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diff --git a/40282-8.txt b/40282-8.txt deleted file mode 100644 index 15613ce..0000000 --- a/40282-8.txt +++ /dev/null @@ -1,6075 +0,0 @@ -The Project Gutenberg EBook of Evolution and Classification of the Pocket -Gophers of the Subfamily Geomyinae, by Robert J. Russell - -This eBook is for the use of anyone anywhere 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 - - -Title: Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae - -Author: Robert J. Russell - -Release Date: July 20, 2012 [EBook #40282] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK POCKET GOPHERS--SUBFAMILY GEOMYINAE *** - - - - -Produced by Chris Curnow, Joseph Cooper, Tom Cosmas and -the Online Distributed Proofreading Team at -http://www.pgdp.net - - - - - - - - -Transcriber's note: - - Text enclosed by underscores is in italics (_italics_). - - Text enclosed by equal signs is in bold face (=bold=). - - Throughout, an asterisk (*) before a name denotes extinct - genera/species. - - - * * * * * - - -UNIVERSITY OF KANSAS PUBLICATIONS - -MUSEUM OF NATURAL HISTORY - - -Vol. 16, No. 6, pp. 473-579, 9 figures in text - -August 5, 1968 - - -Evolution and Classification - -of the Pocket Gophers of the - -Subfamily Geomyinae - - -BY - - -ROBERT J. RUSSELL - - -UNIVERSITY OF KANSAS - -LAWRENCE - -1968 - - - - -UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY - -Editors: E. Raymond Hall, Chairman, Henry S. Fitch, -Frank B. Cross, J. Knox Jones, Jr. - - -Volume 16, No. 6, pp. 473-579, 9 figs. - -Published August 5, 1968 - - -UNIVERSITY OF KANSAS - -Lawrence, Kansas - - -PRINTED BY - -ROBERT R. (BOB) SANDERS, STATE PRINTER - -TOPEKA, KANSAS - -1968 - -[Illustration: Look for the Union label] - -31-4628 - - - - -Evolution and Classification - -of the Pocket Gophers of the - -Subfamily Geomyinae - - -BY - - -ROBERT J. RUSSELL - - - - -CONTENTS - - - PAGE - - INTRODUCTION 477 - - MATERIALS AND ACKNOWLEDGMENTS 477 - - TAXONOMIC CHARACTERS 478 - Prismatic character of molars 478 - Character of enamel patterns 479 - Grooving of incisors 480 - Masseteric ridge and fossa 480 - Basitemporal fossa 481 - Specializations of skull 481 - - FOSSIL RECORD 484 - Miocene 485 - Pliocene 486 - Pleistocene 490 - Thomomys 492 - Zygogeomys 496 - Geomys 496 - Pappogeomys 503 - Orthogeomys 504 - - HISTORY OF CLASSIFICATIONS 505 - - CLASSIFICATION 512 - Family Geomyidae 512 - Subfamily *Entoptychinae 513 - Genus *_Pleurolicus_ 514 - Genus *_Gregorymys_ 514 - Genus *_Grangerimus_ 514 - Genus *_Entoptychus_ 514 - Subfamily Geomyinae 514 - Tribe *Dikkomyini 515 - Genus *_Dikkomys_ 516 - Genus *_Pliosaccomys_ 517 - Tribe Thomomyini 518 - Genus _Thomomys_ 518 - Subgenus *_Pleisothomomys_ 519 - Subgenus _Thomomys_ 520 - Tribe Geomyini 521 - Genus *_Pliogeomys_ 522 - Genus _Zygogeomys_ 523 - Genus _Geomys_ 525 - Genus _Orthogeomys_ 528 - Subgenus _Orthogeomys_ 529 - Subgenus _Heterogeomys_ 530 - Subgenus _Macrogeomys_ 531 - Genus Pappogeomys 532 - Subgenus _Pappogeomys_ 534 - Subgenus _Cratogeomys_ 535 - - PHYLOGENY OF THE GEOMYIDAE 536 - Primitive Morphotype 537 - Entoptychid Radiation 540 - Phyletic Trends in Subfamily Geomyinae 542 - Plio-Pleistocene Radiation of Geomyini 558 - Morphotype 559 - Specializations in Genera 560 - Zygogeomys 564 - Geomys 565 - Orthogeomys 568 - Pappogeomys 569 - - LITERATURE CITED 572 - - - - -INTRODUCTION - - -When C. Hart Merriam wrote his monograph of the subfamily Geomyinae in -1895, he had no opportunity to examine fossil specimens. No doubt his -phylogenetic conclusions and classification would have been greatly -influenced had he enjoyed that opportunity because study of fossil -geomyids reveals the historic sequence of phyletic development, and -this sequence provides a firm basis for distinguishing specialized -from primitive characters. The history of the Geomyinae has been -characterized by the evolution of specializations. These evolutionary -trends begin, as we presently know them, with a generalized ancestral -stock in the early Miocene. The direction, degree, and rate of change, -beginning with the primitive morphotype of the subfamily, has not been -the same in the various lineages. The classification within the -subfamily is based upon the phyletic interpretations of available data -and the relationships they disclose. In turn, a new, and I hope more -realistic, phylogeny and classification is offered. - - -MATERIALS AND ACKNOWLEDGMENTS - -Recent specimens were studied of all the known genera, subgenera and -29 of the 36 living species. Most of the species not studied are -monotypic and have restricted geographic ranges. They are: _Geomys -colonus_, _G. fontanelus_, and _G. cumberlandius_, _Orthogeomys -cuniculus_ and _O. pygacanthus_ of the subgenus _Orthogeomys_, and -_O. dariensis_ and _O. matagalpae_ of the subgenus _Macrogeomys_. -Examination of these modern species would not radically change the -estimation of the degree of phyletic development of the genera and -subgenera involved. All of the major polytypic and widespread species -were studied. - -Specimens of the extinct genera _Dikkomys_, _Pliosaccomys_, -_Pliogeomys_, _Nerterogeomys_, and _Parageomys_ also were studied, -as were examples of the extinct species _Geomys quinni_, _Geomys -tobinensis_, and _Orthogeomys onerosus_. Considerable fossil -material of living species, especially of the genera _Geomys_ -and _Pappogeomys_, was used. - -Inasmuch as the present account concerns mainly structural changes in -the subfamily Geomyinae at the level of subgenera and above, and the -temporal sequence of those changes, no attempt is made in the present -account to revise taxonomy below the level of subgenera. Considerable -modification of the classification below that level (for species and -subspecies) is to be expected in _Orthogeomys_ and Pleistocene taxa of -_Geomys_ when available specimens are studied. - -I thank Prof. Robert W. Wilson for his assistance in securing fossil -geomyids for study, and those in charge of the paleontological -collections at the California Institute of Technology, Prof. Bryan -Patterson, formerly of the Field Museum of Natural History, and Prof. -Claude W. Hibbard of the University of Michigan, Museum of Zoology. -For their kindness in lending Recent species, I thank Mr. Hobart M. -Van Duesen of the American Museum of Natural History, Dr. David H. -Johnson of the U. S. National Museum, and Dr. Oliver P. Pearson of the -California Museum of Vertebrate Zoology, the late Colin C. Sanborn of -the Field Museum of Natural History, and Profs. Emmet T. Hooper and -William H. Burt of the University of Michigan Museum of Zoology. - -I am especially grateful to Prof. E. Raymond Hall for his guidance -and helpful criticisms with the manuscript. For assistance with -paleontological problems, I thank Drs. Robert W. Wilson and William -A. Clemens. Several persons have offered helpful suggestions and -encouragement in the course of my study. For assistance of various -sorts I especially thank Drs. J. Knox Jones, Jr., Rollin H. Baker, -A. Byron Leonard, Sydney Anderson, James S. Findley, Robert L. -Packard, and Robert G. Anderson. Advice concerning the drawings of the -dentitions was generously given by Mr. Victor Hogg, and the drawings -were done by Mrs. Lorna Cordonnier under his direction and by Mr. -Thomas H. Swearingen. For assistance with secretarial tasks I thank -Valerie Stallings, Violet Gourd, Ann Machin, Toni Ward, Sheila Miller, -and my wife, Danna Russell. - - -TAXONOMIC CHARACTERS - -Morphological features of the fossils and their stratigraphic -provenience provide the information upon which phylogenetic -interpretations are based. Although the most critical sequences of -the fossil record are lacking, and although the existing fossils are -mostly fragmentary and therefore seldom furnish ideally suitable data -for the interpretations that have been made, phylogenetic conclusions -drawn from fossil materials are superior to those drawn on other -bases. The especially relevant characters are those disclosing -primary trends in the evolution of the modern assemblages. The higher -systematic categories recognized in the following account are based -primarily upon such characters. - -The most important characters found are in the teeth, although several -structural changes in the lower jaw, especially those associated with -the insertion of cranial musculature, are almost as important. - - -_Prismatic Character of Molars_ - -In primitive geomyines the molar consisted of two columns united at -their mid-points and forming a figure 8 or H-pattern (see Fig. 4B). -Both labial and lingual re-entrant folds were formed between the two -columns. The primitive pattern is retained in the premolars of all -known Geomyinae. Therefore, in the earliest (Miocene) members of the -subfamily, the pattern of the molars was essentially like that of the -premolars. - -In Pliocene Geomyinae the two columns of the molars tend to merge into -one. This is evident on the worn occlusal surface of the teeth; the -lateral re-entrant folds are shallow vertically and progressively -recede laterally until only a slight inflection remains. In the final -stages of attrition, the inflection disappears and the tooth is a -simple elliptical column. In the Pleistocene the monoprismatic pattern -appears at earlier stages of wear owing to the decrease in depth of -the re-entrant folds, and in Geomyinae of Recent time the initial -stages of wear on the enamel cap of infants erase the last vestiges of -two columns in the molar teeth. - -The general trend in evolution, therefore, has been from a bicolumnar -to a monocolumnar pattern. The particular patterns of wear -characterizing each genus are described in detail beyond. - -The third upper molar has evolved less rapidly than the first and -second and in one of the modern lineages (tribe Geomyini) tends to -retain at least a vestige of the primitive bicolumnar pattern in -the final stage of wear. Therefore, the loss of any trace of the -bicolumnar pattern in M3 is considered to be a much specialized -condition. Unfortunately, the fossil record of the third upper molar -is less complete than that for the first molar and second molar; the -tooth drops out of its alveolus more often than does any one of the -other molariform teeth and is seldom recovered. - - -_Character of Enamel Patterns_ - -In the primitive genera the enamel pattern is bilophate and the -enamel loop (see Fig. 4B) is continuous on the occlusal surface of -a worn molar. Concomitant with the union of the double columns, the -bilophodont pattern is reduced to a single loph, but the enamel still -completely encircles the dentine. - -In the molars of modern geomyines, the enamel loop is not continuous -but is interrupted on the sides of the crown by vertical tracts of -dentine that are exposed at the occlusal surface of the tooth during -early stages of wear. Therefore, a continuous enamel band is to be -found only in a juvenal individual whose teeth have been subjected to -only slight attrition on the enamel cap. In molars lacking enamel on -the labial and lingual sides, anterior and posterior enamel plates, or -blades, are found on each molar. The premolar also has an enamel plate -on the anterior surface and another on the posterior surface, and in -addition both re-entrant angles are protected by a V-shaped investment -of enamel. One or the other of the various plates can be reduced or -lost accounting for the several distinctive tooth-patterns of the -modern geomyines. If loss occurs, it usually is the anterior plate in -the lower dentition and the posterior plate in the upper dentition, -including the upper premolar. When reduction of the posterior plate of -the upper cheek teeth occurs, enamel is first lost from the labial -side of the tooth, thus leaving only a short vestigial plate on the -lingual end of the crown. - - -_Grooving of Incisors_ - -The incisors are smooth with no trace of a groove in the ancestral -lineage. In the specialized assemblage (tribe Geomyini) pronounced -grooves are always developed on the anterior face of the upper -incisor. The pattern of grooving is constant in each species and thus -provides characters of taxonomic worth for grouping species into -genera. The only inconstancy noted was an incisor of _Geomys_ from the -Tobin local fauna of the middle Pleistocene which has three grooves -rather than the normal two (No. 6718 KU). The extra groove is an -obvious abnormality, and the tooth was associated with others of the -same species from the same quarry that were normally grooved. - -Grooves on the lower incisors are unknown. The functional significance -of grooving has been debated on numerous occasions in the literature. -Grooves appear in a number of only distantly related rodents and in -lagomorphs. The grooving occurs always in small herbivorous mammals, -and in some way may be related to feeding habits. - -The grooves provide a serrated cutting edge on the occlusal edge of -the upper incisor. In the genus _Geomys_, for example, the two -incisors, including the slight space between them, present a total of -five serrations, which may facilitate cutting and piercing tuberous -and fibrous roots upon which _Geomys_ feeds. Also the sulci would -perform the same function as the longitudinal groove on the side of a -bayonet, and would aid the animal in extracting its upper incisors -from coarse, fibrous material. In gathering food, the gopher sinks its -upper incisors into a root, and then, with the upper incisors firmly -anchored, slices off small chunks by means of the lower incisors. -Therefore, in pocket gophers, grooving may be an adaptation for -feeding on fibrous or woody material. Finally, grooves increase the -enamel surface of the incisor without additional broadening of the -tooth itself. There could be a selective advantage for sulcation if -the extra enamel and the serrate pattern strengthen the incisors, -which are under heavy stress while penetrating or prying off pieces of -coarse material. Few broken incisors of pocket gophers are found. - - -_Masseteric Ridge and Fossa_ - -This ridge and fossa are on the lateral surface of the ramus. The -crest on the ridge begins at the base of the angular process and -terminates slightly anterior to the plane of the lower premolar. The -masseteric fossa receives the insertion of the rostral or superficial -division of the masseter muscle. The mental foramen lies immediately -anterior, or anteroventral, to the fossa. - -In the ancestral lineage, the ridge is distinct but relatively low; -the masseteric fossa is shallow and is a poorly developed area for -attachment of the superficial masseter muscle. In modern Geomyinae the -ridge is massive and forms a high crest, especially anteriorly, and -the masseteric fossa is a deep, prominent cup along the dorsal side -of the crest. The elaboration of the crest and fossa evidently is -associated with an increase in size of the superficial masseter -muscle, which enlarges and provides increased power for the propalinal -type of mastication. A high crest has evolved independently in both -modern lineages, Thomomyini and Geomyini. - - -_Basitemporal Fossa_ - -The name basitemporal fossa is suggested here to denote the deep pit -that lies between the lingual base of the coronoid process and the -third lower molar. The basitemporal fossa receives the insertion of -the temporal muscle. The fossa, which until now has not been named, -is a unique feature in advanced Geomyinae, being unknown in either -primitive Geomyinae or in other rodents. - -The temporal is one of several muscles holding the occlusal surface of -the lower molariform dentition firmly against the upper cheek teeth -during mastication. In primitive geomyines that masticate food -by a planing action, the temporal muscle also moves the mandible -posteriorly and food is ground between the enamel plates when the -lower jaw is retracted as well as when it is moved forward. - -The basitemporal fossa appears in late Pliocene geomyines and -increases the attachment surface of the temporal muscles that powers -the planing action important in utilizing woody and fibrous foods. The -basitemporal fossa developed in only one of the modern lineages (tribe -Geomyini), the same lineage in which grooved incisors evolved. -Both features probably are adaptations for feeding on coarse food. -The fossa is not greatly developed in either the ancestral tribe -Dikkomyini or the modern tribe Thomomyini, although in some specimens -a slight depression marks the site of the