals of erosion
-during which there was great reduction of their total height. What we
-see of them today is the result of more than fifty million years of
-continuous geological activity.
-
-
-PLANT LIFE AND CLIMATE
-
-Some idea of the Mesozoic climate is obtained from the character and
-distribution of the plant life. Triassic floras are not large and there
-is very little fossil evidence for the earlier half of the period. It is
-quite possible that arid or desert conditions prevailed for a time in
-much of North America, as at the close of the Paleozoic era. Plant life
-was at first not abundant, and conditions were unfavorable for the
-production of fossils. In Upper Triassic rocks of Virginia, however,
-there are signs of swampy conditions, with rushes and ferns
-predominating. Adjoining forest areas were well timbered with large
-coniferous evergreens which show no annual growth rings, as similar
-trees do in regions where cold winters alternate with warm summers. This
-suggests, for that time and place at least, a uniformly warm climate,
-lacking seasonal variations. Warm temperature or subtropical climates
-are indicated again by some of the Jurassic and Cretaceous plants, but
-intervals of lower temperatures and variable climates are also apparent.
-Palms, figs, and other trees, very similar to modern types now living
-only in warmer regions, were widely distributed in late Cretaceous time,
-and their range was extended into regions which have since become too
-cold to support such growths.
-
-The trend toward modern forms in the plant world was gradual, but
-throughout the era there were occasional novelties that attract the
-attention of botanists. Ferns and horsetail rushes, reminiscent of the
-Paleozoic forests, soon began to lose their prominence as the
-seed-bearing trees gained the ascendency. Mesozoic time could well be
-called the age of cycads, because of the striking performance of this
-plant group. Different varieties flourished in the three periods, with
-the Jurassic standing out as the time of greatest abundance.
-
-To the uninitiated, the usual cycad fossils resemble "petrified
-pineapples," but these are merely the scarred stems or trunks of small
-to medium-sized trees with a tufted arrangement of leaves at the top,
-and usually without branches. Foliage and habit of growth suggest
-something more like large ferns or low-growing palms, with short, thick
-trunks seldom more than fifteen feet tall and many of them under three
-feet. The leaves are rarely found entire or attached to the trunks, but
-occasional discoveries indicate a leaf-length of about ten feet.
-Although they are classed among the first and lowest of seed-bearing
-plants, and in this respect are related to the conifers, their
-appearance was quite unlike that of the modern cone-bearing evergreens.
-
-More nearly resembling the common conifers of today were the sequoias,
-of early Mesozoic origin and far more abundant during Cretaceous time
-than they are at present. The maidenhair trees, now represented by a
-single species of _Ginkgo_ which is cultivated principally in China and
-Japan, were never very prominent but are of interest as an ancient
-family that persisted throughout the Mesozoic and down to our own time.
-Before the close of the Cretaceous period the flowering plants had
-greatly outnumbered the spore-bearing groups, such as the ferns and
-horsetails which were formerly so abundant. We know little of early
-flowers, however, except in connection with trees, the large gayly
-colored blossoms of the type now conspicuous in woodlands, meadows, and
-gardens being later arrivals and poor subjects for preservation as
-fossils.
-
-Cretaceous floras were surprisingly modern in character, far in advance
-of the animal life. Poplars, plane trees, magnolias, palms, figs, oaks,
-and buckthorns were abundant at the close of the Cretaceous, as
-indicated by fossils of the Laramie formation, which is the surface rock
-in many localities near Denver. Also abundant in various places at this
-time were walnut, hazelnut, laurel, tulip, maple, beech, birch,
-breadfruit, ivy, holly, and many other well-known trees and shrubs.
-Sedges and grasses, which became so important to the herbivorous mammals
-of the next era, made their first appearance in Cretaceous time but were
-then inconspicuous.
-
-
-COAL AND FOSSIL FOOTPRINTS
-
-The abundance of plant life in the Colorado area during the Cretaceous
-period is indicated by the extent of coal deposits of this age. About
-one-fourth the area of the state is underlain by coal seams varying in
-thickness from a few inches to fifty feet or more, most of it being
-Cretaceous. In the northern Colorado district the coal-bearing formation
-is the Laramie. Near Denver there is some coal in the Arapahoe formation
-which overlies the Laramie and is of later age.
-
-Coal mines often produce excellent plant fossils, and occasionally other
-evidence of prehistoric life. In a mine near Canon City, Colorado, a
-series of natural casts of dinosaur feet was taken from the overlying
-rock after the coal had been removed. One of these, in the Denver Museum
-of Natural History, is seen to consist of sandstone inside a very thin
-layer of dark clay. Flattened against the lower surface is the
-carbonized stem of a Cretaceous plant which grew in the swamp where the
-coal deposit was formed.
-
-Since the shape of dinosaur feet is unmistakable we can only assume that
-a large reptile of this type walked over the surface of swampy ground in
-which a great thickness of decaying vegetation had accumulated. A layer
-of mud settled over the top and became sufficiently firm to retain the
-mold of the feet as the animal moved along. Any plant material either on
-the mud or included in it was pushed to the bottom of the impressions
-and flattened out by the weight of the huge creature. Then sand was
-washed into the footprints from some nearby source during a heavy
-rainstorm.
-
-Following these events there was probably a subsidence of the area, and
-a great thickness of rock-making sediments was built over the ancient
-swamp. The buried vegetation gradually became converted into coal, the
-sand consolidated into a firm sandstone, and the mud produced the shales
-forming the roof of the present mine, which is now at an elevation of a
-mile above sea level as a consequence of the general uplifting of the
-Rocky Mountain region during late Cretaceous and subsequent time.
-
-When the coal was removed, the hard sandstone casts separated readily
-from the softer shales surrounding them. A small amount of the shale
-adheres to the sandstone, and some of the flattened vegetation, now in
-the condition of coal, still remains attached.
-
-
-MESOZOIC INVERTEBRATES
-
-As in other eras, the invertebrates fluctuate with the periods.
-Characteristic forms appear, become more or less prominent, then in many
-cases decline or disappear. Variations among the mollusks are
-particularly helpful in the identification of rocks which originated in
-the Cretaceous seas. Clam-like bivalves of the genus _Inoceramus_, the
-straight-shelled ammonids known as _Baculites_, and oysters, are locally
-common in some of the formations exposed a few miles west of Denver.
-
-The ammonids, or "ammonites," were extremely abundant throughout the
-world during Mesozoic time. Their shells were chambered like those of
-the pearly nautilus, a related cephalopod mollusk inhabiting tropical
-seas at the present time. While only four species of the Nautilus tribe
-are living today, thousands of species of ammonids swarmed the
-prehistoric seas. Many new forms came into existence in Triassic time
-but few survived the period. A pronounced revival occurred in the
-Jurassic, only to be followed by a decline and eventual extinction at
-the close of the Cretaceous. Ammonites measuring three or four inches
-across the diameter of the coiled shell were about average size, but
-diameters up to three or four feet were not uncommon. Externally the
-shells were ornamented with ribs, knobs, and spines; inside was a pearly
-lining. The partitions were thin and composed of the same pearl-like
-substance as the lining. Each partition becomes wavy as it approaches
-the shell, and the line of union has a distinct pattern which is seen in
-specimens which have lost the outer shell layer. This wavy suture line
-becomes more complicated in the later members of the race, and the
-peculiar markings developed by the repeated partitions provide a
-convenient method of identification.
-
-The belemnites or ink-fishes, regarded as ancestors of the cuttlefishes
-now living, comprise another group of carnivorous mollusks. These,
-however, had lost the external shell, and the usual fossil is part of an
-internal shell or "skeleton," known as the guard. This limy structure
-has the form of a pointed cigar, and is seldom over a foot long although
-the total body-length of the larger animals was commonly about six to
-eight feet. Several hundred species have been described, the majority
-being of Jurassic age. They declined rapidly toward the close of the
-era.
