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diff --git a/old/55168.txt b/old/55168.txt deleted file mode 100644 index f1fa617..0000000 --- a/old/55168.txt +++ /dev/null @@ -1,3731 +0,0 @@ -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. 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