basitemporal fossa. - - [Illustration: FIG. 1. Types of skulls in the subfamily - Geomyinae. × 1. - - A. and B. Generalized type of skull. _Geomys bursarius lutescens_, - adult, male, No. 77955 KU, 10 mi. N Springview, Keya - Paha Co., Nebraska. - A. Dorsal view of skull. - B. Ventral view of lower jaw. - C. and D. Dolichocephalic type of skull. _Orthogeomys_ (_Orthogeomys_) - _grandis guerrerensis_, adult, female, No. 39807 KU, - 1/2 mi. E La Mira, 300 ft., Michoacán, México. - C. Dorsal view of skull. - D. Ventral view of lower jaw. - E. and F. Platycephalic type of skull. _Pappogeomys_ (_Cratogeomys_) - _gymnurus tellus_, adult, female, No. 33454 KU, 3 mi. W - Tala, 4300 ft., Jalisco, México. - E. Dorsal view of skull. - F. Ventral view of lower jaw. - ] - - -_Specializations of Skull_ - -The skull in most geomyines is generalized, being neither extremely -long and narrow nor short, broad and flat as in specialized skulls -(see Fig. 1). In Pleistocene lineages of the modern tribe Geomyini, -long skulls and broad skulls evolved and have been termed -dolichocephalic and platycephalic specializations, respectively by -Merriam (1895:88-101). He correlated them with two diametrically -different mechanical methods of mastication. - -In animals with dolichocephalic skulls the principal movements of the -mandible in the masticatory process are anteroposterior. The resulting -propalinal action of enamel plates in opposition to each other -characterizes also animals with a generalized skull, and evidently is -the method of mastication in the primitive geomyines, but in animals -with a dolichocephalic skull the method is developed to a high degree -by elongation of the cranium, mandible, and teeth. Both the mandibular -and maxillary tooth-rows are relatively longer than in the generalized -skull, providing a longer block for the planing action of the lower -molariform teeth. All teeth, especially P4 and M3, are longer. In M3 -the heel (posterior loph) in particular is elongated. Both the -anterior and posterior enamel plates usually are retained in M1 and -M2. - -The superficial (or rostral) masseter muscle, originates on the -side of the rostrum and inserts in the masseteric fossa and on the -masseteric ridge. The deep masseter, especially the zygomatic part -having its origin along the zygomatic arch, inserts on the angular -process of the lower jaw. These two divisions of the masseter muscle -have a longer pull (forward) in the dolichocephalic skull than in a -non-dolichocephalic skull. The temporal and diagastric muscles retract -the lower jaws. - -Other, secondary, modifications of the dolichocephalic skull are -shortening of the angular process of the mandible, broadening of the -rostrum, and narrowing of the cranium and zygomata. Depth of the -posterior part of the skull is unchanged. The skull appears to be deep -and of nearly equal breadth from nasals to occiput. A good example of -a dolichocephalic skull is that of _Orthogeomys_ (see Fig. 1, C and -D). - -In the platycephalic skull, the principal masticatory movement of the -mandible is anterooblique, to one side and then to the other. The -oblique passage of the enamel blades of the lower teeth across those -of the upper teeth produces a shearing rather than planing action -(Fig. 1E, F). The anterooblique movement of the lower jaw is possible -because of major architectural changes in the cranium and mandible. -These changes include: (1) Broadening of the postrostral part of the -skull, especially the occiput (mastoidal breadth equals or exceeds -zygomatic breadth in skulls of some taxa); (2) flattening of the -skull; (3) anteroposterior compression of the molariform teeth, -especially the molars. Therefore, the entire maxillary tooth-row is -relatively shorter than in the dolichocephalic skull. Only a vestige -of the heel ordinarily remains on M3. The loss of the posterior enamel -blades of P4, M1, and M2 eliminates unnecessary friction, and each of -these teeth is wider than long. The distance between the posterior -ends of the lower jaws is increased approximately in proportion to the -extent that the occiput is widened. As a result of the flattening of -the skull the angular processes of the lower jaws are lateral to the -zygomatic arches, and approximately on the same vertical level with -them. Consequently the insertions of masticatory muscles are shifted -laterally. This is especially true of the zygomatic division of the -deep masseter, which inserts on the angular process. Contraction of -that muscle division of one side of the skull moves the lower jaws -obliquely forward. The diagastric and temporal muscles of course -retract the lower jaws. - -The platycephalic skull is the most specialized skull in the Geomyinae -and is a result of the new (for the Geomyinae) method of mastication. -The subgenus _Cratogeomys_ (see Fig. 1, E and F) has a platycephalic -skull. The trend toward platycephalic specialization has been the -major feature of evolution in _Cratogeomys_. - - -FOSSIL RECORD - -The fossil record of the subfamily Geomyinae begins in the early -Miocene of western North America. No geomyids have been recovered from -beds of the late Miocene age. Beginning with the early Pliocene the -fossil record becomes progressively more complete, and geomyines are -relatively abundant in deposits of late Pliocene and Pleistocene age. -Although pocket gophers of the subfamily Geomyinae are rare in -lower Miocene deposits, members of the subfamily Entoptychinae are -relatively common and highly diversified. Four genera and a number of -species have been described (see Wood, 1936:4-25), and the subfamily -ranged widely in western North America. I interpret this to mean that -the geomyines were indeed uncommon in the early Miocene and their -distribution restricted since so few of their remains have been -recovered in comparison with entoptychines and the known records are -only from the northern part of the Great Plains. On the other hand, -entoptychines enjoyed a widespread distribution in western North -America (see discussion beyond). Probably the geographic range of the -geomyines was largely allopatric to that of the more specialized -entoptychines. The zone of fossoral adaptation for herbivorous -rodents is ecologically narrow, and as a result competition is severe. -As a rule, the outcome of episodes of intergroup competition is -geographic exclusion. If these rodents were fossorial in the -early Miocene--their morphology suggests they were at least -semi-fossorial--mutually exclusive patterns of distribution are -to be expected. - - -Miocene - -_Dikkomys_ is the only genus of the Geomyinae known from the early -and middle Miocene. _Dikkomys matthewi_ was described by Wood (1936) -on the basis of isolated teeth from lower Harrison deposits -(Arikareean in age) near Agate, Sioux County, Nebraska. Later, -Galbreath (1948:316-317) described the features of an almost complete -mandible recovered from the younger upper Rosebud deposits, now -considered by MacDonald (1963:149-150) to be middle Miocene, near -Wounded Knee, Shannon County, South Dakota. More recently Black -(1961:13) has described a new species, _Dikkomys woodi_, from the -Deep River Formation, Meagher County, Montana. The Deep River Formation -is late Hemingfordian (middle Miocene) in age. No remains of _Dikkomys_ -have been identified in the extensive rodent fauna of the John Day beds -of the lower Miocene of Oregon, although entoptychines are abundant in -these deposits. - -In the present account, _Dikkomys_ is regarded as the ancestor from -which the Pliocene and modern geomyines were derived. These probably -did not evolve from the subfamily Entoptychinae because the dentition -of entoptychines, especially the premolars and third molars, was -already highly specialized by Miocene time. - -The numerous records of _Thomomys_ and especially _Geomys_ reported -from supposed Miocene or Pliocene deposits are without foundation (see -Matthew, 1899:66; 1909:114, 116, 119; 1910:67, 72; 1923a:369; 1924:66; -Matthew and Cook, 1909:382; Cook and Cook, 1933:49; and Simpson, -1945:80). Most of the records of _Geomys_ date back to the description -of _Geomys bisculcatus_ Marsh (1871:121) from the Loup Fork beds of -Nebraska (near Camp Thomas on the Middle Loup River). At first Marsh -and other investigators thought these beds were of the late Miocene -age. Subsequently the Loup Fork fauna was determined by Matthew -(1923b) to be mostly early Pliocene (Clarendonian), but with a later -Pleistocene element. Recently, Schultz and Stout (1948:560) have shown -that the various Loup River faunas and also those from along the -Niobrara River (Hay Springs, Rushville, Gordon local faunas) are of -middle Pleistocene age, the fossil-bearing beds occurring just below -the Pearlette Ash. These beds are those termed the Loup Fork or North -Prong of Middle Loup by the earlier workers who supposed them to be of -Miocene or Pliocene age. Both _Geomys_ and _Thomomys_ have been -recovered from most of these deposits, but they are no older than -middle Pleistocene. This is not surprising in view of the primitive -structure of the geomyids known from Miocene and Pliocene beds, but -the supposed early appearance of _Geomys_ and _Thomomys_ led to much -confusion concerning geomyid evolution in the late Tertiary. - -The dearth of geomyines in the Miocene is counterbalanced by the -relatively abundant and highly differentiated gophers of the subfamily -Entoptychinae. They reached the zenith of their development in this -period. Four genera and a number of species are known from the western -part of the United States, mostly from beds along the Pacific -Coast and in the northern part of the Great Plains. The great -diversification of the group in a relatively short period suggests -prior movement into a new adaptive zone and subsequent specialization -in different subzones and therefore an episode of radial adaptation. -The radiation of the entoptychines is discussed elsewhere in the -account of geomyid phylogeny, but it should be noted here that both -the Geomyinae and the Entoptychinae appear in the fossil record at -about the same time in the early Miocene. The principal distinguishing -features of each of the two lineages were well developed at the time -of their first occurrence, and the entoptychines were the more -successful in early Miocene. The Entoptychinae are known only from -the early and middle Miocene, unless the earlier deposits of the -John Day Formation of Oregon from which mammals have been recovered -are considered to be latest Whitneyian (latest Oligocene); for -correlations, see Wilson (1949:75). Both lineages likely had an -earlier history extending back to their divergence in the Oligocene. - - -Pliocene - -The oldest and most primitive Pliocene geomyine is _Pliosaccomys -dubius_ Wilson (1936:20) from the Smith Valley local fauna of middle -Pliocene (Hemphillian) age in Nevada. According to Wilson (_op. -cit._:15) the beds probably were deposited near the middle of -Hemphillian time. Shotwell (1956:730) recorded _Pliosaccomys dubius_ -from the McKay Reservoir and from the Otis Basin (1963:73) local -faunas of the middle Pliocene (Hemphillian) of Oregon, and Green -(1956:155) has recovered remains of _Pliosaccomys_ (cf. _dubius_) from -the Wolf Creek local fauna, uppermost part of the lower Pliocene (late -Clarendonian in age), of Shannon County, South Dakota. Recently, James -(1963:101) has described a second species, _Pliosaccomys wilsoni_, of -this primitive genus. The new species was found in early Pliocene -deposits (late Clarendonian) from the Nettle Spring local fauna -(Apache Canyon), in the Cuyama Valley, Ventura County, California. -_Pliosaccomys wilsoni_ does not differ greatly from _P. dubius_; -however, the few differences in dental characters seem to warrant -specific recognition. The reduction of cusps on the metalophid of p4 -from three (_dubius_) to two (_wilsoni_) and the lack of accessory -cuspules on the protolophid of p4 in _wilsoni_ are probably -specializations, suggesting that _P. dubius_ even though the more -recent in age is the less advanced of the two. _P. wilsoni_ is known -only from a lower jaw of a young individual that had dp4 in place, -along with m1 and m2. The permanent premolar was in the process of -erupting, and the deciduous tooth was removed so that the unworn -surface of p4 could be examined. - -_Pliosaccomys_ occurred geographically in the area that the -Entoptychinae had occupied in the early Miocene. The Smith Valley -material includes dentitions in almost all stages of wear and the -chronological sequences in the development of the patterns of wear can -be reconstructed. An understanding of the dental patterns of the -primitive geomyines is based mostly on the interpretation of the -stages of wear in _Pliosaccomys_. - -No other pocket gopher is known from the area in which _Pliosaccomys_ -occurred, and it is unknown after middle Hemphillian age. -_Pliosaccomys_ has closer affinities with _Dikkomys_ of the early -Miocene than with any geomyid of the modern assemblage and gives no -clue to the origin of the lineage culminating in the modern pocket -gophers of the tribe Geomyini. - -_Pliogeomys buisi_ Hibbard (1954:353) was found in the Buis Ranch -local fauna, of latest middle Pliocene, on the west side of Buckshot -Arroyo, Beaver County, Oklahoma. The original material included a -right ramus bearing the premolar and first two molars (the holotype) -and five isolated premolars and molars. One of the molars is slightly -worn and from an immature individual. One premolar is a deciduous -tooth. Hibbard (_op. cit._:342) identified the beds from which he -obtained the Buis Ranch local fauna as from the lowermost part of the -Upper Pliocene. Moreover, he judged the Buis Ranch local fauna to be -only slightly older than the Saw Rock Canyon local fauna of Seward -County in southwestern Kansas. Previously (Hibbard, 1953:408-410), -the Saw Rock Canyon local fauna had been assessed as older than -the Rexroad local faunas (latest late Pliocene) and, therefore, -representative of the early part of the late Pliocene. More recently, -Hibbard (1956:164) identified the Buis Ranch beds as part of the -Ogallala Formation, which here occurs unconformably just beneath the -Rexroad Formation (composed of strata nearly all of late Pliocene -age). Therefore, he regarded the Buis Ranch beds as latest middle -Pliocene in age. Hibbard (1954:356) suggested that pocket gopher -remains from the Saw Rock Canyon local fauna were referable to -_Pliogeomys buisi_, and, in effect, tentatively assigned them to -_Pliogeomys_ (in his description of the genus Hibbard remarked that -the upper incisor is bisulcate as in _Geomys_, and the only upper -incisor that he mentions was one of the Saw Rock Canyon fossils -and not part of the Buis Ranch material). _Pliogeomys_ has closer -affinities with modern pocket gophers of the tribe Geomyini than it -does with the middle Pliocene genus _Pliosaccomys_. - -The pocket gopher fauna known from the late Pliocene was more varied -than the faunas known from any earlier time. In addition to the -extinct _Pliogeomys_, which occurs in early late Pliocene (see -discussion above), the living genera _Zygogeomys_, _Geomys_, -_Pappogeomys_ (in the sense used on p. 534), and _Thomomys_ -first appear in the late Pliocene. The only other living genus, -_Orthogeomys_, makes its first appearance in the late Pleistocene. - -The earliest record of the genus _Thomomys_ is based on a fragment of -a left mandibular ramus bearing p4 and m1, _Thomomys gidleyi_ Wilson -(1933b:122), from the Hagerman local fauna of Twin Falls County, -Idaho. Wilson (_loc. cit._) was uncertain as to age (late Pliocene or -early Pleistocene) but subsequently (1937:38 and 67-70) settled on the -middle part of the late Pliocene. Hibbard (1958:11) later considered -the age as early Pleistocene (suggesting that the deposits accumulated -in the Aftonian interglacial interval) but subsequently (Hibbard _et -al._, 1965:512), on the basis of potassium argon age determinations, -also settled on late Pliocene. - -Remains of _Nerterogeomys_ [=_Zygogeomys_] have been found in the -Benson local fauna, Cochise County, Arizona, and the Rexroad local -fauna of Kansas. This early Blancan gopher first was described as -_Geomys minor_ by Gidley (1922:123), and was later referred by Gazin -(1942:487) to his new genus _Nerterogeomys_. Hibbard (1950:138) -identified specimens from the Fox Canyon locality, one of the -localities of Meade