-
-The invertebrate life of the Mesozoic was strongly dominated by
-mollusks, with cephalopods in first place, the bivalve pelecypods and
-the single-valve gastropods or snail-like forms sharing subordinate
-positions. The dominating trilobites, sea-scorpions, and tetracorals of
-the Paleozoic had disappeared, while the brachiopods and crinoids were
-greatly modified and more like the forms which live today.
-
-Crinoids became moderately abundant at various times, but in many ways
-different from their relatives of the preceding era. Some of the largest
-known species, with stems estimated as fifty feet long, have been found
-in lower Jurassic rocks. A great abundance of microscopic life is
-indicated by the frequent occurrence of chalk in the Cretaceous
-formations. Corals of the modern reef-building type (hexacorals) were
-common in the warm seas of a large part of the world.
-
-The insects of the early Mesozoic are represented by few fossils
-although it is evident some new forms were becoming established. The
-warm climates prevailing throughout much of the world appear to have
-been a favorable factor in the progress of insect life. In addition to
-the older cockroach and dragon-fly types may be mentioned the arrival of
-grasshoppers, cicadas, caddis-flies, beetles, and ants.
-
-Several hundred species are found in Jurassic rocks, and by the end of
-the Cretaceous period most of the insect families now known to us were
-probably in existence. The record is seriously obscured by the erosion
-of rocks which so frequently marks the end of a period, also by the
-small size of the subjects, and by the exceptional conditions required
-for the production of such delicate fossils. Among the last of the
-familiar insects to appear were the bees and butterflies. These
-evidently came in with the more advanced types of flowering plants that
-produce the nectar on which many insects feed. It is probable, too, that
-without the arrival of these insects and their service in the
-pollination of flowers, the floras of today would be rather different
-from what they are.
-
-
-EXTINCT BIRDS
-
-It is not surprising to find that birds made their first appearance in
-the Mesozoic era, for of all animals they are most like the reptiles as
-a class. Feathers are about the only dependable characteristic of the
-entire group, nearly every other feature being matched by some reptilian
-creature of great antiquity. The nesting habit, which includes care of
-the young as well as the eggs, is a matter of progress which relates to
-flight and to warmer body temperature. It appears to have been developed
-by forest-dwelling types living among trees and nesting there in
-comparative safety from enemies prowling on the ground.
-
-The oldest known prehistoric birds were found in lithographic stone of
-Upper Jurassic age. _Archaeopterix_ was discovered in 1861 at
-Solenhofen, Germany. Sixteen years later a similar bird in a better
-state of preservation was found in Bavaria. The latter was named
-_Archaeornis_. These Jurassic fossils are regarded as true birds by some
-authorities, while others believe them to be more nearly related to the
-reptiles, the opinions being based on careful studies of the skulls and
-other skeletal features. Both birds had teeth of reptilian character,
-and it is evident that there was no beak, for the jaws were covered with
-scaly skin. The bony construction of the long tail would suggest lizards
-rather than birds, were it not for the presence of feathers which were
-attached at each side. Head, neck, and parts of the body were covered
-with scales. Wings were well provided with stout feathers but the
-skeletal framework indicates that the birds were gliders rather than
-true flyers. Claws on the wings served like fingers to aid in climbing
-among the branches of trees, a practice which is occasionally noted
-among the young of living birds. In adult birds of today, however, the
-claw-like appendages of the fore limbs are greatly reduced and of little
-service.
-
-The next fossil birds of importance have been found in Cretaceous rocks
-of Kansas, both of them fishers of the seas instead of forest
-inhabitants. _Ichthyornis_ was a small bird, standing about eight inches
-in height, a powerful flyer with reptilian jaws and teeth. _Hesperornis_
-was built for diving and swimming, like the loon, but was somewhat
-larger and provided with teeth. Its wings were too poorly developed to
-be of use in flying.
-
-Toothed birds became extinct with the close of Cretaceous time, and the
-ancestors of modern types were in existence before the Age of Mammals,
-but fossil remains are few and poorly preserved. Large ostrich-like
-birds, however, are known to have lived in North America during the
-Eocene period. One of these, named _Diatryma_, stands nearly seven feet
-tall in the reconstructed skeleton. Its legs are heavy, wings greatly
-reduced, beak massive. In its relation to modern birds it is possibly
-nearer to the cranes than the ostriches.
-
-Flightless birds of large size are known from many parts of the world
-and seem to have been prominent throughout the Cenozoic era, as they are
-today in the southern hemisphere. _Aepyornis_ lived in Madagascar during
-the Pleistocene period and may have become extinct quite recently. Its
-eggs are the largest known among fossils, several times the size of an
-ostrich egg. Also in this period the moas were living in New Zealand
-where their remains are still abundant. One of the largest, known as
-_Dinornis_, had about the same form as _Diatryma_ but the neck was
-longer, head and beak smaller, legs better fitted for running, height
-about eleven feet.
-
-A much smaller flightless bird, the dodo, became extinct in modern time.
-This former inhabitant of Mauritius and other islands of the Indian
-Ocean was related to the doves and pigeons, and had lost its power of
-flight through disuse of the wings. It was a clumsy, defenseless bird
-weighing possibly as much as fifty pounds. Actual remains are few and
-incomplete, and descriptions published by the explorers who knew the
-bird two centuries ago are not entirely trustworthy. In the Pleistocene
-Rancho la Brea beds of California the largest of all prehistoric flying
-birds has been found, a vulture bearing the name of _Teratornis_.
-Re-assembled skeletons show them to be slightly bigger than existing
-condors.
-
-
-ANCESTORS OF THE MAMMALS
-
-The monotremes or egg-laying types of mammals such as the duck-bill and
-spiny anteaters which now inhabit Australia are almost unknown as
-fossils. Marsupials, the next higher living group, which includes the
-opossum and kangaroo, appeared at the end of Cretaceous time along with
-the placentals or higher mammals which dominate the history of the
-Cenozoic era. Nevertheless, there are a few teeth and jaws from rocks of
-Triassic and Jurassic age to indicate that small mammals, from the size
-of mice to slightly larger than rats, existed throughout most of the Age
-of Reptiles. There is no complete skeleton of any of the earlier forms,
-and little is known of their relationships either with living orders of
-mammals or with probable ancestors among the reptiles. The record
-becomes somewhat clearer toward the end of the era but it is obscured
-again by the great disturbances which followed.
-
-Looking back among earlier land animals for the origin of the first
-mammalian stock it is necessary to go as far as Permian or even
-Carboniferous time. Reptiles then living had many structural features in
-common with mammals, and mammal-like forms continued to flourish until
-late in the Triassic. An interesting group of such animals, named
-therapsids, was one of the earliest reptilian stocks to appear, and is
-well known from fossils found in the Red Beds of Texas and New Mexico,
-in Europe, South Africa, and Asia. Quite a variety of types is included
-in this group, with many advances in dentition, and modifications of the
-skull, limbs and pelvic construction which strongly suggests a
-relationship to the mammals.
-
-    [Illustration: Murals Over Fossil Exhibits, Hall of Mammals
-    Top: Eocene; Protylopus, Tanyorhinus, Patriofelis, Uintatherium,
-    Turtle, Crocodile, Eohippus.
-    Middle: Upper Oligocene; Mesohippus, Merycoidodon, Hoplophoneus,
-    Metamynodon, Poebrotherium, Trigonias.
-    Bottom: Pliocene; Teleoceras, Turtle, Synthetoceras, Amebelodon,
-    Teleoceras.]
-
-
-
-
-                           THE AGE OF MAMMALS
-
-
-The striking feature of life development in the Cenozoic era is the
-great progress and expansion over the earth of the mammalian races. The
-division of the era into periods, however, was based largely on a study
-of fossil mollusks. In the Paris basin of France, it was noticed by the
-geologists of a century ago that the youngest of the sedimentary beds
-contained the greatest number of recent or still living species.