County, Kansas, where the Rexroad local fauna is -preserved, as _Nerterogeomys_, and tentatively referred them to the -species _N. minor_. _Nerterogeomys_ cf. _minor_ has been recovered -also from Locality 3 of the Rexroad local fauna (Hibbard, 1950:171) -of Meade County, Kansas. Apparently these are also the small gophers -about which Franzen (1947:58) wrote. She assigned them to the genus -_Geomys_, and they may actually be a primitive form of _Geomys_ that -represents an intermediate stage in the development of the enamel -pattern from the uninterrupted loops of the ancestor to the -discontinuous pattern of modern _Geomys_. I favor this interpretation; -the evidence, however, is inconclusive, and I have, therefore, -reluctantly allocated them, along with the other specimens of -_Nerterogeomys_, to the genus _Zygogeomys_. In an early paper, -Hibbard (1938:244) erroneously referred the same specimens, two upper -premolars of a young individual, to the genus _Thomomys_, and the same -material was identified with the genus _Geomys_, also without specific -assignment, in a later paper (Hibbard, 1941b:278). _Thomomys_ is -unknown from the late Pliocene of the Great Plains. The specimens -previously referred to _Nerterogeomys_ are assigned to the genus -_Zygogeomys_ for the first time in this report; for a discussion of -the systematic arrangement see the accounts beyond. The type and -paratype of _Nerterogeomys_ from the Benson local fauna of Arizona -have no indication of enamel reduction. - -Specimens of the genus _Geomys_ from the late Pliocene were referred -to the large _Geomys quinni_ McGrew, first by Franzen (1947:55) and -later by Hibbard and Riggs (1949:835) and Hibbard (1950:171). _Geomys -quinni_ has been obtained from the Fox Canyon locality and Locality 3 -of the Rexroad local fauna. At Locality 3, both _Zygogeomys_ (cf. -_minor_) and _Geomys quinni_ have been found together, but _Geomys -quinni_ can be distinguished by its much larger size and the advanced -enamel pattern of the cheek teeth (see systematic accounts beyond). -All age classes are represented among the specimens of _Geomys -quinni_; therefore, it seems unlikely that the smaller gophers -referred to _Zygogeomys_ are actually the young of _Geomys quinni_. -Hibbard (personal communication, May, 1966) informed me that specimens -of _Geomys_ from the late Pliocene (Fox Canyon and Rexroad Locality 3) -are erroneously referred to _G. quinni_. According to Hibbard, this -material represents instead two distinct undescribed species, -descriptions of which have been submitted by him for publication. -Allocation of late Pliocene specimens of _Geomys quinni_ to other -species will restrict _quinni_ to the early Pleistocene. - -_Cratogeomys bensoni_ Gidley (1922:123) was of medium size. The name -was based on an upper incisor bearing a single median sulcus and an -associated lower jaw containing all of the cheek teeth from the Benson -local fauna, Cochise County, Arizona. Additional lower jaws carrying -various teeth also were recovered. The specimens might just as well -have been assigned to the genus _Pappogeomys_ since the lower -dentitions of all the genera of the tribe Geomyini have the same -enamel pattern, and the subgenera _Pappogeomys_ and _Cratogeomys_ -have upper incisors with median grooves. The specimens are too -fragmentary to warrant more than generic identification. Mainly -because of their late Pliocene age and primitive traits the specimens -are here regarded as early representatives of the subgenus -_Pappogeomys_. Discovery of the upper molariform dentition would make -a more precise assignment possible. - - -Pleistocene - -Numerous specimens of geomyids from many localities and horizons are -available from the Pleistocene of North America. Specimens of the -genera _Geomys_ and _Thomomys_ are especially common. Few specimens -are known of the genera _Orthogeomys_ and _Pappogeomys_, especially -from the early and middle Pleistocene, owing, probably, to slight -knowledge of the early Pleistocene of México where these two genera -are thought to have evolved (see map, Figure 2). This lack of -knowledge about early Pleistocene deposits in México is a handicap in -the present instance since the center of differentiation for several -of the modern genera is judged to have been in México, probably on, -and at the edge of, the Central Plateau. The relative abundance of the -remains of _Geomys_ and _Thomomys_ from Pleistocene deposits farther -north, and the marked absence of other genera, may mean that -_Orthogeomys_ and _Pappogeomys_ did not range northward from southern -and central México in most of the Pleistocene. One species of -_Pappogeomys_ eventually ranged into the southwestern United States in -the late Pleistocene (toward the end of the Wisconsin) and it occurs -there today, but the genus is essentially Mexican. - -The fossil record of _Zygogeomys_, as the genus is here understood, -evidently continued in the United States will into the Middle -Pleistocene, depending upon the stratigraphic interpretation of the -age of the Curtis Ranch local fauna from southeastern Arizona. Hibbard -(1958:25) regarded the Curtis Ranch local fauna as Irvingtonian in -age, a local fauna that lived either in the late Kansan glacial or the -Yarmouthian interglacial, and his correlation is tentatively followed -here. In deposits laid down later than those of Irvingtonian age no -remains of _Zygogeomys_ have been found. Today a single species exists -as a relic in the mountains of central México and _Zygogeomys_ may -have retreated southward to its present refugium in the late -Pleistocene. Perhaps, _Zygogeomys_ occurred in northern México and the -southwestern United States in the early and middle Pleistocene (see -Fig. 2), occupying the area between the ranges of _Pappogeomys_ to the -south and _Geomys_ to the north. Competition with _Pappogeomys_, and -especially _Geomys_, during Irvingtonian time may have extirpated -_Zygogeomys_ over most of this area, and by late Pleistocene -(Sangamon) much of the former range of _Zygogeomys_ came to be -occupied by one or the other of its competitors. The occurrence of -_Geomys garbanii_ in southern California (see White and Downs, 1961) -and the unidentified species of _Geomys_ in Aguascalientes (Mooser, -1959; for faunal correlation, see Hibbard and Mooser, 1963), both -from deposits of Irvingtonian age, supports this suggestion. - - [Illustration: FIG. 2. Probable distribution of the Subfamily - Geomyinae in the early Pleistocene (late Blancan), depicting - major areas of differentiation of the modern genera. - - 1. _Thomomys_ - 2. _Geomys_ - 3. _Zygogeomys_ - 4. _Pappogeomys_ - 5. _Orthogeomys_ - ] - - -_Thomomys_ - -The earliest Pleistocene records of _Thomomys_ are mostly isolated -teeth. Although they can be identified as genus _Thomomys_, most of -the materials are too fragmentary to be identified to species. In -_Thomomys_ two distinct patterns of occlusal surfaces of the molars -can be recognized: the generalized elliptical pattern in the subgenus -_Pleisothomomys_, not unlike the pattern in other geomyids, and the -pear-shaped pattern in the subgenus _Thomomys_, which results from -constriction of the upper molars on the labial side and constriction -of the lower molars on the lingual side. Some fossils assigned to -_Thomomys_ were not examined with this distinction in mind by -the persons who made the assignments. Consequently some of the -identifications now in the literature may be subject to change. - -Three occurrences of _Thomomys_ are from the early and middle -Pleistocene, with a possible fourth (depending upon the age of the Hay -Springs local fauna of Nebraska). The earliest Pleistocene record is -from the Broadwater-Lisco beds along the North Platte River in Morrill -County, western Nebraska. Possibly the specimen from there was -misidentified. Those beds are Lower Pleistocene, and are regarded by -Schultz and Stout (1948:560-561, 573) and by Hibbard (1958:11), as -having been deposited mostly during the Aftonian interglacial. There -is also some indication that some of the strata were deposited late -in the Nebraskan glaciation. There are no other early Pleistocene -records of _Thomomys_. Savage (1951:228) reported the genus from -the Irvington local fauna, Alameda County, California. The specimens -were not identified to species, although they were described as -indistinguishable from _Thomomys bottae_. Paulson (1961:137) recorded -specimens from the Cudahy local fauna, Meade County, Kansas. These -fragmentary specimens are referable to the subgenus _Thomomys_, owing -to the strong constriction of the molars, but have not been identified -to species. The Cudahy is an Irvingtonian local fauna, and is -considered to have been deposited during the late Kansan glaciation. -The stratum containing the Cudahy local fauna immediately underlies -the Pearlette Ash. The Cudahy material includes five isolated molars -and a fragmentary ramus bearing only the premolar. The genus -_Thomomys_ has been recovered also from the Hay Springs local fauna -in Sheridan County, northwestern Nebraska, by Shultz and Tanner -(1957:71). The Hay Springs local fauna is considered to have been -deposited in late Kansan glaciation or in early Yarmouth interglacial -by Shultz and Tanner (_op. cit._:69), or of Irvingtonian age; however, -Hibbard (1958:25) regarded the beds containing this fauna as Illinoian -(thus post-Irvingtonian in age), and equivalent in age to the Berends -local fauna of Oklahoma and the Butler Springs and Mt. Scott local -faunas of Kansas. The _Thomomys_ from Hay Springs local fauna has not -been referred to species. - -The relative abundance of _Geomys_, and rarity of _Thomomys_, in Great -Plains fossil beds of early and middle Pleistocene is probably due to -allopatric distributions of the two genera. The Great Plains area was -evidently the center of distribution and differentiation of _Geomys_. -Perhaps _Thomomys_ evolved earlier to the west, in the Great Basin and -Pacific Coastal regions, and not on the Great Plains. - -Upper Pleistocene records of _Thomomys_ are more common. The genus -was widespread in beds identified with the Illinoian and Sangamon -and extended its range eastward to the Atlantic Coast. Stephens -(1960:1961) reported _Thomomys_ from the Doby Springs local fauna, -Harper County, northwestern Oklahoma. The material (34 isolated teeth) -was too fragmentary to permit assignment to species. The molars are -constricted on one side, indicative of the subgenus _Thomomys_, like -the Cudahy specimens reported by Paulson (see discussion above). -Stephens erroneously mentioned that the enamel plate on the posterior -face of the upper premolar is unique in _Thomomys_; this plate occurs -also in _Zygogeomys_. The Doby Springs local fauna was recovered from -beds that have been identified as Illinoian deposits, and it is -correlated with the Berends local fauna in Beaver County, Oklahoma, -and the Butler Springs local fauna in Meade County, Kansas (see -Stephens, _op. cit._: 1700). - -Local faunas in Maryland and Florida of Rancholabrean age -include _Thomomys_, in every instance referable to the subgenus -_Pleisothomomys_ on the basis of unconstricted molars. _Thomomys -potomacensis_ (Gidley and Gazin, 1933), from Cumberland Cave local -fauna, Allegany County in western Maryland, is the type of the genus -_Pleisothomomys_ Gidley and Gazin (1933:354). _Pleisothomomys_ is here -regarded as a subgenus. The material used in the original description -included four lower jaws, one with a complete dentition. Hibbard -(1958:25) pointed out that the Cumberland Cave assemblage is a -composite fauna including both glacial and interglacial forms. He -placed the stratigraphic position of the fauna as definitely Upper -Pleistocene, probably deposited in both Illinoian glaciation and -during the Sangamon interglacial. _T. potomacensis_ is significantly -larger than _T. orientalis_ Simpson (1928:6), from the Saber-tooth -Cave local fauna, Citrus County, Florida. Simpson's material included -a rostral fragment with an incisor, premolar, and first molar. The -Saber-tooth Cave local fauna is regarded by Kurten (1965:219) as -having been recovered from Sangamon deposits. _Thomomys_ is unknown -from Wisconsin deposits in the eastern United States, and today the -genus does not occur east of the Great Plains. - -_Thomomys_ of Rancholabrean provincial age from the western United -States and México is known only from Wisconsin beds. - -Three extinct species of _Thomomys_, all referable to the subgenus -_Thomomys_, have been described. _Thomomys microdon_ Sinclair -(1905:146), based on the rostral portion of a skull without a -mandible, is from the Potter Creek Cave local fauna, Shasta County, -California, and has been recovered also from Samwel Cave, Shasta -County, California. _T. microdon_ closely resembles _Thomomys -monticola_ that lives in the area today. _Thomomys scudderi_ Hay -(1921:614) is from the Fossil Lake (or Christmas Lake) local fauna in -central Oregon. Elftman (1931:10-11) referred these specimens to -_Thomomys townsendii_, and he considered _T. scudderi_ to be a synonym -of _T. townsendii_. Davis (1937:156-158) disagreed with Elftman -concerning the taxonomic status of _T. scudderi_, which he regarded as -a valid species. According to Davis, _T. scudderi_ is more closely -allied to _Thomomys bottae_ than to _T. townsendii_. Cope (1878:389; -1889:160-165) had referred the same specimens to _Thomomys clusius_ -(now _Thomomys talpoides clusius_). Cope considered the beds to be -Pliocene in age. In all accounts of the Fossil Lake local fauna up to -Hay (1921), the specimens of _Thomomys_ were referred to the species -_clusius_, _talpoides_, or _bulbivorus_ (see Elftman, _loc. cit._). -The Fossil Lake local fauna is currently considered as being of -Rancholabrean provincial age, probably dating from the Wisconsin -glacial maximum when the lake reached its greatest size. The third -extinct species described from the Wisconsin is _Thomomys vetus_ Davis -(1937:156), also from the Fossil Lake local fauna in Lake County, -Oregon. Davis pointed out that _T. vetus_ differs from _T. scudderi_ -Hay, of the same fauna, in larger size and other cranial details, and -that it is closely allied to the living species _Thomomys townsendii_, -and not to _Thomomys talpoides_, which is the only species of -_Thomomys_ living in the area today. - -_Thomomys townsendii_ was recovered by Gazin (1935:299) from the -American Falls beds (probably Wisconsin deposits) in Idaho. - -_Thomomys talpoides_ is reported from the Howard Ranch local fauna in -Hardeman County, western Texas, by Dalquest (1965:69-70), who referred -the isolated teeth to _T. talpoides_ on geographic grounds, apparently -on the erroneous assumption that _T. talpoides_ was the species of -_Thomomys_ nearest geographically to Hardeman County. Hay (1927:259) -reported _Thomomys fuscus_ [= _Thomomys talpoides_] from late -Pleistocene beds near Wenatchee, Chelan County, Washington. Hibbard -(1951:229) recorded _Thomomys talpoides_ from late Pleistocene -deposits in Greeley County, Kansas, and Walters (1957:540) reported -the same species from late Pleistocene deposits in Clark County, -Kansas. According to Hibbard (1958:14) other remains reported as _T. -talpoides_ have been recovered from numerous areas of Wisconsin -glacial drift in western North America. - -_Thomomys bottae_ has been identified from Wisconsin age deposits in -western North America, as follows: Burnet Cave, Gaudalupe Mt., New -Mexico (Schultz and Howard, 1935:280); Carpinteria Asphalt, California -(Wilson, 1933a:70); McKittrick Asphalt, Kern County, California (J. R. -Schultz, 1938:206); Rancho La Brea, Los Angeles County, California -(Dice, 1925:125--specimens described as a new subspecies, _T. b. -occipitalis_); Papago Springs Cave, Santa Cruz County, Arizona -(Skinner, 1942:150 and 158--probably _bottae_, but possibly _umbrinus_ -on the assumption that the two are specifically instead of -subspecifically distinct); Isleta Cave, Bernalillo County, New Mexico -(Harris and Findley, 1964:115--some of these fossils may be -post-Wisconsin in age); Potter Creek Cave and Samwel Cave, Shasta -County, California (Sinclair, 1905:146--identified as _T. leucodon_, -now a subspecies of _T. bottae_; also see Hay, 1927:214-215). - -_Thomomys umbrinus_ has been reported from San Josecito Cave, Nuevo -León, México (Russell, 1960:542); Upper Bercerra, México (Hibbard, -1955a:51--identified only as _Thomomys_ sp., but undoubtedly referable -to _T. umbrinus_). Post-Wisconsin remains of _Thomomys umbrinus_ are -reported by Alvarez (1964:6) from capa II and capa III of the Cueva La -Nopalera, southwestern Hidalgo. Hay (1927:222-223) reported specimens -of the genus _Thomomys_ from Wisconsin deposits in Hawver Cave, -Eldorado County, California, but did not assign them to species. -Gilmore (1947:158) found the remains of _Thomomys umbrinus_ in cave -deposits near Quatro Ciénegas in central Coahuila. These cave deposits -may have been laid down during the Wisconsin, but more likely -accumulated in the post-Wisconsin. - - -_Zygogeomys_ - -Remains found in the Curtis Ranch local fauna, Cochise County, in -southeastern Arizona are regarded as of middle Pleistocene age. See -Gazin (1942:481-484), Wilson (1937:39-40), Hibbard (1958:25), and -Hibbard _et al._ (1965:510-511). Although some question as to the -exact age of the Curtis Ranch local fauna still seems to exist, most -authorities on the Pleistocene agree that the age is not Pliocene and -that it is older than Rancholabrean. Gidley (1922:122) described the -pocket gopher found in the Curtis Ranch beds as _Geomys parvidens_, -which is preoccupied by _Geomys parvidens_ Brown (1908:194), a name -proposed for the pocket gopher from the Conard Fissure of Arkansas; -therefore, Hay (1927:136) proposed the name _Geomys persimilis_ for -the Curtis Ranch species to replace _Geomys parvidens_ Gidley. -_Geomys persimilis_ Hay became the type species of Gazin's genus -_Nerterogeomys_ (1942:507). In this paper, _Nerterogeomys_ is -considered to be a junior synonym of _Zygogeomys_. - -_Zygogeomys persimilis_ is represented by a rostral fragment bearing -all the cheek teeth on the left side and the upper incisors. In -addition, two lower jaws, one with the first three cheek teeth, -are referred to the species (see Gazin, 1942:507). The fossils -identified as _Geomys_ from the Arroyo San Francisco, Cedazo fauna, -in Aguascalientes, México, by Mooser (1959:413) may be referable -instead to _Zygogeomys_. I have not seen the specimens and no figures -are available; Mooser states that a cranium was recovered. If either -the upper premolar or third molar is in place, generic identification -could be made with reasonable certainty. No other fossils of -_Zygogeomys_ have been uncovered in late Pleistocene deposits and the -significance of the absence of _Zygogeomys_ has been discussed in an -earlier paragraph of this section. _Geomys_ has not been found so far -south as Aguascalientes, but _Zygogeomys_ occurs farther south now and -presumably had a more extensive range on the plateau to the north in -the Pleistocene. - - -_Geomys_ - -_Geomys_ is common in Pleistocene deposits, especially on the Great -Plains. Certainly the center of differentiation for _Geomys_ was -in this region, although at times, probably when conditions were -favorable, _Geomys_ expanded its range into adjacent areas, reaching -the Pacific Coast in Irvingtonian times and the Atlantic Coast at the -time of the Illinoian glaciation. The earliest Pleistocene records -of the genus are from the Great Plains. McGrew (1944:49) described -_Geomys quinni_ from the Sand Draw local fauna, Brown County, -Nebraska, considered by Hibbard (1958:11) to be Nebraskan in age. As -mentioned in the account of Pliocene geomyids, _Geomys quinni_ occurs -also in the late Pliocene deposits of southwestern Kansas. Also, -_Geomys quinni_ occurs in the Broadwater-Lisco local fauna of Morrill -and Garden counties, western Nebraska (Barbour and Schultz, 1937:3; -Schultz and Stout, 1948:560-563; Schultz _et al._, 1951: table 1). The -Broadwater-Lisco is currently regarded as Aftonian deposits (Schultz -and Stout, _loc. cit._; Hibbard, 1958:11). Hibbard (1956:174) -identified _Geomys quinni_ from the Deer Park local fauna, probably -deposited during the early Aftonian interglacial, of Meade County, -Kansas. Strain (1966:36) described _Geomys paenebursarius_ on the -basis of fossils obtained from early Pleistocene deposits of the -Hudspeth local fauna from western Hudspeth County in the Trans-Pecos -of Texas. The Hudspeth fossils were probably deposited during the -Aftonian interglacial. From Kingman County, Kansas, Hibbard (_op. -cit._: 164) recovered isolated teeth of _Geomys_ from the Dixon local -fauna, regarded by him (_op. cit._:153-154) as deposited during the -latest Nebraskan glaciation, and correlated by him with the Sand Draw -local fauna of Nebraska. Hibbard (1958:11) later regarded the Dixon as -a transitional fauna between Nebraskan and Aftonian. The remains of -_Geomys_ from the Dixon are known only from isolated teeth. The teeth -are small, and suggest that a smaller species of _Geomys_ may have -occurred along with the more common and larger _G. quinni_ during the -early Pleistocene (see discussion beyond of the Saunders _Geomys_). -_Geomys quinni_ was widespread and common throughout the central Great -Plains from the late Pliocene (Rexroad fauna) through the early -Pleistocene (Nebraskan and Aftonian deposits). - -Hibbard (1956:179) referred the pocket gopher remains taken from the -Saunders local fauna in Meade County, Kansas, to _Geomys tobinensis_, -a small species having continuous enamel bands around the lower -premolar in younger specimens. The Saunders local fauna was deposited -in the late Aftonian and is younger than the Deer Park local fauna -discussed above. Paulson (1961:138) later pointed out that the -Saunders _Geomys_ is distinct from _Geomys tobinensis_; hence, the -small pocket gopher from the Saunders local fauna is probably an -unnamed species, perhaps more closely allied to _paenebursarius_ -than to _quinni_. The small _Geomys_ reported from the Aftonian -Broadwater-Lisco local fauna of Nebraska (Schultz and Stout, 1948:563) -may also be the same as the Saunders pocket gopher, but the smaller -adult specimens occurring in the same bed with larger specimens -probably are females and the larger specimens males. In all living -Geomyini females have smaller skulls than males. - -The Irvingtonian provincial age is currently regarded as Middle -Pleistocene and includes the late Kansan glaciation (that part -occurring after the glacial maximum) and the Yarmouthian interglacial -(see Hibbard _et al._, 1965:512-514). The Irvingtonian provincial -age, therefore, follows the late Blancan provincial age of the early -Pleistocene and is succeeded by the Rancholabrean provincial age of -the late Pleistocene. No specimen of an Irvingtonian _Geomys_ is -referable to any living species. Two Irvingtonian species have been -described. Hibbard (1944:735) named _Parageomys tobinensis_ [= _Geomys -tobinensis_] from the Tobin local fauna of Russell County, Kansas. -This species since has been reported from the Cudahy local fauna of -Meade County, Kansas (Paulson, 1961:137). Hibbard (1956:183) also -identified as _Geomys tobinensis_ the pocket gopher recovered from the -Saunders local fauna, a late Aftonian deposit of Meade County, Kansas, -and reduced the technical name _Parageomys_ from generic to subgeneric -rank. Paulson (_op. cit._:138) pointed out that the Saunders specimens -differ from _G. tobinensis_, and he, therefore, restricted the name -to the small _Geomys_ of the Cudahy and Tobin local faunas of -Irvingtonian provincial age. _G. tobinensis_ is markedly smaller than -the Blancan _G. quinni_. The Cudahy and Tobin local faunas are of -approximately the same age, and presently both are included in one -unit, the Cudahy fauna. The Cudahy fauna is considered to have been -deposited in late Kansan as it occurs in strata immediately below the -Pearlette ash. - -Recently, White and Downs (1961:8) described a new Irvingtonian -species, _Geomys garbanii_, from the middle Pleistocene Vallecito -Creek local fauna of San Diego County, California. Many well preserved -fossils of the new species were recovered. _Geomys garbanii_ is of -medium size (approximately the size of one of the larger subspecies of -_G. bursarius_), and significantly larger than the Irvingtonian -_Geomys tobinensis_ of the Great Plains. The Vallecito Creek -occurrence of _Geomys_ is the first authenticated record from the -Pacific Coast region. Matthew (1902:320) erroneously referred remains -of _Thomomys_ to the genus _Geomys_ in his revised list of Cope's -earlier report on the Fossil Lake (or Silver Lake) fauna (see -discussion of _Thomomys_ above). - -A number of Irvingtonian fossil remains of _Geomys_ have not been -identified with particular species. Hibbard (1941a:206) found _Geomys_ -in the Borchers local fauna (deposited in the time of the Yarmouthian -interglacial) of Meade County, Kansas. Also, _Geomys_ has been -reported from several sites in Nebraska. Schultz and Tanner (1957:67) -reported _Geomys_ from the Angus fossil quarry in Nuckolls County, -south-central Nebraska. The Angus fossils were found in sediments -of the Sappa Formation considered by Schultz and Tanner to be a -Yarmouthian deposit. Fossil quarries (Hay Springs, Rushville, and -Gordon) along the south side of the Niobrara River Valley in Sheridan -County, Nebraska, have also provided records of geomyids. Both a large -and small species of _Geomys_ have been reported from the more -recently excavated Rushville and Gordon sites (Schultz and Stout, -1948:562-567, and table 3). In view of the great disparity in size -owing to sex, these may actually be males and females of the same -species, as mentioned above. The name Hay Springs has been used in -reference to all three sites. The ages of the Hay Springs sites are -approximately the same, but their correlation is presently under -debate. Schultz and Tanner (1957:68-71) maintain that the fossils are -distinctly middle Pleistocene, and that they were deposited during -late Kansan glaciation, or perhaps from early Yarmouthian into early -Illinoian, with the largest concentration coming from the Sappa sands -of pre-Illinoian (Yarmouth) age. Hibbard (1958:25), basing his opinion -on the presence of _Microtus pennsylvanicus_, and the stage of -evolution of other species in the assemblage, regards the Hay Springs -sites as probably Illinoian deposits, but certainly no older than -that. - -Mooser (1959:413) identified as _Geomys_ the pocket gopher from -Irvingtonian deposits in Arroyo San Francisco (loc. no. 5) near the -city of Aguascalientes, México. As suggested elsewhere in this -account, these fossils may be referable to _Zygogeomys_ rather -than _Geomys_. The Irvingtonian provincial age of this fauna was -established by Hibbard and Mooser (1963:245-250). Other alleged -occurrences have recently been compiled by Alvarez (1965:19-20). -Maldonado-Koerdell (1948:20) noted four fossil occurrences of the -genus _Geomys_ in México. Two of these from San Josecito Cave in -Nuevo León have since been identified with the genera _Orthogeomys_ -and _Pappogeomys_ (Russell, 1960:543-548); the third listed by -Maldonado-Koerdell from "near Ameca, Jalisco," was based on Brown's -(1912:167) mention of some bones supposedly of the family "Geomyidae," -and the fourth refers to pocket gopher remains from the "Hochtals von -Mexiko" listed as _Geomys_ by Freudenberg (1921:139). His generic -identification is doubtful and the specimens should be compared with -Mexican genera of the Geomyinae. - -Upper Pleistocene records of _Geomys_ also are common. Upper -Pleistocene is here understood to include late Illinoian, -Sangamon and Wisconsin deposits; all are considered to be of -Rancholabrean provincial age (see Hibbard _et al._, 1965:512-515) and -post-Irvingtonian. The presence of remains of _Bison_ and/or _Microtus -pennsylvanicus_ are currently considered mammalian index fossils of -Rancholabrean faunas. In the Illinoian, _Geomys_ extended its range to -the Atlantic Coast in the southeastern United States. The eastern and -western species-groups evidently were isolated throughout much of the -late Pleistocene, and, therefore, evolved separately. Of the two, the -eastern, or _pinetis_, species-group seems to have remained somewhat -more generalized, and the western, or _bursarius_, species-group has -become more specialized. The Rancholabrean _Geomys_ from deposits in -the southeastern United States are referable (see Ray, 1963:325) to -_Geomys pinetis_. - -Marsh (1871:121) described _Geomys bisulcatus_ from the North Prong of -the Loup River (near Camp Thomas), Nebraska. These beds are also -termed the Loup Fork or Loup River fossil beds (see discussion on p. -485), and they lie along the upper reaches of the Middle Loup River -in Thomas County (near Senea), Hooker County (near Mullen), and -southeastern Cherry County (probably the North Prong beds northwest -of Mullen). These beds were at first thought to be of Miocene age, -but later were regarded as early Pliocene (see Schultz and Stout, -1948:562-566 for a historical account of expeditions to these fossil -sites). Schultz and Tanner (1957:71-72) pointed out that the principal -fossiliferous beds in the Middle Loup region are of middle to late -Pleistocene age, with most of the fossils coming from the Crete sand -and silt beds which are probably early Illinoian deposits, and, -therefore, younger than the Hay Springs faunas. Some fossils may have -come from the Sappa deposits dated by Schultz and Tanner (_loc. cit._) -as mostly Yarmouthian deposits. _Geomys bisulcatus_, judging from -the original description and Hibbard's discussion of the cotypes -(1954:357), does not differ significantly from _Geomys bursarius_. -However, _Geomys bisulcatus_ is tentatively retained as a valid -species. Based on the evidence cited above it seems unlikely that -_Geomys bisulcatus_ occurred in pre-Irvingtonian times as often -suggested in the literature. - -The genus _Geomys_ has been identified in several faunas of Illinoian -age, all from the Great Plains. Stephens (1960:1961) reported the -genus from the Doby Springs local fauna in Harper County, Oklahoma, -and Starrett (1956:1188) reported it from the Berends local fauna in -Beaver County, Oklahoma. Schultz (1965:249) assigned 21 isolated -teeth, including six incisors, from Butler Springs local fauna -(considered by him to be late Illinoian, following the glacial -maximum) to _Geomys_ cf. _bursarius_. Hibbard and Taylor (1960:167) -reported a baculum tentatively identified as that of _Geomys_ from the -early Illinoian Butler Springs local fauna (including the Adams fauna) -of Meade County, Kansas. Hibbard (1963:206) recorded the genus -_Geomys_ from the Mt. Scott local fauna (late Illinoian deposits) of -Meade County, Kansas; the specimens probably are referable to the -living species _bursarius_. From McPherson County, Kansas, Hibbard -(1952:7) reported the genus _Geomys_ from the Kentuck Assemblage, -which he (1958:25) regarded as a composite of Illinoian and Sangamon -species. Specific identification of the Illinoian pocket gophers is -uncertain, primarily due to the fragmentary nature of the material. On -the basis of dental characters alone most specimens could be referred -to _G. bursarius_; however the taxonomic status of _G. bisulcatus_ -is in doubt, and more complete material may indicate that the -Illinoian gophers are specifically distinct from the living species. -Consequently, most authors, including myself, have made no attempt to -refer these specimens to species. Nevertheless, the Illinoian _Geomys_ -from the Great Plains is more closely allied to the living species of -_Geomys_ than it is to the earlier Irvingtonian species. - -_Geomys bursarius_ has been collected from a number of Sangamon fossil -sites on the Great Plains. Although specific identification of -specimens of _Geomys_ from Illinoian faunas is uncertain, the Great -Plains _Geomys_ from Sangamon and later deposits probably is referable -to the living species as Hibbard and Taylor (1960:165) pointed out. -They found no difference between _Geomys_ recovered from the Cragin -Quarry local fauna (early Sangamon) of Meade County, Kansas, and the -living species _Geomys bursarius_. Isolated teeth of the same species -were collected from the Jinglebob local fauna of Meade County, Kansas -(Hibbard, 1955b:206), a fauna of the late Sangamon. Hibbard (1943:240) -also recorded the genus _Geomys_ (referable to _G. bursarius_) from -the Rezabek local fauna of Lincoln County, Kansas. According to -Schultz _et al._ (1951:6 and table 1) the genus _Geomys_ occurs -in buried or "fossil" soils of Sangamon age, lying just above the -Loveland Loess, in Nebraska. No specific localities were given by -them, nor were any particular specimens mentioned. Dalquest