-Successively downward into the older beds the percentage of recent
-species decreased until there were practically no living species
-represented in the oldest rocks of the series. From the percentage of
-recent forms among prehistoric ones it was proposed that the following
-division be made: Eocene, meaning _dawn of the recent_; Miocene, meaning
-less recent; and Pliocene, meaning _more recent_. Sometime later it was
-suggested that another period be added, and to this was given the name
-Pleistocene, meaning _most recent_. In 1854, the older Miocene
-formations were segregated and referred to a newly provided Oligocene
-period, this name meaning _little of the recent_.
-
-Early geologists grouped the rocks in three great divisions, applying
-the names Primary, Secondary, and Tertiary. To these was added
-afterwards the name Quaternary, which applied to the youngest formations
-of the earth. Only two of these terms remain in common use at present:
-it is a frequent practice to refer to the combined Eocene, Oligocene,
-Miocene and Pliocene periods as the "Tertiary" division of Cenozoic
-time; to the Pleistocene and Recent periods as the "Quaternary"
-division. The geology of some remote future may be clearer with regard
-to the full significance of this subdivision of the Age of Mammals into
-two parts. It may be that a great era was concluded at the end of
-Pliocene time as others have been concluded, by the usual earth
-disturbances and climatic changes and by the decline of animals once
-prominent in the faunas of the world. Events of such character have
-registered their occurrence but may eventually prove to have been a
-series of minor events not comparable with the revolutionary changes
-that terminated other great time divisions. The favored practice of
-including ourselves and our times in the Cenozoic is based on a trend of
-opinion which holds that no great era has been ended since the Age of
-Reptiles was concluded.
-
-    [Illustration: Skulls of the clumsy, six-horned uintathere and the
-    early, hornless titanothere form part of this Eocene display. In the
-    mural these animals and the little "three-toed" Eohippus, smallest
-    of horses, are pictured with a contemporary turtle and crocodile.]
-
-The oldest of Eocene rocks show a great variety of mammals and a strange
-assortment of forms far in advance of the Mesozoic record. Ancestries
-and successive stages of development have been only partially worked
-out, though details have been better preserved for some of the groups
-than for others. Some oddly shaped creatures such as the uintatheres
-reached their full development in a relatively short time and passed out
-of the picture before the end of the period. These animals, represented
-in our collection by _Uintacolotherium_, acquired large bodies and many
-horns, but a peculiar tooth equipment fitted them for a special diet
-which apparently failed to be supplied in sufficient abundance at a
-critical time.
-
-On the other hand, we find in this period the ancestors of more
-successful groups, some of which continue on into modern times. Only a
-few of these histories can be traced in a brief sketch, but in a general
-way it may be stated that the successful races had modest beginnings and
-that they developed very slowly into what they are now, by a process of
-adjusting themselves, or by becoming better adapted to new or previously
-unused conditions in their respective environments. In their early
-stages the various types had much in common; they were generalized,
-rather than specialized for any particular kind of existence. What they
-were fitted for is best indicated by their teeth and feet, though other
-structural features frequently contribute valuable information. The rise
-of mentality is indicated by skull capacities and the increasing
-development of the upper lobes of the brain, as revealed by casts taken
-from the interior of skulls.
-
-The creodonts were the earliest and most primitive of the flesh-eating
-mammals or carnivores. Many of them were small of body and brain, and
-equipped with teeth that indicate a mixed or largely insectivorous diet,
-or possibly the habit of feeding on carrion. Although there was
-considerable variation among them, and some tendency toward
-specialization, there was little to suggest the coming of more
-progressive groups such as the cats and dogs, with teeth perfected for
-the tearing and cutting of flesh, and feet especially fitted for the
-life of hunters. The ancestry of the cats cannot be traced farther back
-than the Oligocene but it probably connects somewhere prior to that time
-with the creodonts.
-
-_Cynodictis_, an Oligocene carnivore slightly under two feet in length
-is commonly regarded as a primitive dog, but its characters are so
-generalized that it probably differs but little from the ancestors of
-many other carnivores. The skeleton of this animal suggests a slender
-and flexible body like that of the weasel, with somewhat shortened limbs
-and a long tail. It lived in forested regions and was probably more or
-less of a tree dweller. The more advanced carnivores required longer
-legs, better adapted for running and overtaking the prey, which is the
-dog's way of hunting, or for stalking and springing upon the quarry,
-which is the method of the cat.
-
-    [Illustration: Moropus (_Moropus cooki_)
-
-    Though its teeth clearly indicate a diet of plant material, this
-    strange animal had claws on its toes, like the carnivores. It is
-    probable that the claws were used in digging for roots, as indicated
-    by the artist.]
-
-A prominent group of mammals today is that known as the ungulates, or
-hoofed animals, which includes the horses, cattle, deer, swine,
-rhinoceroses, tapirs, and other types both living and extinct. Their
-probable ancestors were the condylarths, primitive ungulates of the
-Eocene period. One of these, known as _Phenacodus_, serves well to
-illustrate the general character of the early hoofed mammals. It was
-about five and one-half feet long, rather large for its time, with long
-tail and short limbs, low elongated skull and small brain, very similar
-in many respects to the creodonts or ancestral carnivores. The teeth,
-however, were partially of the grinding type so essential to the welfare
-of plant feeders.
-
-The condylarths were five-toed animals and evidently provided with small
-hoofs, but the more progressive ungulates soon lost one or more of the
-toes, and a division of the group into odd-toed and even-toed branches
-became firmly established. Consequently, the families of ungulates
-having one, three, or five toes are classed together as being closely
-related to one another, and those having two or four toes are segregated
-in a second lot. The odd-toed clan, known as perissodactyls, included
-such animals as the horses, rhinoceroses, tapirs, and titanotheres, each
-of these types being placed in a separate family. The even-toed clan has
-been treated in a similar way and named the artiodactyls. In this
-division are such families as swine, cattle, deer, camels, oreodonts,
-and others. The odd-toed group dominated among the larger animals of
-North America for a time but has been completely replaced by the
-even-toed division which is still flourishing, although some of the
-older families have become extinct.
-
-Among the exhibits of the Denver Museum of Natural History may be seen
-complete skeletons of extinct horses, rhinoceroses, titanotheres, and
-chalicotheres representing the perissodactyls. _Moropus_ was one of the
-chalicotheres, an exceptional family which never became very prominent
-although it had a prolonged history and persisted in Europe and Asia
-after its extinction in North America. The family is grouped with the
-ungulates because of many similarities found in the molar teeth, skulls,
-and other parts of the skeletons, but the toes were provided with claws
-instead of hoofs. The use of these claws is somewhat of a puzzle:
-possibly for defense against carnivorous enemies, for dragging down
-branches in order to obtain food, or for digging roots which may have
-been an important part of the diet.
-
-Titanotheres are represented in our collection by the skeletons of the
-large, horned type which was the last of the race and destined to
-extinction by the middle of the Oligocene period. Smaller hornless
-varieties of Eocene time are illustrated by skulls. This family of
-ungulates had an unprogressive dental equipment, and a small brain in a
-flattened skull. The molar teeth readily distinguish the group from
-other ungulates and enable us to trace the relationship between earlier
-and later varieties. These teeth were of a type which is soon destroyed
-by wear, and it is evident that the animals survived only so long as
-their environment provided them with an abundance of soft vegetation.
-
-    [Illustration: Titanotheres of Oligocene Time
-
-    The name of these animals refers to the large size though they were
-    greatly exceeded in bulk by the mastodons and mammoths of later
-    periods. Ancestral titanotheres, dating back to the Eocene, were
-    hornless animals of much smaller size. These splendid specimens were
-    obtained in Weld County, Colorado.]