reported -_Geomys bursarius_ from two Sangamon faunas in northern Texas. The -species is represented in the Ward Quarry local fauna of Cooke County, -Texas (1962a:42), and the Good Creek local fauna of Foard County, -Texas (1962b:575). - -_Geomys bursarius_ has been reported from Wisconsin fossil deposits -of the Great Plains and adjacent areas as follows: Jones local fauna, -Meade County, Kansas (Hibbard and Taylor, 1960:64-66); Two Creeks -Forest beds of the third interstadial soils formed between Cary and -Mankato glaciations, late Wisconsin (Schultz _et al._, 1951:8 and -table 1); Cita Canyon local fauna in the northern part of the -Panhandle of Texas (Johnson and Savage, 1955:39); Howard Ranch local -fauna of Hardeman County in northwestern Texas (Dalquest, 1965:70); -Quitaque local fauna of Motley County, Texas (Dalquest, 1964:501); -Clear Creek local fauna of Denton County in north-central Texas -(Slaughter and Ritchie, 1963:120); Ben Franklin local fauna, of late -Wisconsin beds along the North Sulphur River in Delta County, NE Texas -(Slaughter and Hoover, 1963:137); Bulverde Cave (Hay, 1920:140; -1924:247) and Friesenhahn Cave (Tamsitt, 1957:321), both in Bexar -County, south-central Texas; Alton, Illinois (Hay, 1923:338-339); -Wisconsin drift of Illinois, without mention of specific locality -(Bader and Techter, 1959:172); Wisconsin drift of southwestern -Wisconsin and northeastern Iowa (Hay, _op. cit._:343); Wisconsin drift -near Galena, Illinois, and mouth of Platte River in eastern Nebraska -(Leidy, 1869:406). - -Brown (1908:194) described _Geomys parvidens_ from the Conard Fissure, -in northern Arkansas. Hibbard (1958:25) concluded that the Conard -Fissure fauna represents a glacial stage, probably the Illinoian, and -Hibbard _et al._ (1965:510-511) regarded the fauna as a composite -including both Irvingtonian and Rancholabrean elements. White and -Downs (1961:21) considered _G. parvidens_ to be a subspecies of -_Geomys bursarius_. - -The first Pleistocene occurrence of _Geomys_ in the southeastern -United States is from the Reddick I deposits reported by Gut and Ray -(1963:325), who found the remains of _Geomys pinetis_ among the -fossils comprising the "rodent beds" of Marion County, Florida. Gut -and Ray tentatively identified the beds as Illinoian, but Kurten -(1965:219) regarded the Reddick I fauna as early Sangamon. Simpson -(1928:2) reported _Geomys floridanus_ [= _pinetis_] from Saber-tooth -Cave deposits of Citrus County, Florida. The Saber-tooth Cave (or -Lecanto Cave) local fauna is considered by Kurten (_op. cit._:219) -also to be a Sangamon deposit. _Geomys floridanus_ [= _pinetis_] was -reported from the Seminole Field deposits by Simpson (1929:563); both -Simpson and Kurten (_op. cit._:221) agreed that the Seminole Field -fauna is mainly late Wisconsin, although sub-Recent fossils occur at -the tops of the beds. Ray (1958:430) collected remains of _Geomys -pinetis_ from the Melbourne Bone Bed of Brevard County, Florida. The -Melbourne local fauna is considered to be from Wisconsin deposits by -Kurten (_op. cit._:220). The eastern species of _Geomys_ were probably -derived from Great Plains stock that reached the southeastern Coastal -Plains in early Rancholabrean (Illinoian) time. Presently there is no -contact between the eastern and western populations of the genus, and -it is assumed that disjunction occurred as a result of Wisconsin -glaciation. It is interesting to note that the genus _Thomomys_ -occurred in this region at approximately the same time; both genera -occur in Saber-tooth Cave deposits. - - -_Pappogeomys_ - -The genus _Pappogeomys_ is not known from Pleistocene deposits older -than the Wisconsin glaciation, but a pre-Pleistocene occurrence in the -Benson beds of Arizona (see discussion of the Pliocene above) shows -that _Pappogeomys_ had been differentiated by late Pliocene time. -The absence of _Pappogeomys_, beginning in the early Pleistocene and -continuing well into the late Pleistocene, is attributed to the -southern distribution of the genus, where its range probably was -centered on the Central Plateau of México. The paucity of early and -middle Pleistocene deposits from this critical region prevents any -definite statements about phyletic development within the genus. All -of the late Pleistocene records pertain to the subgenus _Cratogeomys_ -(long in use as a generic name but in the present paper reduced to -subgeneric rank in the genus _Pappogeomys_). Schultz and Howard -(1935:280) found _Cratogeomys_ [= _Pappogeomys_] _castanops_ in -Burnett Cave in the Guadalupe Mountains of south-central New Mexico. -The Burnett deposits are probably late Wisconsin (see Schultz and -Tanner, 1957:75, for discussion of the age of these deposits based -on carbon-14 tests). These writers (_loc. cit._) also referred -the mandible of a small pocket gopher to the genus _Pappogeomys_ -[= subgenus _Pappogeomys_]. However, neither genera nor subgenera of -the tribe Geomyini can be distinguished on the basis of their inferior -dentitions. Judging from the distribution of the modern geomyines, it -seems unlikely that the subgenus _Pappogeomys_ has occurred beyond its -present range in the late Pleistocene; therefore the small mandible is -most likely that of a young individual of _Pappogeomys castanops_. -Russell (1960:543) referred specimens collected at San Josecito Cave -in Nuevo León, México, to the group of small subspecies _Cratogeomys_ -[= _Pappogeomys_] _castanops_. Also, Russell (_loc. cit._) identified -a rostral fragment as of the genus _Cratogeomys_ [= subgenus -_Cratogeomys_] although the fragment had a combination of features -different than in any named species of the genus; he did not name the -fragment as a new species, preferring to wait for additional material -that could clarify its taxonomic relationships. - -Hibbard (1955a:52-53) identified _Cratogeomys_ [= _Pappogeomys_] -_tylorhinus_ from the Becerra Superior deposits in the valley of -Tequixquic in the northern part of the state of México. The Wisconsin -age of these beds suggests an earlier Pleistocene derivation of the -_gymnurus_-group of species. - -Several specimens of the subgenus _Cratogeomys_ have been reported -from beds of latest Wisconsin (certainly after the glacial maximum) -or post-Wisconsin age. Gilmore (1947:158) found fossil remains of -_Cratogeomys_ [= _Pappogeomys_] _castanops_ commonly in Quaternary -cave deposits on the mountain slopes in the vicinity of Cuatro -Ciénegas, in central Coahuila. These deposits actually may be of -post-Wisconsin origin (see discussion above). Alvarez (1964:8) -obtained fragments of _Cratogeomys_ [= _Pappogeomys_] _tylorhinus_ -from sub-Recent deposits of Capa III in the Cueva La Nopalera in -southwestern Hidalgo, México. _Pappogeomys merriami_ lives in the -area today. Mayer-Oakes (1959:373) reported remains of _Cratogeomys_ -[= _Pappogeomys_] _merriami_ from levels eight and eleven of the -excavations at El Risco II, in the northern part of Mexico City. The -ages of these deposits are unknown to me, but they probably are no -older than late Wisconsin with most of the beds dating from the -post-Wisconsin. - - -_Orthogeomys_ - -This genus is not known from the Pleistocene, except for its -occurrence in the San Josecito cave deposits of southwestern Nuevo -León, México (Russell, 1960:544). Although _Orthogeomys_ does not -occur in the immediate vicinity of the cave at the present time, the -northern limits of its range is nearby in southern Tamaulipas. The -_Orthogeomys_ from San Josecito Cave differs from living species, -and has been named _Heterogeomys_ [= _Orthogeomys_] _onerosus_ -Russell (_loc. cit._), and is evidently referable to the subgenus -_Heterogeomys_. As mentioned before, the San Josecito Cave local -fauna represents deposits of Wisconsin glaciation. - - - - -HISTORY OF CLASSIFICATION - - -The account of the Tucan or Indian mole by Hernandez (sometimes listed -as Fernandez) in 1651 probably is the earliest published one of a -geomyid (see Merriam, 1895:201; Coues, 1877:607-608). Linnaeus in 1758 -did not mention geomyids. In 1772, Kerr described Hernandez's Tucan -under the name _Sorex mexicana_ on the basis of Hernandez's account -without having seen any specimens. Lichtenstein in 1827 applied the -technical name _Ascomys mexicana_ to three specimens collected by -Deppe from unknown localities on the tableland of México. Merriam -(_loc. cit._) pointed out that the name _mexicanus_ of Lichtenstein in -1827 is a _nomen nudum_, and that it is preoccupied by _mexicanus_ -used by Kerr in 1792. The latter can not be technically identified -with any particular species of geomyid. - -Bartram in 1791 wrote of the pocket gopher of Florida, without -formally describing it. The first available technical name is _Mus -bursarius_ of Shaw in 1800. Rafinesque in 1817 proposed the first -generic names for the geomyids when he described _Geomys_ and -_Diplostoma_. In 1839, Waterhouse referred the genus _Geomys_ to his -family Arvicolidae, considered by him to be a subgroup of muroids. In -1841, he suggested that _Geomys_ was related to _Bathyergus_ and -_Spalax_. Waterhouse in 1848 (p. 8) treated the pocket gophers as a -subgroup of rodents under the group name Saccomyina, in which he -included the genera _Heteromys_, _Saccomys_, _Perognathus_, and -_Dipodomys_. Hence, Waterhouse was the first to recognize the -relationship between the heteromyids and geomyids. In the next year -Gervais erected the family Pseudostomidae for a group of specialized -squirrels to include _Geomys_ and _Thomomys_ and the same genera (at -least in part) of heteromyids that Waterhouse classified in the -"family" Saccomyina. - -In 1839 the name _Thomomys_ was proposed by Maximilian (Wied-Neuwied). -All of the generic names previously proposed for pocket gophers were -considered by subsequent authors to be synonyms of _Geomys_. - -A third family name, Sciurospalacoides, was proposed by Brandt -(1855:188) who referred _Geomys_ and _Thomomys_ to that family. He -placed his new family phylogenetically between the family Sciuridae -and the family Spalacoides (a group in which Brandt included the -genera _Spalax_, _Sipheus_, and _Ellobius_). Brandt took exception -to the classification of Waterhouse (1848), who united the geomyids -and heteromyids in one family. Brandt placed the heteromyid genera -in other groups: _Perognathus_ in the Muridae, and _Macrocolus_ -[= _Dipodomys_] in the Macrolini, a subfamily of the family Dipodoides. - -Modern classification of the pocket gophers begins with Baird in 1858. -The important classifications are summarized in Table 1; a few that do -not depart essentially from those listed have been omitted owing to -limited space for the tabular arrangement, but are discussed in the -following account. - -Baird probably was strongly influenced by the arrangement proposed by -Waterhouse in 1848, but was opposed to separating geomyids from -heteromyids as was done by Brandt. Baird was convinced of the close -relationship of the geomyids and heteromyids, and referred both groups -to one family, the Saccomyidae, as Waterhouse had done earlier. In -order to recognize the morphological specializations he used two -subfamilies, Geomyinae and the Saccomyinae. In the 20 years that -followed, some authors followed Brandt and others followed Baird. - -Gill, in 1872 (p. 71), proposed a classification essentially like -Baird's of 1858, but Gill raised Baird's subfamilies to the rank -of family (see Table 1). In referring all pocket gophers to the -Geomyidae, Gill used that name as a family term for the first time. -Also he established the superfamily Saccomyoidea to include his two -families, Geomyidae and Saccomyidae; therefore, the Saccomyoidea was -equivalent to the group Saccomyina of Waterhouse (1848) and the -Saccomyidae of Baird (1858). Coues (1877), in his classic monograph of -the Geomyidae followed the arrangement proposed by Gill in treating -the pocket gophers as a family. Alston in 1876 proposed another -classification based on Baird (1858), with two subfamilies, the -Geomyinae and the Heteromyinae, united together in the family -Geomyidae; thus, he recognized that the genus _Saccomys_ Frédéric -Cuvier, 1823, was a synonym of _Heteromys_ Desmarest, 1817, as had -been pointed out by Gray (1868:201) and Peters (1874:356). Coues -(1877:487-490) acknowledged the invalidity of the genus _Saccomys_, -but refused to give up the name in supergeneric classification. Winge, -first in 1887 and subsequently in 1924, classified the geomyids and -heteromyids together in the family Saccomyidae as did Baird in 1858, -and like Coues, Winge too ignored the synonymy of _Saccomys_ with -_Heteromys_ and insisted on retaining the technical terms Saccomyidae -and Saccomyini. - -Up to the time of Merriam's classic revision of the Recent Geomyidae -in 1895 all the known species of living pocket gophers were referred -to two genera, _Geomys_ and _Thomomys_. Merriam described much new -material, especially from México and Central America, and proposed -seven new genera (see Table 1). His complete and detailed study of the -dentitions and osteology of the skull remains today as the definitive -work on this subject, and is the point where most studies of the -Geomyidae must begin. His treatment of the Recent genera survived for -52 years without change until Hooper (1946:397) arranged _Platygeomys_ -as a synonym of _Cratogeomys_. However, Merriam's genera have been -recognized in all subsequent classifications except for the current -review (see Table 1). - -Cope described the first known fossil geomyids in 1878, and published -an excellent review of the two genera, _Pleurolicus_ and _Entoptycus_, -in 1884 (pp. 855-870, pl. 64, figs. 1-9). Both genera were recovered -from the John Day Miocene deposits of Oregon. Cope did not propose a -new systematic arrangement of these geomyids, but referred them to the -family Saccomyidae and mentioned that the Saccomyidae was equivalent -to the family Geomyidae of Alston. Winge, in 1887, followed Cope in -referring _Pleurolicus_ and _Entoptycus_ to the Saccomyidae along with -the living genera _Thomomys_ and _Geomys_. Miller and Gidley (1918), -in their synopsis of the supergeneric groups of rodents, proposed a -new subfamily, Entoptychinae, to include the divergent Miocene pocket -gophers. Miller and Gidley also revived the old subfamily Geomyinae of -Baird (1858), but restricted its application to the modern pocket -gophers and their immediate ancestors. In 1936, A. E. Wood revised the -taxa of the subfamily Entoptychinae, and described the first Miocene -genus, _Dikkomys_, of the Geomyinae. He followed the supergeneric -classification of Miller and Gidley (1918). - -The recent classifications of Simpson (1945) and Wood (1955) have -combined the classifications of Merriam (1895) and Wood (1936). Wood -(1955) brought up to date the list of genera, including those that -were described after the publication of Simpson's classification -(1945). In Table 1, the list of genera is principally from Simpson -(1945) but generic names used by Wood (1955) are included. This is the -currently accepted classification. - -The new classification proposed in this paper (see Table 1) includes -three tribes proposed as vertical units; they are intended to stress -the phyletic trends in the known evolutionary sequences by placing -immediate ancestors together with their descendants. - -_Pliogeomys_ is placed in the same tribe (Geomyini) as _Zygogeomys_, -_Geomys_, _Orthogeomys_, and _Pappogeomys_. That tribe includes the -most specialized Geomyinae. _Zygogeomys_, _Geomys_, _Orthogeomys_, and -_Pappogeomys_ are lineages resulting from a Pleistocene radiation in -which all the lineages diverged from a common Pliocene ancestor. The -radiation of the Geomyini was well under way by the close of the late -Pliocene. Although _Pliogeomys_ may not be the actual ancestor, it -closely resembles the primitive morphotype. - -TABLE 1.--History of the classification of the Superfamily Geomyoidea - - ===============+==============+==================+================ - Baird 1858 | Gill 1872 | Winge 1887 | Merriam 1895 - | Coues 1877 | and 1924 | Ellerman 1940 - ---------------+--------------+------------------+---------------- - Family | Family | Family | Family - Saccomyidae | Geomyidae | Saccomyidae | Geomyidae - ---------------+--------------+------------------+---------------- - Subfamily | | "Group" | - Geomyinae | | Geomyini | - -- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- - - | | | - | | | - | | | - | | | - | | | - | | | - | | | - | | | - | | | - | | | - _Thomomys_ | _Thomomys_ | _Thomomys_ | _Thomomys_ - | | | - | | | - | | | - | | | - | | | - | | | _Zygogeomys_ - | | | - | | | - _Geomys_ | _Geomys_ | _Geomys_ | _Geomys_ - | | | - | | | _Orthogeomys_ - | | | _Heterogeomys_ - | | | _Macrogeomys_ - | | | - | | | _Pappogeomys_ - | | | _Cratogeomys_ - | | | _Platygeomys_ - -- -- -- -- -- +-- -- -- -- --+-- -- -- -- -- -- +-- -- -- -- -- - - | | | - | | | - | | | - | | *_Pleurolicus_ | - | | | - | | | - | | *_Entoptychus_ | - ---------------+--------------+------------------+---------------- - | | | - | | | - | | | - | | "Group" | - | | Gymnoptychine** | - | | _Gymnoptychus_ | - ---------------+--------------+------------------+---------------- - Subfamily | Family | "Group" | - Saccomyinae | Saccomyidae | Saccomyini | - -------------+-------------+------------------+---------------- - - =====================+=====================+=================== - Wood 1935 | Simpson 1945 | Names used in - Wood 1936 | Wood 1955 | present paper - ---------------------+---------------------+------------------- - Family | Family | Family - Geomyidae | Geomyidae | Geomyidae - ---------------------+---------------------+------------------- - Subfamily | Subfamily | Subfamily - Geomyinae | Geomyinae | Geomyinae - -- -- -- -- -- -- -- +-- -- -- -- -- -- -- +-- -- -- -- -- -- - - | | Tribe - | | Dikkomyini - | | - *_Dikkomys_ | *_Dikkomys_ | *_Dikkomys_ - | *_Pliosaccomys_ | *_Pliosaccomys_ - | | - | | Tribe - | | Thomomyini - | | - *_Pleisothomomys_ | *_Pleisothomomys_ | } - _Thomomys_ | _Thomomys_ | } _Thomomys_ - | | - | | Tribe - | | Geomyini - | | - | *_Pliogeomys_ | *_Pliogeomys_ - _Zygogeomys_ | _Zygogeomys_ | } - | *_Nerterogeomys_ | } _Zygogeomys_ - | | - _Geomys_ | _Geomys_ | } - | *_Parageomys_ | } _Geomys_ - _Orthogeomys_ | _Orthogeomys_ | } - _Heterogeomys_ | _Heterogeomys_ | } _Orthogeomys_ - _Macrogeomys_ | _Macrogeomys_ | } - | | - _Pappogeomys_ | _Pappogeomys_ | } - _Cratogeomys_ | _Cratogeomys_ | } _Pappogeomys_ - _Platygeomys_ | _Platygeomys_ | } - -- -- -- -- -- -- -- +-- -- -- -- -- -- -- +-- -- -- -- -- -- - - Subfamily | Subfamily | Subfamily - Entoptychinae | Entoptychinae | Entoptychinae - | | - *_Pleurolicus_ | *_Pleurolicus_ | *_Pleurolicus_ - *_Gregorymys_ | *_Gregorymys_ | *_Gregorymys_ - *_Grangerimus_ | *_Grangerimus_ | *_Grangerimus_ - *_Entoptychus_ | *_Entoptychus_ | *_Entoptychus_ - ---------------------+---------------------+------------------- - | Geomyidae | Geomyidae - | _incertae sedis_ | _incertae sedis_ - | | - | | - *_Gidleumys_ | *_Diplolophus_ | *_Diplolophus_ - | *_Griphomys_ | *_Griphomys_ - ---------------------+---------------------+------------------- - Family | Family | Family - Heteromyidae | Heteromyidae | Heteromyidae - ---------------------+---------------------+------------------- - - * Denotes extinct genera. - - ** Winge included in his family Saccomyidae the "group" - Gymnoptychine and the contained genus _Gymnoptychus_ Cope, 1873, - which genus currently is placed in the family Eomyidae. The type - of _Gymnoptychus_ Cope, 1873, is synonymous with _Ischyromys_ - Leidy, 1856, and the valid name for the genus is _Adjidaumo_ - Hay, 1899. - -_Pliosaccomys_, on the other hand, represents the terminal stages of a -long trend that began with the _Dikkomys_-like Geomyinae of the early -Miocene. In this lineage, the rate of evolution in the dentition and -the skull was slow; therefore, the differences between early Miocene -(_Dikkomys_) and middle Pliocene (_Pliosaccomys_) are not great and -the two are united into the tribe Dikkomyini. The Dikkomyini is the -ancestral geomyinen trunk from which the modern groups have diverged. - -The Pliocene ancestor of _Thomomys_ is unknown but probably resembled -_Pliosaccomys_, with which it may have been a contemporary. _Thomomys_ -is the least specialized of the modern Geomyinae, and, consequently, -shows the most resemblance to the ancestral tribe. The specializations -of _Thomomys_, however, clearly preclude its reference to the tribe -Dikkomyini; therefore, it is set apart in the monotypic tribe -Thomomyini. That tribe has not undergone an adaptive radiation -comparable to that of the tribe Geomyini or that of the Entoptychinae -in the early Miocene. Here, for the first time, _Thomomys_ is set -apart in classification from the other living pocket gophers. - -Merriam's genera _Orthogeomys_, _Heterogeomys_, and _Macrogeomys_ are -closely related. Each of these taxa is retained as a subgenus of a -single genus, _Orthogeomys_. Some species of _Macrogeomys_ seem to be -more closely allied to the subgenus _Orthogeomys_ and others to the -subgenus _Heterogeomys_. A revision of the genus is needed; it might -show that the currently recognized subgenera are artificial, and that -a different arrangement of the species would more clearly express -their evolutionary relationships. The subgenus _Heterogeomys_ seems to -be the most nearly uniform of the subgenera, and it is the least -specialized. Radiation within the genus may have begun relatively -recently, but the many special adaptations for tropical environments -suggest that the genus has been in the Neotropical Zone a long time. -Therefore, discovery of an early dichotomy from the common ancestral -stock of the tribe would come as no surprise. - -_Nerterogeomys_ Gazin here is arranged as a junior synonym of -_Zygogeomys_. Both are less specialized than any of the other -Geomyini, except _Pliogeomys_. The single living species (_Zygogeomys -tricopus_) is obviously a relic. Its range is small. The two -subspecies differ only in minor features. The living species does -have a few unique characteristics, only to be expected in the -surviving species of a long phyletic lineage. Some of these are -specializations. Otherwise, _Zygogeomys_ and _Nerterogeomys_ are -closely related and the latter is best placed as a synonym of the -former. Both are admittedly closely related to _Geomys_. _Zygogeomys_ -and _Geomys_ share several characters, particularly primitive ones; -there is considerable parallelism, especially marked in Irvingtonian -species of _Geomys_. Nevertheless, _Geomys_ is more specialized, -particularly in the dentition, and it has developed some -_Pappogeomys_-like specializations. _Zygogeomys_ has retained more of -the primitive characters of the tribe. A strong case could be made for -recognizing only one genus, _Geomys_, containing _Zygogeomys_ as one -of two subgenera. Nevertheless, the characters separating _Zygogeomys_ -and _Geomys_ are of considerable importance and I consider the two -kinds to be distinct genera. - -The species of _Geomys_, both living and extinct, form a distinct and -well-marked group. The genus is less primitive in most respects than -_Zygogeomys_ and _Orthogeomys_ and it is less specialized than -_Pappogeomys_, excluding the ancestral stock (subgenus _Pappogeomys_). -Some specimens of species of Irvingtonian age (_Geomys tobinensis_ and -_Geomys garbanii_, especially the former) retain primitive enamel -plates as does _Zygogeomys_; but this is true of only a small -percentage of the individuals. Also the adult dental pattern developed -somewhat later in ontogeny in these middle Pleistocene species of -_Geomys_ than in either Recent or late Pliocene and early Pleistocene -representatives (_Geomys paenebursarius_, _Geomys quinni_) of the -genus. Whether these features represent a stage in the evolution of -the late Pleistocene and Recent species or a terminal stage in members -of a sterile and primitive branch of the main line of evolution of -_Geomys_ is uncertain. At present I favor the latter explanation, and -view _G. paenebursarius_ and _G. quinni_ as early progressive species -that evolved dental specializations that were maintained in the main -line of phylogeny. - -Hibbard proposed the generic name _Parageomys_ (1944:55), but later -regarded it as a subgenus of _Geomys_ (1956:182) that includes those -species retaining continuous enamel bands until relatively late in -ontogeny; no other differences have been noted. When the early -phylogeny of _Geomys_ is better understood, _Parageomys_ may serve as -a subgeneric taxon in which the primitive species of _Geomys_ can be -grouped, but as of now _Parageomys_ is arranged as a synonym of -_Geomys_. - -_Pappogeomys_ and _Cratogeomys_ also form a natural group. Their close -relationship is best reflected in formal taxonomy by including them in -the same genus. Their dissimilarities are of the sort that separate a -primitive ancestral lineage from a divergent and progressively more -specialized assemblage. The fossil record is inadequate, and I -can only speculate that _Cratogeomys_ diverged from primitive -_Pappogeomys_-stock in the earlier Pleistocene, at least before the -end of the Irvingtonian. _Cratogeomys_ probably originated on the -Mexican Plateau and probably underwent its subsequent evolution there. -The living species of the subgenus _Pappogeomys_ are evidently relics -of the ancestral stock of the genus. Hooper (1946:397), I think -correctly, considered _Platygeomys_ as congeneric with _Cratogeomys_, -although the highest degree of specialization of the genus is attained -in those species formerly classed in the genus _Platygeomys_. Even so, -in my opinion, the differences are insufficient to warrant even -subgeneric recognition. - - - - -CLASSIFICATION - - -Family GEOMYIDAE Gill, 1872 - -Rodents of the superfamily Geomyoidea specialized for completely -fossorial life (early Pliocene to Recent); specialized earlier (late? -Oligocene and early Miocene) for semi-fossorial habits; body thickset, -fusiform without apparent neck (in modern geomyids); legs short; -forelegs especially stout; eyes and ears small (pinna reduced to -inconspicuous crest concealed beneath pelage); tail tactile, shorter -than head and body; lips closing behind incisors; cheek pouches -external, fur-lined; baculum rodlike, arched, having expanded -quadriform platelike base; pelage long, soft without underfur, -covering body in thick coat (in some species of _Orthogeomys_ scant, -harsh or scattered bristles); color varying from pale tints of buffy -(almost white) to metallic black. - -Skull thick-walled, massive, angular, relatively broad, and flattened; -distinctly murine form, but having zygomasseteric structure of -advanced sciuromorphs, including small infraorbital canal (that -transmits no part of masseter muscle) and well-developed, broad -zygomatic plate; zygomata massive and widely flaring, jugals stout; -rostrum robust, relatively broad and deep, and without evidence of -transverse canal (as in Heteromyidae); anterior projection of nasals -only slightly exceeding that of upper incisors; interorbital region -usually constricted, narrower than rostrum; anterior opening of -infraorbital canal far forward on side of rostrum, about half -way between zygomatic plate and upper incisor and just behind -premaxillary-maxillary suture, its opening countersunk in oblique -sulcus (for protection from muscle contraction); postorbital process -lacking, except for rudimentary knoblike projection in subgenus -_Macrogeomys_; palate relatively narrow, its deeply sculptured surface -sloping steeply downward posteriorly causing region supporting -maxillary tooth-row to be markedly depressed; palatine bone reduced, -forming, on two abruptly different levels, posterior margin of hard -palate behind tooth-rows; parietals compressed and narrow, and most of -cerebral cavity roofed by squamosals (in some species squamosals -overlap lateral parts of parietals); tympanic bullae completely -inferior in position and fully ossified, external meatus being -developed laterally as elongated tube; mastoid not inflated, but -broadly exposed at posterolateral margin of the skull; occiput large, -its surface usually rugose, and paroccipital processes large and -flangelike, at least in advanced groups (early Pliocene to Recent); -ramus relatively short and stout, having distinct crest and ridges -for muscle attachments; coronoid process well developed, erect; -articular condyle prominent; angular process prominent, reflected -laterally, and in modern groups lateral extension protruding from -posterior border of ramus nearly at right angle; capsule for root of -lower incisor, prominent between angular process and articular -condyle. - -Anterior surface of incisors broad and flat, always smooth on lower -teeth, but either smooth or grooved on upper teeth depending on taxon; -cheek teeth hypsodont, becoming progressively higher crowned in modern -groups, rooted in primitive groups (late? Oligocene to middle -Pliocene), rootless and ever-growing in modern groups (late Pliocene -to Recent); upper and lower premolars persistently bicolumnar; upper -and lower molars bicolumnar only in primitive groups (late? Oligocene -and early Miocene), becoming progressively monocolumnar in advanced -groups (early Pliocene to Recent), primitive bicolumnar pattern being -retained on occlusal surface only in early stages of ontogeny and in -third molar throughout life; enamel pattern of occlusal surface of -cheek teeth based on sextituberculate prototype (see Wood and Wilson, -1936:388-391), having cusps arranged in two transverse rows of three -cusps each, excepting three anterior cusps of premolars that are -arranged in trefoil, especially on p4 (sometimes only one or two, -rather than three, cusps develop in a particular set, especially in -p4), conules absent; protostyle and endostyle in upper teeth and -protostylid and hypostylid in lower teeth formed from cingulum; cusps -of each row uniting with wear into transverse enamel lophs (or -lophids), each tooth having two lophs, one on anterior column, -protoloph and protolophid, and one on posterior column, hypoloph and -hypolophid, that unite with additional wear forming continuous enamel -band; enamel lacking on sides of each column in advanced lineages, -thereby restricting enamel to anterior and posterior walls; with -extreme reduction, posterior plates of upper teeth and, more commonly, -anterior plates of lower molars, missing. Dental formula: 1/1, 0/0, -1/1, 3/3. - -Key to the Subfamilies of Geomyidae - - A Angular process of ramus mostly below alveolar level of - mandibular tooth-row; pattern of premolar like that of molars, - consisting of two subequal crests united at one or both margins - of tooth; molars persistently bicolumnar; molariform teeth - always rooted. Subfamily Entoptychinae p. 513 - - A´ Angular process of ramus mostly above level of mandibular - tooth-row; pattern of permolar unlike that of molars, - consisting of two prisms differing in size and united at their - mid-points but never at either margin; molars progressively - monocolumnar, except for early Miocene forms; molariform teeth - rooted only in primitive genera (late? Oligocene to middle - Pliocene), and rootless and ever-growing in later genera (late - Pliocene to Recent). Subfamily Geomyinae p. 514 - - -Subfamily ENTOPTYCHINAE Miller and Gidley, 1918 - -Anterior face of upper incisor usually smooth, sometimes bearing faint -groove in center or near medial margin of tooth, at least in -_Gregorymys_; cheek teeth hypsodont, medium to high crowned, and -rooted in all but _Entoptychus_ (has rootless, ever-growing teeth); -cheek teeth identical in form, premolars resembling molars and lower -cheek teeth mirror images of upper teeth; crowns biprismatic, having -two columns joined at edge of protomeres (for description of term, see -discussion of primitive morphotype on page 537) and with persistent -lateral fissure between them; lateral re-entrant fold deep, -penetrating at least half width of crown, from external side in upper -teeth and internal side in lower teeth (in specialized genus -_Entoptychus_ lophs, upon additional wear, join also at edge of -parameres, thus uniting columns at both ends and thereby enclosing -interior part of lateral fissure as a transverse fossette in center of -tooth); enamel investment of prisms usually complete, including -inflection bordering re-entrant folds, occlusal pattern becoming -interrupted with wear only in _Entoptychus_, where enamel disappears -first from sides of crowns (following union of anterior and posterior -columns at both sides) and later, in final stages of attrition, from -anterior wall of lower molars and posterior wall of upper molars. - -Maxillary bone without pronounced vertical depth in part supporting -cheek teeth, its inferior border only slightly lower than inferior -border of premaxillary and