-
-The large assortment of rhinoceros material provides an idea of the
-great abundance and variety of forms in this family which was once
-prominent in North America but no longer among the inhabitants of that
-continent. Some of the mounted skeletons have been restored on one side
-to show how these animals appeared in the flesh.
-
-Of the even-toed ungulates there are also several types illustrated by
-complete skeletons. _Merycochoerus_, the subject of one of our mounted
-groups, represents the oreodonts, a large family of mammals whose
-history begins with the Upper Eocene and ends in the Lower Pliocene. The
-oreodonts were small animals, rather pig-like in form and quite common
-in the western plains region shortly after the time of the titanotheres.
-Ancient swine are represented in our exhibits by two mounted skeletons
-which were obtained from northeastern Colorado, where the bones were
-found associated with rhinoceros and titanothere remains. Some of these
-animals were of very large proportions, and the entire family is
-commonly known as the "giant pigs."
-
-Camels and closely related forms were quite abundant in North America
-from early Oligocene to comparatively recent time. Numerous types were
-developed during the course of their history, some small and delicately
-formed, others tall and clumsy and much like the giraffe in structure.
-Parts of many of these creatures have been found but the only completely
-prepared skeletons in our collection are of the little gazelle-camel,
-_Stenomylus_, from Lower Miocene deposits in northwestern Nebraska.
-Pleistocene bisons are represented by several complete skeletons and
-numerous skulls and horncores, some of the species showing an extreme
-development in the length of horns. With two of the bison skeletons are
-shown prehistoric weapon points, found with the bones and indicating
-that these animals were hunted by primitive men at some time near the
-close of the Ice Age. The artifacts first discovered near Folsom, New
-Mexico, by field workers of our Museum, have become known to
-archeologists as Folsom points.
-
-
-PREHISTORIC HORSES
-
-The past history of horses is well known from an abundance of fossil
-material, ranging in age from the Eocene down to the present. Modern
-horses have only one toe in each foot, but there are remnants of two
-additional toes which may be seen only in the bony structure underlying
-the skin. Most of their ancestral relatives were three-toed as far back
-as the Oligocene period. During Eocene time, however, there was a stage
-which may be regarded as four-toed although it was evidently a temporary
-condition, linking known horses with more remote forms having five toes.
-
-    [Illustration: Oligocene Mammals From Weld County, Colorado
-
-    The giant pigs (_Archaeotherium mortoni_) at the left of the group,
-    and the rhinoceros (_Trigonias osborni_) were common animals of the
-    western plains region at one time.]
-
-_Eohippus_, the "dawn horse" as it has been called, is one of the oldest
-and best known of the American horses. Its relation to existing members
-of the family can be traced by means of changes in tooth structure as
-well as in the gradual reduction in the number of toes that is seen
-among intermediate forms. Its ancestors some day may be positively
-identified in that group of generalized, primitive, five-toed, hoofed
-mammals which are known to have lived at the beginning of the mammalian
-era, but such identification has not yet been established. Even
-_Eohippus_ bore little resemblance to the familiar horse of today. Its
-height was only eleven inches, and in body form it had much of the
-appearance of a modern dog. There were four toes on the front foot, one
-of them decidedly shorter than the others but complete in all its parts,
-and evidently capable of service in carrying a portion of the animal's
-weight. The hind foot had three complete toes and a tiny remnant of a
-fourth which could not have been apparent externally.
-
-As changes in the structure of the feet progressed, the central toe of
-the original five continued to increase in size while the adjacent
-digits became relatively shorter and eventually so reduced in length
-that they could touch the ground no longer. The smaller bones at the
-extremities, corresponding to the joints of our fingers and toes,
-eventually disappeared from the side toes. Then the longer bones of the
-outer digits lost the broadened supporting surface, where the missing
-toes had been attached, and became reduced to pointed remnants known as
-splints. Extreme shortening of the splint bones eventually leaves only a
-small knob which is often referred to as a rudimentary toe. In the
-skeleton of a large horse the splints are readily seen, but in some of
-the earlier species they are so small that they may easily be destroyed
-or overlooked by the collector who removes the fossilized material from
-the surrounding rocks. Even then, the bones of the wrist and ankle may
-indicate in an unmistakable manner that an additional toe once was
-present, for each bone is supported by another, and at the point of
-attachment there is a characteristic surface whose purpose is usually
-obvious.
-
-Throughout the Cenozoic era the changes continued. Among the horses of
-the North American Oligocene were _Mesohippus_, approximately the size
-of a collie dog, and _Miohippus_ which was slightly larger. Both were
-three-toed, but the rudimentary splint of a fourth toe was still present
-in the front foot. _Parahippus_ and _Merychippus_ carried on during the
-Miocene period, the latter being characteristic of the time, and
-showing, in addition to other progress, a decided trend toward the
-modern structure of molar teeth. There was some increase in size but the
-largest horse of that period was hardly more than a small pony.
-
-    [Illustration: A Pleistocene Horse of the Texas Plains (_Equus
-    scotti_)]
-
-_Hipparion_ and _Protohippus_, living during Upper Miocene and Pliocene
-time, represent later stages of the three-toed condition. The side toes
-were completely formed but greatly shortened, only the central toe
-touching the ground. In some of the species the outer toes had also
-become very slender, approaching the splint condition. By this time the
-molar teeth were longer and better adapted for feeding on grasses which
-were becoming sufficiently abundant to attract some of the forest
-dwellers into the open country.
-
-During the Pliocene period, in the genus _Pliohippus_ and also in
-_Hipparion_, the feet were far advanced in structure, with most of the
-species single-toed, the side digits having reached the splint stage.
-Pleistocene horses of the genus _Equus_, like living species of that
-genus, were strictly one-toed animals, ranging over grassy areas and
-highly specialized for a life in that kind of environment.
-
-Specialization is to be noted partly in the foot and leg structures
-where the modifications have contributed to greater speed and travelling
-ability. This is of great service to an animal of the plains where food
-and water are often scarce, and great distances frequently have to be
-covered in order to obtain sustenance. The horse, as we know it, is
-built for speed, its limbs and feet being elongated to permit a greater
-stride, and also modified to decrease the weight without loss of
-strength. The ordinary ball-and-socket joint is replaced by a
-pulley-like construction which limits the direction of movement but
-provides an excellent mechanism for locomotion, especially over flat,
-open ground. Flexibility in other directions is sacrificed for greater
-strength, and the foot incidentally becomes less suited for other
-purposes.
-
-This is what is meant by "specialization"--a departure from
-"generalization." The study of fossils provides numerous illustrations
-of specialized development which contributes greatly to an interest in
-prehistoric life. Any specialized structure or habit which increases
-fitness for a particular way of living is also known as an "adaptation."
-Quite in line with the idea of specialization and adaptation is the
-change which occurred in the construction of the horses' teeth, for the
-dental equipment of the modern grazing animals differs widely from that
-of the browsing creatures which lived on the soft leaves and other plant
-substances of the forests.
-
-    [Illustration: The Structure of Molar Teeth
-
-    The large lower molar of a long-jawed mastodon shows worn and unworn
-    cusps, with the enamel layer forming a heavy border around the
-    central dentine where the surface covering has been worn through. In
-    the grinding teeth of rhinoceroses (illustrated at the right) the
-    crown pattern is quite different, but both types are adapted for
-    softer foods and are similar in having the protective enamel on the
-    outside only. The central tooth shows the condition after the
-    shallow surface depressions have been removed by wear.]