alveolar lips of molariform teeth -consequently approximately level with, or slightly below, alveolar lip -of upper incisor; squamosal without lateral expansion, therefore, -meatal tube of auditory bulla separated from zygomatic process of -squamosal by deep, well-developed postglenoid notch; angular part of -mandible below alveolar level of mandibular cheek teeth; angular -process only slightly reflected laterally; coronoid process low, tip -only slightly above condyle. - -For information concerning the structure and relationships of the -known genera, and for accounts of species, see Wood (1936). A list -of the named genera in order of specialization is as follows: - - *_Pleurolicus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:66. - - *_Gregorymys_ Wood, 1936. Amer. Mus. Novit., 866:9. - - *_Grangerimus_ Wood, 1936. Amer. Mus. Novit., 866:13. - - *_Entoptychus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:64. - -Five new species have been described since Wood's (1936) revision. -They are: _Pleurolicus clasoni_ MacDonald (1963:180); _Gregorymys -kayi_ Wood (1950:335); _Gregorymys montanensis_ Hibbard and Keenmon -(1950:198); _Grangerimus dakotensis_ MacDonald (1963:182); -_Grangerimus sellardsi_ Hibbard and Wilson (1950:623). - - -Subfamily GEOMYINAE Baird, 1858 - -Anterior face of upper incisor primitively smooth, grooves -consistently developed only in one modern lineage (Geomyini); cheek -teeth hypsodont, primitively rooted and having crown of medium height -(late Oligocene to middle Pliocene), being higher crowned, rootless -and ever-growing in modern lineages (late Pliocene to Recent); -primitively crowns of cheek teeth biprismatic, having two columns -joined at mid-points by narrow isthmus and entire crown sheathed in -continuous band of enamel; premolars retaining primitive biprismatic -form, anterior and posterior columns never uniting at edge of -protomeres or parameres, and with both lateral re-entrant folds -persistent throughout life; primitive biprismatic pattern becoming -decidedly modified in molars (except in M3), having two prisms -progressively uniting into one column by reduction and loss of lateral -inflections, primitive biprismatic patterns being retained only in -early stages of ontogeny; third upper molars retaining, at least -partially, primitive bicolumnar pattern (except in Thomomyini), with -relatively broad isthmus and horizontally shallow re-entrant folds, -lingual fold sometimes wanting; enamel pattern becoming discontinuous -(late Pliocene to Recent) owing to loss of enamel from sides of each -column; remaining enamel restricted to anterior and posterior plates, -or cutting blades, and enamel bordering lateral inflections in -premolars (considering both sides together, these plates constitute -essentially two transverse cutting blades); enamel pattern of M3 -varying, depending on taxon; with specialization, anterior plates of -lower molars and posterior plates of upper premolar and molars may be -reduced or lost; except in primitive species (early Miocene), no -enamel fossettes retained in adult dentitions. - -Maxillary bone having pronounced vertical depth in part supporting -cheek teeth, inferior border arching downward well below inferior -border of premaxillary; consequently, alveolar lips of molariform -teeth decidedly below level of alveolar lip of upper incisor; -squamosal with marked lateral expansion at expense of postglenoid -notch; notch compressed and reduced between meatal tube of auditory -bulla and zygomatic process of squamosal; angular part of mandible -mostly above alveolar level of mandibular cheek teeth; angular process -reflected laterally at right angles to axis of ramus and developed -into heavy knoblike projection; coronoid process well developed, tip -decidedly higher than condyle; fossorial specializations remarkably -well developed in advanced lineages, degree of specialization of -primitive Miocene species unknown but probably only semi-fossorial as -in Entoptychinae. - -Key to the Tribes of the Geomyinae - - A Enamel investment complete and uninterrupted, even in final - (adult) stages of wear; cheek teeth rooted, with crowns of medium - height; third lower molar biprismatic, the two columns separated - by inner and outer re-entrant folds as in lower premolar. - Tribe Dikkomyini p. 515 - - A´ Enamel investment incomplete and discontinuous, reduced, at least - in final (adult) stages of wear, to interrupted enamel plates; - cheek teeth rootless and ever-growing (except in extinct genus - _Pliogeomys_), crowns of maximum height; third lower molar - monoprismatic, without trace of inner and outer re-entrant folds - as in first and second lower molars. - - B Upper incisors smooth, occasionally with a fine indistinct - groove near inner margin of tooth; form of third upper molar - same as M1 and M2, monoprismatic, anteroposteriorly compressed, - and having transverse enamel plates on both anterior and - posterior faces, and without suggestion of either labial or - lingual re-entrant folds; basitemporal fossa absent (except - for a shallow depression in one Recent species, _T. townsendii_); - forefoot small and narrow with claws not elongated for digging. - Tribe Thomomyini p. 518 - - B´ Upper incisors grooved, bearing either one or two sulci; form of - third upper molar distinctly different from M1 and M2, fully or - partially biprismatic (with a few exceptions discussed beyond), - without marked anteroposterior compression (either subtriangular, - elongated, suborbicular or quadriform in cross-section, but not - elliptical as in M1 and M2), and having typical transverse - anterior plate and two lateral plates (varying in their - development, depending on taxa), but no posterior plate, and with - lateral re-entrant folds usually developed, especially labial - inflection (although sometimes minute in a few species, as - described beyond); basitemporal fossa well-developed, although - occasionally shallow or absent (primitive species of _Zygogeomys_); - forefoot large and broad, with elongated claws for digging. - Tribe Geomyini p. 521 - - -Tribe DIKKOMYINI, new tribe - -_Genotype._--_Dikkomys_ Wood, 1936. - -_Chronologic and geographic range._--Early to Middle Pliocene (early -Arikareean to mid-Hemphillian) in western United States. Known from -Miocene fossil sites in Montana, South Dakota, and Nebraska and -Pliocene sites in South Dakota, Oregon, Nevada, and southern -California. For precise localities see accounts of _Dikkomys_ and -_Pliosaccomys_ beyond. - -_Diagnosis._--Small Geomyinae; lacking specializations of more -advanced tribes; upper incisors smooth, at least in _Pliosaccomys_; -molariform teeth always rooted and having crowns of medium height; -enamel investment of cheek teeth complete and uninterrupted in all -stages of wear; crowns of molars primitively biprismatic, having two -columns united at mid-points, thus forming narrow isthmus separating -lateral re-entrant folds as in premolars, and, with wear, also uniting -secondarily at protomeres (with exception of third lower molars), -consequently, isolating remnant of that inflection as shallow fossette -(columns uniting first at protomeres in _Pliosaccomys_); anterior and -posterior columns of first and second molars, both above and below, -becoming progressively united into one column in advanced Dikkomyini -(early and middle Pliocene), but m3 (M3 unknown) retaining primitive -biprismatic pattern, with columns joined at centers but never at -protomeres (for details of dentition see generic accounts); mandible -stout, its angle mostly above mandibular tooth-row; masseteric ridge -low; basitemporal fossa barely discernable in some fragments of -_Pliosaccomys_; postcranial skeleton unknown. - -Key to the Genera of the Tribe Dikkomyini - - A Molars biprismatic throughout life; anterior and posterior - lophs of first and second molars in pre-final stages of wear - uniting first at their mid-points and later at edge of - protomeres; anterior lophid of lower premolar having distinct - anteroexternal inflection. Genus _Dikkomys_ p. 516 - - A' First and second molars becoming monoprismatic in final - (adult?) stages of wear, biprismatic only in pre-final stages - of wear; third molars persistently biprismatic; anterior and - posterior lophs of first and second molars uniting first at - edge of protomeres; anterior lophid of lower premolar lacking - anteroexternal inflection. Genus _Pliosaccomys_ p. 517 - - -Genus =Dikkomys= Wood - - 1936. _Dikkomys_ Wood, Amer. Mus. Novit., 866:26, July 2. - -_Type._--_Dikkomys matthewi_ Wood, 1936, from Lower Harrison deposits -near Agate, Sioux County, Nebraska. - -_Chronologic range._--Early Miocene, from early Arikareean (Lower -Harrison local fauna of Nebraska) to middle Miocene, late -Hemingfordian (Upper Rosebud local fauna, South Dakota, and the Deep -River Formation, Montana). According to MacDonald (1963:149-150), the -Upper Rosebud is middle Miocene rather than early Miocene. - -_Description._--Size small, about as in small kinds of _Thomomys_; -known only from fragmentary mandible, including molariform dentition -in place, and isolated cheek teeth, including M1 (see Wood, 1936:26-28 -and fig. 32; Galbreath, 1948:316-317 and fig. 1; and Black, 1961:13-14 -and fig. 58); upper incisors unknown; cheek teeth hyposodont, -persistently rooted, and having crowns of medium height compared with -Recent geomyids; enamel investment complete and uninterrupted in all -molariform teeth in all stages of wear; P4 unknown, but probably -formed like p4; p4 persistently biprismatic, two crowns joined at -mid-points by relatively narrow isthmus separating lateral re-entrant -folds; anterior lophid of p4 having distinct anteroexternal -inflection; molars also biprismatic throughout life; two lophids of -lower molars first uniting at mid-points as in p4, and, with -additional wear, m1 and m2 secondarily uniting at edge of protomeres -and forming isolated enamel fossette between point of connection -(detailed description of stages of wear discussed in account of -phylogeny of subfamily); m3 permanently joined at mid-point only, -without lateral union at edge of protomeres; upper molars, judging by -M1 (M2 and M3 unknown), having same pattern as lower molars, but first -union of lophs decidedly on lingual side of center, consequently, -lingual re-entrant fold small; M1 probably developing U-pattern in -advanced stages of wear by union of protomeres, with minute lingual -fossette developing in transition as lophs secondarily become united -at lingual edge of columns; mandible stout and geomyidlike; masseteric -ridge weakly developed; basitemporal fossa absent. - -Evidently, _Dikkomys matthewi_ is more primitive than _Dikkomys -woodi_. The modified H-pattern in m1 and m2, with the metalophid and -hypolophid joined at both their mid-points and also at their -protomeres (by union of the protostylid and hypostylid in the lower -dentition), is persistent throughout life. Therefore, the enclosed -enamel fossette is not eradicated with wear. In m1 and m2 of _Dikkomys -woodi_, the fossette is shallower, and, at least in advanced stages of -wear, it would disappear, therefore, forming a U-pattern on the -occlusal surface, as in M1 and M2, but lateral inflection horizontally -shallow rather than deep as in entoptychines. - -Specimen (No. P 26284 FMNH) reported as _Dikkomys matthewi_ by -Galbreath (1948:316) is referable to the recently described species -_Dikkomys woodi_ Black, 1961. - -_Specimens examined._--One, no. P 26284, Field Mus. Nat. Hist., from -upper Rosebud, Shannon Co., South Dakota. - -_Referred species._--two: - - _Dikkomys matthewi_ Wood, 1936. Amer. Mus. Novit., 866:26, July. - Type from early Arikareean Lower Harrison deposits (early Miocene) - near Agate, Sioux County, Nebraska. - - _Dikkomys woodi_ Black, 1961. Postilla, Yale Peabody Museum, 48:13, - January 16. Type from Deep River Formation, late Hemingfordian - (middle Miocene), Meagher County, Montana; also known from Upper - Rosebud deposits (middle Miocene) near Wounded Knee, Shannon - County, South Dakota. - - -Genus =Pliosaccomys= Wilson - - 1936. _Pliosaccomys_ Wilson, Carnegie Inst. Washington Publ., - 473:20, May 21. - -_Type._--_Pliosaccomys dubius_ Wilson, 1936, from Smiths Valley local -fauna in Lyon County, Nevada. - -_Chronologic range._--Early Pliocene, late Clarendonian (Wolf Creek -local fauna, South Dakota, and Nettle Springs local fauna, California) -to Middle Pliocene, middle part of Hemphillian (Smiths Valley local -fauna, Nevada, and McKay Reservoir and Otis Basin local faunas, -Oregon). - -_Description._--Size small (alveolar length of mandibular tooth-row -measuring 6.0 in holotype), about as in _Thomomys monticola_; upper -incisor relatively broad and flat, having anterior face smooth, -without trace of grooving; crowns of cheek teeth of medium height and -rooted; enamel investment continuous and uninterrupted in all stages -of wear; premolars permanently, biprismatic; P4 having anterior prism -subtriangular and decidedly smaller that sub-crescentic posterior -prism, and joined near centers by narrow, obliquely oriented isthmus; -p4 having anterior prism subovate, posterior prism strongly compressed -anteroposteriorly, and joined at mid-points by relatively broad and -straight isthmus; first and second molars, both above and below, -monoprismatic in final (?adult) stage of wear, derived ontogenetically -from primitive bilophate pattern by coalescence of two columns into -one; M1 and M2 mirror images of m1 and m2 in pre-final stages of wear, -two columns first uniting at edge of protomeres forming U-pattern, and -primitive H-pattern never developing in either series (for detailed -description of stages of wear, see account of phylogeny, p. 546); m3 -(M3 unknown, but probably with same form as in Geomyini, see p. 552) -persistently biprismatic, two columns joined by relatively broad -isthmus at centers, consequently, forming H-pattern of primitive -ancestors; rostrum heavy and broad as in modern geomyids; palate -narrow and strongly ribbed; mandible stout; masseteric ridge and fossa -well developed; basitemporal fossa absent. - -_Specimens examined._--Six, nos. 1796 (holotype)--1799, 1804 and 1806 -(CIT) now in the Los Angeles County Museum, all from Smiths Valley -local fauna, Middle Pliocene, Nevada. - -_Referred species._--two: - - *_Pliosaccomys dubius_ Wilson, 1936. Carnegie Inst. Washington Publ., - 743:20, May 21. Known from early and middle Pliocene faunas - including Wolf Creek local fauna (late Clarendonian), Shannon - County, South Dakota; McKay Reservoir local fauna and Otis Basin - local fauna (Hemphillian), Oregon; type from Smiths Valley local - fauna (probably middle Hemphillian), Lyon County, Nevada. - - *_Pliosaccomys wilsoni_ James, 1963. Univ. California Publ. Geol. Sci., - 45:101, June 26. Type from Nettle Springs local fauna of late - Clarendonian (early Pliocene), Ventura County, California. - - -Tribe THOMOMYINI, new tribe - -_Type._--_Thomomys_ Wied-Neuwied, 1839. - -_Chronologic and geographic range._--Known from late Pliocene (early -Blancan) to Recent. Known primarily from western North America from -southern Canada south to Central México in Pliocene, Pleistocene and -Recent and in middle and late Pleistocene of Maryland and Florida. - -_Diagnosis._--Size small to medium (basilar length exclusive of _T. -bulbivorus_, measuring from approximately 24 to 45, including both -males and females); upper incisors without grooving, excepting fine, -indistinct sulcus rarely near inner margin (grooving more common in -_T. monticola_ than in other Recent species); crowns of cheek teeth -high, rooted and ever-growing; all molars, including M3, monoprismatic -and anteroposteriorly compressed, sometimes (especially in subadults) -having slight inflection on labial side in upper teeth and lingual -side in lower teeth; molars bicolumnar in pre-final stages of wear -(seen in juvenal teeth only), patterns of wear in both upper and lower -molars resembling those of _Pliosaccomys_, except that crowns of m3 -and M3 unite into single column in final stages of wear; enamel -pattern interrupted in all cheek teeth, loss occurring only at sides -of each column; transverse enamel blade completely covering posterior -face of both P4 and p4; all upper and lower molars with two transverse -enamel blades, one on anterior surface and one on posterior surface, -of each tooth, including M3; small third plate sometimes persistent on -broad side of tooth, labial side in upper molars and lingual side in -lower molars (_T. bulbivorus_); skull generalized, neither unusually -narrow and deep or broad and flat; usually without marked cresting or -rugosity; masseteric ridge well developed and massive; basitemporal -fossa absent, sometimes shallow depression forming in _T. townsendii_; -pelage soft, never harsh or hispid, covering body with thick coat of -hair; forefoot exceptionally small for fossorial mammal, claws not -especially long; body form remarkably fossorial. - -The tribe Thomomyini is monotypic, including only the genus -_Thomomys_. - - -Genus =Thomomys= Wied-Neuwied - - 1839. _Thomomys_ Wied-Neuwied, Nova Acta Phys. Med. Acad. Caesar. - Leop.