-
-The cheek teeth or grinding equipment of the horses underwent as
-complete a change as the feet. Modification resulted in a new type of
-tooth which enabled herbivorous animals to take advantage of a kind of
-vegetation which was late in arriving and has since become the principal
-diet of the ungulates. The grasses are coarse and harsh as compared with
-the leaves of forest shrubbery, requiring more thorough grinding to make
-them digestible. In addition they contain minute particles of silica,
-which is a highly abrasive mineral that quickly wears down the tooth
-substance, especially the softer materials found in tooth construction.
-An increase in the length of the tooth would offset the excessive wear
-but would not necessarily produce a better mechanism for grinding.
-
-The fulfillment of the new requirements is to be seen in the change from
-what is known as the low-crowned, browsing type of molar, to the
-high-crowned, grazing type. Details of the changes that may be traced
-through millions of years of gradual adjustment become apparent only
-from the examination of a great deal of fossil material. As compared
-with earlier types of construction, a modern molar tooth may appear
-extremely complicated, but the process which brought about the improved
-quality is very simple. A little discussion of tooth structure, however,
-is required to make this clear.
-
-A tooth, as everyone knows, is partly imbedded in the jaw, partly
-exposed outside the gum. In a short-crowned tooth the exposed portion is
-known as the crown, and the part imbedded in the jaw consists of one or
-more roots which are comparatively long. The crown is nearly always
-protected by a thin layer of hard enamel. In a grinding tooth, the
-working surface has a number of more or less prominent elevations known
-as cusps. The enamel layer completely covers this surface until wear
-begins. As the tooth goes into service the signs of use begin to appear;
-the enamel is soon worn from the tops of the cusps, and the underlying
-substance, called dentine, becomes exposed. This is far less resistant
-to wear, and as the enamel continues to be reduced the tooth becomes
-less efficient as a grinding device, partly because of the smoothing off
-of the surface, partly because of the relative softness of the inner
-material which is being exposed in increasing quantity. A very old molar
-tooth of the low-crowned type has a smooth surface from which almost the
-last trace of the enamel has been removed. In many prehistoric animals
-the enamel is of a darker color than the dentine or cement, this
-difference in color enabling one to see at a glance how the teeth are
-constructed.
-
-    [Illustration: Grazing Type of Molar Teeth
-
-    The side view of the bison's molar and premolar equipment
-    illustrates the elongated construction which is common among grazing
-    animals. In the pattern of the grinding surface may be seen a cross
-    section of the enamel layers. One layer surrounds each tooth while
-    two folded "cylinders" of the same material occupy the interior.]
-
-In a long-crowned tooth the roots are usually very short, for much of
-the crown itself is imbedded in jaw bone, and the longer roots are not
-required. Growth of the tooth is usually completed after a few years;
-then as it is gradually worn away it is continuously moved upward by the
-production of new bone under the roots, which slowly fills the bottom of
-the socket and continues to provide the necessary support. An equally
-important difference between the two types of teeth, however, is to be
-seen in the arrangement of the enamel, the long-crowned type being
-provided with this durable substance on the inside of the crown instead
-of having a mere protective cap on the outside.
-
-The more complicated structure was developed from the simpler form by
-the easy method of deepening certain depressions located between cusps
-at the top of the tooth. As the crown of the tooth increased its length
-these depressions remained tucked in, and eventually became deep pits
-roughly cylindrical in shape. In addition to the enamel and dentine, a
-third tooth substance, known as the cement, made its appearance at about
-this time, and we find that quantities of this new material were
-deposited outside the crown enamel and also inside the enamel walls of
-the pit, in this way producing a firmly consolidated structure otherwise
-weakened by deep channels and hollow pockets. The cement differs only
-slightly from the dentine but is deposited while the uncut tooth is in
-the gum tissues of the mouth, the enamel and dentine elements being
-formed earlier in the embryonic tooth before it emerges from the jaw
-bone.
-
-A tooth constructed by such a process, if cross-sectioned through the
-crown, will be found to consist of successive layers of hard and softer
-materials. In living animals the top of the tooth soon wears off and the
-enamel layers stand in higher relief because of their greater resistance
-to wear. A roughened surface of excellent grinding quality is thus
-provided, and as long as the wear continues there remains the same
-relative amount of enamel to retain the roughness, and resist abrasion.
-
-Among the various types of grazing animals there is a marked difference
-in the arrangement and form of the enamel layers. Within a species of
-genus, however, the complicated enamel patterns of the molar teeth are
-consistently similar. In the case of horses especially, these patterns
-provide a most helpful key to the identification of extinct forms. The
-general pattern, in any of the more modern horses, may be understood
-more readily if the wavy enamel layers be regarded as forming a set of
-cylinders with deeply crinkled walls. Near the outer border of the
-tooth, surrounded by a thin layer of cement, is the enclosing cylinder
-which represents the enamel cap of the old-fashioned, low-crowned tooth.
-Inside of this is the central mass of dentine which has been penetrated
-by two of the deep pits previously mentioned. The original enamel cap
-has been depressed into these pits, forming two inner cylinders which
-are filled with cement. Instead of being circular in outline, when the
-cap is worn through at the grinding surface these inner cylinder walls
-are seen to be wrinkled and folded so as to produce a most irregular
-pattern. However, if several teeth of the same kind of horse are
-compared, it will be found that the edges of these cylinders produce
-figures which are remarkably uniform and characteristic for that
-species.
-
-    [Illustration: American Mastodon (_Mastodon americanus_)
-    A true mastodon of the short-jawed type.]
-
-
-MASTODONS AND MAMMOTHS
-
-Elephant-like mammals both living and extinct are classed together in a
-single order bearing the name Proboscidea. Living members of the group
-are the elephants, of which the large Indian and African species are
-best known. Among prehistoric representatives the most frequently
-mentioned in the popular literature of North American animals are the
-following:
-
-The American Mastodon, an immigrant from Siberia which ranged over
-nearly all of the United States and Canada. It was principally a forest
-dweller, rarely found in plains regions, was abundant during the
-Pleistocene period and may have been known to the early American
-Indians;
-
-The Woolly Mammoth, which was about nine feet tall. It ranged over
-British Columbia into the United States and across to the Atlantic,
-disappearing in late Pleistocene time;
-
-The Columbian Mammoth, about eleven feet tall, lived in the early half
-of the Pleistocene period, ranging over the warmer portions of North
-America, including practically all of the United States and much of
-Mexico;
-
-The Imperial Mammoth, reaching a height of more than thirteen feet, and
-becoming extinct in the Middle Pleistocene. It was a western form,
-remains being found from Nebraska to Mexico City.
-
-Originally placed in the genus _Elephas_, the mammoths are referred to
-commonly as elephants, though technically they should not be regarded as
-such. Recent explorations and researches have added greatly to our
-knowledge of these animals but have also caused much confusion with
-regard to scientific names, for many new subdivisions of the larger
-group are now recognized, and it has become necessary to change some of
-the older nomenclature.
-
-    [Illustration: A Long-Jawed Mastodont (_Trilophodon phippsi_).
-    One of the Early American Proboscideans]
-
-The large mammoth exhibited by the Museum bears the impressive name of
-_Archidiskodon meridionalis nebrascensis_. Fifty years ago it might have
-been identified simply as a specimen of the imperial elephant and in
-such case would have received the old name of that species, which was
-_Elephas imperator_. But late in the last century it was proposed that
-the mammoths be recognized by some other name to distinguish them more
-sharply from living elephants. The name suggested for the new genus thus
-established was _Archidiskodon_, in recognition of the more archaic or
-primitive construction of the enamel plates in the mammoths molar teeth.
-The specific name, _meridionalis_, had been given to a kind of mammoth
-which is well known from the southern part of Europe, and the Latin
-name, signifying "southern," had been applied to differentiate this
-species from the northern or woolly mammoth.
-
-This mammoth, however, had disappeared from southern Europe and for many
-years its subsequent history remained a mystery. The late Dr. Henry
-Fairfield Osborn had been engaged in an extensive study of the subject,
-and when the nearly perfect skeleton from Angus, Nebraska, was brought
-to his attention he recognized it as being closely related to
-_meridionalis_, and considered it to be a record of the migration of
-that species into North America. Because of minor variations from the
-typical mammoth of southern Europe he regarded it as a variety or
-subspecies which had descended from the latter, and the subspecific
-name, _nebrascensis_, was added to take care of this situation, using a
-Latinized form of the name of the State in which the skeleton was found.
-With the knowledge we now have of these mammoths it becomes apparent
-that _Archidiskodon meridionalis nebrascensis_ is an ancestor of the
-imperial mammoth, currently known as _Archidiskodon imperator_, and not
-identical with it.
-
-This instance is typical of the manner in which prehistoric animals
-obtain their names. Although given a Latin form, these technical names
-are derived from many languages, and the root words are applied with
-reference to anything that happens to appeal to the author as
-significant. Consequently there is seldom a name of this kind which may
-be translated directly into natural history or science. It is a mistake
-to believe that these strange phrases conceal important technical
-information which is available only to those who are familiar with dead
-and foreign languages. Actually they contain nothing of the sort, and
-the most enlightened of the Greeks and Romans could not find it there.
-When a name is needed there is none better than the one provided by the
-specialist who is skilled in the business of naming things. Some
-technical ability is required, to apply the name where it properly
-belongs, but technical knowledge is not obtained from such sources.
-Names, in any form, have another purpose to serve. There is no magic in
-them and there need be no mystery about them.
-
-    [Illustration: Molar Tooth of Mammoth
-
-    This type of tooth is constructed for long continued use and will
-    withstand the wear of more abrasive foods. The position of the white
-    enamel plates is seen in this view of the grinding surface. These
-    plates extend all the way to the base of the tooth, which is of the
-    long-crowned variety and not to be destroyed by the wearing away of
-    a single outside layer of enamel.]
-
-Other specimens in the Museum collection are the long-jawed mastodonts,
-so named because of the elongated jaws and protruding chin which is
-often mistaken for a tusk. Early members of this group had more cheek
-teeth than later types of mastodons, and longer jaws were required for
-their accommodation. Some of them had flattened lower tusks which
-evidently were used for digging purposes. These are popularly known as
-"shovel tuskers." The more modern American mastodon had shorter jaws
-and, like the mammoths and elephants, only one pair of tusks. Both the
-long-jawed and short-jawed types are represented by complete skeletons,
-and also by tusks, jaws, and teeth of many individuals. The American
-mastodons and mastodonts were of about the same size as the smaller
-mammoths.
-
-The difference between mastodons and mammoths is most readily recognized
-in the structure of the grinding teeth, the molars and pre-molars. In
-the mastodon these teeth are of the short-crowned type, while in the
-mammoths, as in the modern elephants, they are long-crowned. The
-difference between these two types of molars has been described with
-reference to horses, and the change from the older to the modern form
-may be regarded as coming about in the same general way, through a
-series of gradual modifications. In both horse and mammoth the final
-development shows internal enamel extending from the grinding surface
-nearly to the roots. Otherwise, however, there is almost no resemblance,
-for the mammoth tooth is made up of flattened enamel plates, the number
-of which is variable for different species. In the jaws of a very young
-individual these plates may be seen as separate parts. As the tooth
-continues to grow, the plates become cemented together, and when the
-ends of the plate are worn down it may be observed that each consists of
-a layer of enamel surrounding a flat central core of dentine. The type
-of construction is rather more obvious in the mammoth tooth than in that
-of a horse, partly because of the larger size, and partly because of the
-relative simplicity of construction.
-
-The earlier history of the Proboscidea is not recorded in the rocks of
-North America, for the group was of African origin and its migrations
-did not extend as far as the New World until middle Cenozoic times. The
-mastodons and mammoths were the largest of land animals since the Age of
-Reptiles, but their Old World ancestors were not conspicuous because of
-their bulk. Many of these ancient forms, even in the earliest stages,
-reveal some of the prominent characters that dominate the entire group.
-None of them, however, should be regarded as a miniature mammoth or
-mastodon, for these highly specialized types were perfected only at a
-comparatively recent date, and by a process that works very slowly.
-Among the earlier forms there were also some oddities which failed to
-survive or to produce a successful branch of the stock such as the
-elephants.
-
-    [Illustration: Nebraska Mammoth
-    (_Archidiskodon meridionalis nebrascensis_)]
-
-The earliest known member of the order was _Moeritherium_, an animal of
-the size of a tapir, living in Egypt during the late Eocene and early
-Oligocene time. At this stage the characteristic specializations leading
-to the mastodons and mammoths were apparent but not far advanced. The
-proboscis was probably much like the flexible snout of modern tapirs,
-for the need of a long trunk had not yet arrived. In upper and lower
-jaws the second pair of incisor teeth were becoming large and prominent.
-The enormous tusks of the mammoths later developed from the enlargement
-of the same pair of upper incisors, and in some of the long-jawed
-mastodonts the lower pair also produced large tusks, though frequently
-the lower tusks were not prominent.
-
-_Dinotherium_ had downward-growing tusks in the lower jaws, none in the
-upper. This genus was fairly common in the Miocene of Europe, Asia, and
-Africa. In the tropics it survived throughout the Pliocene and possibly
-into the Pleistocene. Some of the species acquired the size of
-elephants, but it is apparent that they were not ancestral to any of the
-more progressive types. They are to be regarded rather as an offshoot
-from the main line of descent.
-
-In 1859 only ten species of the elephant-like mammals were known, and
-all were referred to a single genus. At the present time eleven genera
-appear to be well founded, and the number of recognized species has
-reached a hundred, if it has not already passed that figure. New
-discoveries are expected to add to the existing total. With this mass of
-material before us we note certain definite trends among the more
-progressive types. The increasing weight was accompanied by the
-development of strong, upright limbs in which the bones have a columnar
-position instead of the angular assembly which prevails among most of
-the mammals. As the tusks increased in size there was a shortening of
-both skull and neck to bring the weight closer to the point of support.
-The front teeth disappeared except the second pair of upper incisors
-which remain as tusks in the modern elephant. The cheek teeth present in
-the shortened jaws of the mammoth were reduced to one pair at a time in
-the upper set and another pair below. From a simple, low-crowned origin
-these grinding teeth developed into the more successful high-crowned
-pattern with numerous plates of enamel inside. A prehensile upper lip
-acquired the length and usefulness of the elephants trunk.
-
-    [Illustration: Rancho la Brea Fossils
-
-    One of the most unusual of the many animals that have been taken
-    from the tar pits is the large ground sloth, seen at the left in
-    this group. Such sloths were very abundant during Pleistocene time,
-    and some may have lived up to a few thousand years ago.
-    Archaeologists have found indications that these creatures may have
-    been hunted by cave-dwelling peoples of the American Southwest.
-
-    Other skeletons include the saber-tooth tiger, characterized by the
-    long curved upper canine teeth which undoubtedly were used for
-    stabbing and slashing, and the dire wolf, the smaller of the two
-    which are facing the sloth. The artist's reconstruction of this
-    scene also shows the great vulture, Teratornis, which is the largest
-    known bird of flight.]
-
-Over-specialization in the production of tusks appears to have been the
-principal factor in the downfall of the mammoths. The large size of the
-animals and the difficulties of finding sufficient food to sustain life
-must have been a serious handicap at times, but their ability and
-inclination to travel over long distances enabled some of them to find
-tolerable living conditions until the end of the Glacial Period. They
-are now extinct and the nearest living relatives are the elephants,
-somewhat reduced in size of tusks and body but otherwise very similar.
-
-There are many other tribes of mammals whose ancient history is
-partially known though broken by periods of time for which there is no
-fossil evidence. All have undergone changes in which various forms and
-degrees of specialization are featured; this general process is best
-revealed by the horses and elephant-like animals which have left a
-clearer record. For other groups the story would differ but little
-except as to names and specific details.
-
-
-THE RANCHO LA BREA FOSSIL PITS
-
-The La Brea tar pits, as they are often called, provide a remarkable
-record of Pleistocene life in southwestern North America. Scattered over
-an area of about thirty acres just off Wilshire Boulevard in Los
-Angeles, these bone deposits were known, as far back as 1875, to contain
-the remains of prehistoric animals. It was not until 1905, however, that
-their value was recognized by paleontologists. In that year the
-University of California began an investigation, and excavations were
-carried on at intervals by various institutions during the next ten
-years. A great deal of material was acquired by the Los Angeles Museum
-of History, Science, and Art, where many skeletons, skulls, and other
-interesting specimens have been placed on exhibition.
-
-The pits have the form of small craters formed by the seeping of oil
-from the underlying rocks. The seeps appear to have been active during
-part of the Pleistocene period but apparently not at the beginning. The
-oil is rich in asphalt which has served as a preservative for the bones,
-and owing to its sticky properties has been an effective animal trap for
-thousands of years.
-
-The fossil beds at present are of oil-soaked earth and sand. In past
-times there must have been a greater percentage of oil, often concealed
-by a layer of dust or pools of water. The large number of carnivorous
-animals found in the deposits suggests that they were attracted by the
-cries and struggles of creatures wandering carelessly into the asphalt
-and serving as live bait to keep the traps in continuous operation.
-
-Animals found there include many species still living in the locality,
-some that have migrated to other territory, and a large number that have
-become extinct. Among the latter may be mentioned species that differ
-but slightly from living relatives, others that have left no
-descendants. Horses, bison, and wolves, though extinct species, were of
-relatively modern types. On the other hand the large sloths and
-saber-tooth cats seem rather out of place. True cats are represented by
-the mountain-lion, bob-cat, and a species of lion which is nearly
-one-fourth larger than any of the great cats of the Old World. A
-long-legged camel, with a height of approximately eight feet to the top
-of the head, was among the native animals of the district. Skunks,
-weasels, badgers, squirrels, rabbits, bear, deer, and antelope were more
-or less abundant.
-
-The La Brea group exhibited by the Denver Museum of Natural History
-includes the following species: horse (_Equus occidentalis_), bison
-(_Bison antiquus_), wolf (_Aenocyon dirus_), saber tooth (_Smilodon
-californicus_), sloth (_Mylodon harlani_). Horses had entirely
-disappeared from the North American continent by the time the first
-white man arrived. _Equus occidentalis_ was one of the several species
-living during the Pleistocene period, this one apparently being
-restricted to California and perhaps adjacent states. _Bison antiquus_
-was slightly larger than the plains bison of recent times and had it
-horns set at a characteristic different angle. The species was first
-described from Kentucky and appears to have had a wide distribution.
-
-The wolves in this group are about the size of timber wolves, but have
-heavier skulls with less brain capacity, massive teeth especially
-adapted to biting and crushing large bones, and limbs of rather light
-construction. They probably assembled in packs where meat was abundant
-and, hunting in this fashion, were able to attack and overcome the
-larger ungulates and edentates. To most visitors the large ground sloth
-is the most interesting animal of the group. This edentate animal is
-shown at the edge of the pool with one foot stuck in the "tar."
-
-The edentates are a group of primitive animals with very simple teeth,
-if any. Teeth are usually lacking in the front part of the mouth,
-sometimes entirely absent, as among anteaters. Better known living
-representatives of the group are the tree sloths, armadillos, and
-anteaters of South America. Ground-sloths were prominent among South
-American mammals during much of Cenozoic time. During Pliocene and
-Miocene time there was a marked tendency to large size, and it was
-principally during these two periods that they appeared in the United
-States area.
-
-_Mylodon_ was one of the larger North American ground-sloths. Its teeth,
-without the protective enamel which is present among higher mammals, are
-restricted to the cheek region, and have the form of simple pegs;
-instead of being specialized they stand close to the extreme of
-generalization. The construction of the entire skeleton is massive,
-suggesting great strength with slow movements. The hands are well
-developed, provided with stout claws, and must have served the creature
-well as protection against attacks by predatory neighbors. We have some
-idea as to what caused the extermination of the ground-sloths in this
-particular region, but the complete disappearance of such a large and
-widely distributed group at the close of the Pleistocene period is a
-mystery that may never be explained.
-
-    [Illustration: The Folsom, New Mexico, Bison (_Bison taylori_)]
-
-The saber-tooth cat, sometimes referred to as a tiger, was specialized
-as a meat eater though hardly as a hunting animal. In the La Brea region
-its principal food was probably the flesh of the sluggish ground-sloths.
-The size was equal to that of the African lion, with hind limbs slightly
-longer and the front legs more powerfully developed. The most remarkable
-characteristic is to be found in the development of the upper canine
-teeth and modifications of the skull which were necessary to enable the
-animal to use these teeth as weapons.
-
-In order to make the "sabers" effective it was necessary to get the
-lower jaws out of the way, and this was provided for in an unusual type
-of hinge which enabled the mouth to open wider than is possible in the
-case of the less specialized carnivores. Judging by all the structural
-features of the skeleton, _Smilodon_ could not have lived well on small
-animals, for it was not equipped to capture that kind of prey. It is
-evident that large mammals were preferred, and that the method of attack
-was to spring upon the victim and cling there with the powerfully
-developed fore limbs until the kill was completed by stabbing into a
-vulnerable spot. That the position of the large sabers near the front of
-the mouth interfered with normal feeding, is a reasonable conclusion.
-There are also anatomical features which lead to the belief that this
-carnivore was a blood sucker, perhaps more than it was meat-eater.
-
-If most of these conclusions are correct we have here another case of
-over-specialization and a possible explanation of the extinction of two
-species. Such evidence as we have is far from conclusive, for there is
-no proof that Rancho La Brea was the last stand of either the
-saber-tooth or the ground-sloth. Both races were widely distributed and
-their living conditions could not have been exactly duplicated in other
-localities. It has been suggested, however, that _Smilodon_ ate the last
-of _Mylodon_, and starved soon afterward because it had become unable to
-partake of other foods. The conjecture is offered for what it is worth,
-together with the facts on which the story has been based.
-
-The geological record for Pleistocene time is not as complete as one
-might imagine. Numerous localities have produced representative fossils
-but the yield is rarely large enough to solve many of the riddles which
-are constantly arising as investigation proceeds. Aside from those areas
-which bordered the retreating ice cap and where living conditions were
-far from favorable, the sedimentary deposits of this period are not
-continuous over large areas. Many Pleistocene fossils are found in
-stream channel beds which are always subject to removal by subsequent
-floods.
-
-    [Illustration: Early Man in North America
-
-    There is abundant evidence to indicate that the great elephants of
-    Pleistocene time were hunted by primitive Americans whose only
-    weapons were darts or spears tipped with points of stone. A skull
-    and the lower jaws of several mammoths are shown here.]
-
-Isolated patches of fossil-bearing sediments frequently record the
-migration of animals in unmistakable terms, but the details of the
-wanderings and the conditions encountered in the newly established
-habitats are often left in doubt. To correlate the facts revealed at one
-locality with findings at other places and, if possible, to date all
-prehistoric events with a greater degree of accuracy are among the major
-tasks of current investigations.
-
-
-
-
-                             THE AGE OF MAN
-
-
-The Pleistocene or "Ice Age," and the Recent period in which we are
-living at the present moment are not sharply separated by any event
-readily recognized or dated, and the two combined are of very short
-duration as compared with other periods more clearly established by the
-passing of centuries. Together they comprise the Age of Man as commonly
-recognized, with about a million years representing the Pleistocene
-period, some ten to twenty thousand years the Recent. When geologists of
-the nineteenth century suggested that the coming of man should be
-regarded as the beginning of a new era, the name Psychozoic was
-proposed, and to some extent this term has been applied to the present
-period. More in keeping with other period names is Holocene, meaning
-_entirely recent_. Common usage, however, applies the simple term Recent
-to this unfinished chapter which is also without a clear-cut beginning.
-
-Zoologically, man is merely one of the creatures that arrived in the
-course of time, along with other mammals. Just when he arrived and how
-he looked at the time of his coming cannot be determined from a study of
-fossils. Perhaps it is of no importance. There is nothing to indicate
-his existence before the Cenozoic, no completely satisfactory proof of
-existence before the Pleistocene period. As with other inhabitants of
-the earth, it is probable that he became prominent only after a great
-deal of competition with other creatures which kept his ancestors
-submerged for thousands of years. The Ice Age, with its check upon the
-progress of competing animals, undoubtedly gave him an advantage. His
-superior mentality enabled him to overcome adversity by methods not
-available to other mammals; his inventive and mechanical genius must
-have been greatly strengthened by his experience during this interval.
-
-At about this point, where prehistory begins to merge into history, the
-geologist and paleontologist must let other interpreters carry on.
-Archeologists and anthropologists take up the work, and through their
-efforts many details have been added to our knowledge of the human race.
-The study of biology, which is the science of life, has provided an
-instructive viewpoint that enables us to see ourselves against the vast
-background built up by investigations into the nature of the earth and
-its ancient inhabitants. This science deals with living creatures as
-_organisms_--plants and animals so organized as to be capable of
-existence only in an environment which provides exact life requirements.
-
-The Age of Man has been variously characterized as an age of soul, of
-higher intelligence, of culture, and finally, of civilization, freedom
-and democracy. The "crowning glory" of the organic world is pictured in
-history as a creature who has busied himself for thousands of years with
-the building up and tearing down of civilizations. Prehistory reveals
-this habit as something unique in the human character, for there is no
-other organism that has specialized so persistently in the creation of
-its own environment, no other that has had the combined power and talent
-to produce so much change.
-
-More than anything else, the prehistoric record is a lesson in
-adaptation, which in its broadest sense means fitness for life under
-particular conditions, and always subject to organic law. Man's efforts
-to bring about an adjustment between himself and his civilization have
-centered largely on the method of forcing himself into the mold that
-happens to be present, one pattern today, another tomorrow. No creature
-of the past has had to adapt itself to anything so radically new or so
-thoroughly revolutionary. The vital problem now is whether this man-made
-environment will prove helpful or disastrous.
-
-Though one of its names is "culture," it has grown sporadically and
-unevenly, with little evidence of the cultivation that is implied and
-required. Parts have been expanded to extraordinary proportions while
-others equally essential have been retarded in their growth. A more
-intelligent handling of this environment factor seems to be possible,
-and the present mania for "organization" may become tempered with an
-awakening consciousness of organic requirements where organism and
-environment are involved. Once we grasp the idea that "culture" results
-from man's effort to improve his living, by putting into his environment
-something that was not there before--then, surely, this history of a
-billion years of living, and as many "ways of life," should teach us
-something we ought to know as we go into an all-out endeavor to teach a
-whole world how to obtain a one-and-only way.
-
-We may stand at the beginning of an era for which an appropriate name
-has not yet been suggested. Civilization, on the other hand, may provide
-only a minor epoch to be added in some remote time to the story of
-fossils.
-
-
-
-
-                         SUPPLEMENTARY READING
-
-
-The literature pertaining to fossils is widely scattered and usually too
-technical for the layman. It is better to use the resources of the
-nearest library than to feel that a specified list of books is
-necessary.
-
-Any textbook on geology, zoology, or botany will provide helpful
-information. Most books of this type will be found interesting and
-readable if used to solve definite problems suggested by the student's
-immediate curiosity. Very few can be read from beginning to end without
-a great deal of effort and discouragement.
-
-The following have been prominent among the books consulted by the
-author:
-
-_Textbook of Geology_; by Pirsson and Schuchert. This work has undergone
-several revisions and currently appears in two volumes: _Physical
-Geology_ by Longwell, Knopf, and Flint; _Historical Geology_ by C. O.
-Dunbar. Published by John Wiley & Sons. (Historical geology covers the
-entire range of prehistoric life--plant, invertebrate, and vertebrate.)
-
-_Historical Geology_ (The Geologic History of North America); by Russell
-C. Hussey. Published by McGraw-Hill. Concise, interesting, and
-informative.
-
-_Geology and Natural Resources of Colorado_; by R. D. George. Published
-by the University of Colorado. Contains an excellent summary of the
-historical geology and sedimentary formations of Colorado.
-
-_Vertebrate Paleontology_; by Alfred Sherwood Romer. Published by the
-University of Chicago Press. This is one of the most comprehensive and
-up-to-date treatments of the subject for students desiring to go beyond
-the elementary stage.
-
-_A History of Land Mammals in the Western Hemisphere_; by William
-Berryman Scott. Published by The Macmillan Company. This well-known
-account of living and extinct mammals is one of the favorites among
-students.
-
-_The Age of Mammals_; by Henry Fairfield Osborn. A classic in this field
-of literature, but for advanced reading. The book is now out of print.
-
-_The Dinosaur Book_; by Edwin H. Colbert. Published by the American
-Museum of Natural History, New York. An illustrated story of amphibian
-and reptilian evolution.
-
-_Down to Earth_; by Carey Croneis and William C. Krumbein. Published by
-the University of Chicago Press. An excellent popularization of the
-earth sciences--geology and paleontology.
-
-_Lexicon of Geologic Names of the United States_; compiled by M. Grace
-Wilmarth. Bulletin 896 (in two parts) of the United States Geological
-Survey. A rich source of information concerning the age, character, and
-distribution of geologic formations, with numerous references to
-fossil-bearing beds.
-
-_Bibliography of North American Geology_ (including paleontology);
-various bulletins of the United States Geological Survey. Where library
-facilities provide access to the technical literature of museums,
-universities, and scientific societies, this is a valuable aid in
-locating publications dealing with original work in paleontology.
-Bulletins 746 and 747 cover the years between 1785 and 1918; Bul. 823
-(1918-1928); Bul. 937 (1929-1939); Bul. 938 (1940-1941); Bul. 949
-(1942-1943); Bul. 952 (1944-1945); Bul. 958 (1940-1947); Bul. 968
-(1948); Bul 977 (1949). Preparation is a continuous process with recent
-bulletins appearing at one or two year intervals.
-
-_Ancient Man in North America and Prehistoric Indians of the Southwest_;
-by H. M. Wormington. Published by Denver Museum of Natural History, City
-Park, Denver 6, Colorado. Both volumes contain authentic and up-to-date
-accounts of early American cultures.
-
-
-MAPS
-
-_Geologic Maps._ United States Geological Survey: map of the United
-States (1932); map of Colorado (1935). Geologic maps of a few other
-states are available; information regarding these may be obtained from
-state universities or state geological surveys.
-
-  Note: Bulletins of the U.S.G.S. are purchasable from the
-  Superintendent of Documents, Washington, D. C. Maps are sold by the
-  Director of the Geological Survey, Washington, D. C.
-
-
-
-
-                          Transcriber's Notes
-
-
---Silently corrected a few typos
-
---Restored one accidental omission in the Table of Illustrations
-
---Retained publication information from the printed edition: this eBook
-  is public-domain in the country of publication.
-
---In the text versions only, text in italics is delimited by
-  _underscores_.
-
-
-
-
-
-
-
-End of the Project Gutenberg EBook of Fossils: A Story of the Rocks and
-Their Record of Prehistoric Life, by Harvey C. Markman
-
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