-Carol., 19(1):377. - - 1836. _Oryctomys_ Eydoux and Gervais (in part), Mag. de Zool., 6:20, - pl. 21. Type: _Oryctomys_ (_Saccophorus_) _bottae_, from - coast of California, probably near Monterey. - - 1903. _Megascapheus_ Elliot, Field Columb. Mus., Publ. 76, Zool. - Ser., 3(11):190, July 25. Type: _Diplostoma bulbivorum_ - Richardson, from Columbia River, probably near Portland, Ore. - - 1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm. 14:354. Type: - _Pleisothomomys potomacensis_ Gidley and Gazin, from - Pleistocene, Cumberland Cave local fauna, Allegany County, - Maryland. - -_Chronologic range._--Known from late Pliocene to Recent. - -_Description._--Same as that given for the tribe Thomomyini above. - -_Discussion._--Features characterizing _Thomomys_ and the tribe -Thomomyini are more advanced than those characterizing the tribe -Dikkomyini. Also, the Thomomyini retain more of the primitive -features of the Geomyinae than do the more specialized tribe Geomyini. - -Specializations are few, but include the third molar being a single -column both above and below, enamel plates, and a masseteric ridge. - - -Key to the Subgenera of _Thomomys_ - - A Molars sub-crescent or ovate in cross-section, not - becoming abruptly narrower at one end of tooth. - Subgenus _Pleisothomomys_ p. 519 - - A´ Molars pear-shaped, not sub-crescent or ovate, in - cross-section, crown becoming abruptly narrow at one - end of tooth. Subgenus _Thomomys_ p. 520 - - -Subgenus =Pleisothomomys= Gidley and Gazin - - 1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm., 14:354, - November 13. - -_Type._--_Pleisothomomys potomacensis_ Gidley and Gazin, 1933. - -_Chronologic range._--Late Pliocene (Hagerman local fauna, Idaho) to -late Pleistocene. The latest records are from the fauna of Saber-tooth -Cave, Florida, a late Pleistocene assemblage that probably was -deposited in the Sangamon. The middle and late Pleistocene records are -from the eastern United States, suggesting that the subgenus -_Pleisothomomys_ was restricted to that region while the subgenus -_Thomomys_ occupied the western United States and parts of Canada and -México as it does today. - -_Description and Comparison._--Separated from subgenus _Thomomys_ only -on basis of sub-crescentic shaped molars (only jaw fragments and -isolated teeth known), seemingly a primitive feature of the genus. -This dental structure continued into the late Pleistocene; none of the -Recent species expresses this feature of the molars, although the -molars of _Thomomys vetus_ of the late Pleistocene (Wisconsin -deposits), referred to the subgenus _Thomomys_ on the basis of -its alleged relationship to _Thomomys townsendii_ (see Davis, -1937:156-158), are less distinctly pear-shaped, and are more -sub-crescentic, than in any other known species of the subgenus -_Thomomys_. _Pleisothomomys_ Gidley and Gazin (_loc. cit._) was -proposed as a genus but is here considered as of no more than -subgeneric worth, and is recognized because of the apparent constancy -of the sub-crescentic molars in the earlier members of the genus and -in those populations of _Thomomys_ occurring in Pleistocene times in -the eastern United States. - -_Referred species._--Three (all extinct): - - *_Thomomys gidleyi_ Wilson, 1933. Carnegie Inst. Washington Publ. - 440:122, December. Type from Hagerman beds, late Pliocene, - Idaho. - - *_Thomomys potomacensis_ Gidley and Gazin, 1933. Jour. Mamm., - 14:354, November 13. Type from Cumberland Cave, middle and late - Pleistocene, Maryland. - - *_Thomomys orientalis_ Simpson, 1928. Amer. Mus. Novit., 328:6, - October 26. Type from Saber-tooth Cave, late Pleistocene, - Florida. - - -Subgenus =Thomomys= Wied-Neuwied - - 1839. _Thomomys_ Wied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar. - Leop. Carol., 19(1):377. - - 1903. _Megascapheus_ Elliot, Field Columb. Mus., Publ. 76, Zool. - Ser., 3 (11):190, July 25. Type: _Diplostoma bulbivorum_ - Richardson, from Columbia River, probably near Portland, Oregon. - -_Type._--_Thomomys rufescens_ Wied-Neuwied, 1839. - -_Chronologic range._--Early Pleistocene (Broadwater-Lisco local fauna, -Nebraska) to Recent. Numerous records, mostly isolated teeth, from -nearly all stratigraphic levels of the Pleistocene (for details, see -account of fossil record). - -_Description._--Molars pear-shaped in cross-section, becoming abruptly -narrow at one end of the tooth. The teeth of the late Pleistocene -species _Thomomys vetus_ are less distinctly pear-shaped than other -referred species (see remarks in the description of the subgenus -_Pleisothomomys_). - -Essentially on the basis of its significantly larger size and details -of the skull, Elliott (1903:190) proposed subgeneric recognition of -_Thomomys bulbivorus_ and described the subgenus _Megascapheus_ to -include it. Also the molars of _Thomomys bulbivorus_ usually have a -small enamel plate, both above and below, bordering the persistent -inflection on the protomere end of the tooth; each lateral plate is -isolated from the transverse plates on the anterior and posterior -walls of the tooth. In my opinion these features do not warrant -subgeneric recognition; however, these characters do distinctly -separate _Thomomys bulbivorus_ from other groups of species, and the -character of the molars suggests retention of a primitive trait. -Therefore, I propose that the unique structure of this species be -recognized by setting it apart in the _bulbivorus_ species-group. - -_Referred species._--Ten species, three extinct, placed in three -species-groups (the numerous subspecies of this genus are listed in -Miller and Kellogg, 1955:276-332, and Hall and Kelson, 1959:412-447). - - -_bulbivorus_ species-group - - _Thomomys bulbivorus_ (Richardson, 1829). Fauna Boreali-Americana, - 1:206. Type from Columbia River, probably near Portland, - Oregon. - - -_umbrinus_ species-group - - *_Thomomys scudderi_ Hay, 1921. Proc. U. S. Nat. Mus., 49:614. - Type from Fossil Lake beds, late Pleistocene, Oregon. - - _Thomomys umbrinus_ (Richardson, 1829). Fauna Boreali-Americana, - 1:202. Type from southern México, probably near Boca de Monte, - Veracruz. - - _Thomomys bottae_ (Eydoux and Gervais, 1836). Mag. de Zool., Paris, - 6:23. Type from coast of California, probably near Monterey. - - *_Thomomys vetus_ Davis, 1937. Jour. Mamm., 18:156, May 12. Type - from Fossil Lake beds, late Pleistocene, Oregon. - - _Thomomys townsendii_ (Bachman, 1839). Jour. Acad. Nat. Sci. - Philadelphia, 8:105. Type probably from near Nampa, Canyon Co., - Idaho (erroneously given as "Columbia River"). - - -_talpoides_ species-group - - *_Thomomys microdon_ Sinclair, 1905. Bull. Dept. Geol. Univ. - California, 4:145-161. Type from Potter Creek Cave, late - Pleistocene, California. - - _Thomomys monticola_ J. A. Allen, 1893. Bull. Amer. Mus. Nat. Hist., - 5:48, April 28. Type from Mt. Tallac, 7500 ft., El Dorado Co., - California. - - _Thomomys talpoides_ (Richardson, 1828). Zool. Jour., 3:518. Type - locality fixed at near Fort Carlton (Carlton House), - Saskatchewan River, Saskatchewan, Canada. - - _Thomomys mazama_ Merriam, 1897. Proc. Biol. Soc. Washington, - 11:214, July 15. Type from Anna Creek, 6000 ft., near Crater - Lake, Mt. Mazama, Klamath Co., Washington. - - -Tribe GEOMYINI, new tribe - -_Genotype._--_Geomys_ Rafinesque, 1817. - -_Chronologic and geographic range._--Known from late middle Pliocene -deposits to Recent. The range of living members extends from extreme -southern Manitoba and the southeastern United States south to southern -Panamá, and probably northern Colombia, South America. - -_Diagnosis._--Size small to large (condylobasal length of skull 33.0 -to 73.0 in adults, including both sexes); sexual dimorphism marked, -sometimes strongly, females being smaller than males, especially in -cranial dimensions; upper incisors invariably grooved, number and -position of grooves varying according to genus; cheek teeth -high-crowned and ever-growing, except in one primitive genus -(_Pliogeomys_); all three lower molars and M1 and M2 monoprismatic, -and elliptical in cross-section in final stages of wear (teeth of -young, subadult, and adult animals); primitive biprismatic patterns -(as known from Recent specimens) occurring only in pre-final stages of -wear (teeth of juveniles only); biprismatic patterns of lower molars -as in _Dikkomys_, and upper molars as in _Pliosaccomys_ (for detailed -description of these patterns, see account beyond of the phylogeny of -the Geomyinae); m3 becoming monoprismatic, anteroposteriorly -compressed and elliptical in cross-section like m1 and m2, but M3 -remaining, with rare exceptions (see accounts of _Geomys_ and -_Pappogeomys_ beyond), at least partially biprismatic throughout life, -having one or both lateral inflections usually persisting (with -exceptions) and developing various occlusal shapes (subtriangular, -elongate, obcordate, suborbiculate, or quadriform) but never -elliptical. - -Enamel of cheek teeth reduced to interrupted plates, with exception of -p4 in _Pliogeomys_; plate on posterior wall of P4 variable, occurring -completely across posterior surface in primitive members, but -progressively reduced to lingual side only or completely lost in -modern genera (see generic accounts beyond for detailed description); -both anterior and posterior plates usually retained in M1 and M2, -posterior plate sometimes reduced to lingual side or completely lost -(as in _Pappogeomys_) but anterior plate always completely retained; -M3 usually having three plates, one anterior and two lateral; -posterior plate wanting (sometimes lingual plate moved to posterior -position); plates retained completely across posterior walls of all -lower cheek teeth with no reduction, but anterior plates of m1-3 -always lacking, except in primitive genus _Pliogeomys_ (only Geomyini -having both anterior and posterior enamel plates on lower molars). - -Skull primitively generalized, but becoming specialized towards either -dolichocephaly (_Orthogeomys_) or platycephaly (_Pappogeomys_) in two -modern genera; skull highly specialized for fossorial life; mandible -stout and deep, angular process being high and diverging laterally at -right angles to ramus; masseteric ridge and fossa weakly developed in -primitive members, becoming well developed and massive in modern -genera; basitemporal fossa absent in primitive forms (_Pliogeomys_ and -early members of _Zygogeomys_); pelage usually soft, but harsh and -hispid in some genera; forefeet broad and massive, claws long and -stout for digging; body form remarkably fossorial. - -The tribe Geomyini includes the most highly specialized members of the -subfamily Geomyinae. - - -Key to the Genera of the Tribe Geomyini - - A Cheek teeth rooted; p4 with uninterrupted enamel loop; enamel - plates on both anterior and posterior walls of m1 and m2; - masseteric ridge weakly developed, low, not massive. - Genus _Pliogeomys_ p. 522 - - A´ Cheek teeth rootless, ever-growing; p4 with enamel investment - interrupted at ends of columns, consequently, forming four - isloted plates; enamel plate retained only on posterior wall - of m1 and m2, anterior wall without trace of enamel (except - rarely in pre-final stage of wear in _Geomys tobinensis_ of - middle Pleistocene); masseteric crest strongly developed and - massive. - - B Enamel plate on posterior wall of P4, but usually - restricted to lingual end of tooth (usually absent in - subgenus _Orthogeomys_ of genus _Orthogeomys_); - M3 conspicuously bicolumnar, longer than wide owing to - elongation of posterior loph. - - C Upper incisor bisulcate; skull generalized; rostrum - relatively narrow; length of labial enamel plate of - M3 decidedly less than length of lingual plate; - pelage soft and thick. Genus _Zygogeomys_ p. 523 - - C´ Upper incisor unisulcate; skull strongly - dolichocephalic; rostrum remarkably broad and massive; - length of lingual plate of M3 approximately equal to, - or greater than, length of labial plate; pelage harsh, - often hispid and scant. Genus _Orthogeomys_ p. 528 - - B´ Posterior wall of P4 without trace of enamel; M3 not - strongly bicolumnar, having shallow re-entrant fold on - labial side, and crown no longer than wide owing to - shortness of posterior loph. - - D Upper incisor bisulcate; skull generalized; both - anterior and posterior walls of M1 and M2 having - complete enamel plates. Genus _Geomys_ p. 525 - - D´ Upper incisor unisulcate; skull generalized or - tending towards platycephaly; enamel plate on - posterior wall of M1 usually reduced to lingual - side or absent (complete only in one species, - _Pappogeomys bulleri_); enamel plate on posterior - wall of M2 also absent in advanced species - (subgenus _Cratogeomys_). Genus _Pappogeomys_ p. 532 - - -Genus =Pliogeomys= Hibbard - - 1954. _Pliogeomys_ Hibbard, Michigan Acad. Sci., Arts and - Letters, 39:353. - -_Genotype._--_Pliogeomys buisi_ Hibbard, 1954, from Buis Ranch local -fauna (middle Pliocene), Beaver County, Oklahoma. - -_Chronologic range._--Latest Middle Pliocene, known only from the -highest part of the Hemphillian mammalian fauna (Buis Ranch local -fauna, Oklahoma). Professor Hibbard informs me (personal -communication) that he found the type, a right ramus, lying on the -surface near the base of the fossil beds. The isolated teeth of small -geomyids from the Saw Rock Canyon local fauna (see Hibbard, 1953:392) -may also be referable to this genus. The Saw Rock Canyon local fauna -may also be middle Pliocene in age but is considered to be from the -later part of the late Pliocene, and, therefore, somewhat younger than -the Buis Ranch local fauna (Hibbard, _op. cit._:342). - -_Description and discussion._--The size of members of this small genus -of the Geomyinae is about the same as in smaller adults of _Geomys -bursarius_. According to Hibbard (_op. cit._:353), the holotype is -smaller than specimens from the Rexroad local fauna referred to -_Geomys quinni_ and larger than specimens referred to _Zygogeomys_ cf. -_minor_. The cheek teeth are rooted, and the crowns are as high as -those of living geomyids. The upper incisor is bisulcate, and the -inner groove is fine and indistinct in places. - -Of the molariform dentition only the lower premolar and first two -lower molars are known. The enamel investment of p4 is complete, and -would not be subject to interruption at any stage of wear; the two -prisms are joined at their mid-points, and the isthmus of dentine is -relatively broad (as in _Pliosaccomys_) when compared with modern -pocket gophers of this tribe. Also, the re-entrant folds, rather than -having parallel sides, diverge broadly to the sides. The divergence is -especially noticeable in the labial fold. The lower deciduous premolar -would have formed essentially the same enamel pattern with wear as -observed in _Nerterogeomys_ [= _Zygogeomys_] cf. _minor_ (see Hibbard, -1954:fig. 5, A and B) and _Pliosaccomys dubius_ (see Wilson, 1936; pl. -1, fig. 1). Each molar is a single column in the final stages of wear; -pre-final stages are unknown. Anterior and posterior enamel plates are -present on m1 and m2 (m3 has not been recovered). The dentine tracts -of m1 are exposed over a relatively wide surface; therefore, the -enamel plates are distinctly separated. The tracts of dentine of m2 -are much narrower than in m1 and the enamel plates are barely -separated at the anterolateral margin of the tooth. Possibly the -enamel band of m2 was continuous in an earlier stage of wear. - -The mandible is stout and its general construction not unlike that in -modern geomyines. The capsule at the base of the angular process that -receives the terminal end of the lower incisor is well developed. The -base of the angular processes is preserved, and suggests that the -process was short and decidedly smaller than in living examples of the -tribe. The masseteric ridge is distinct but weakly developed, and not -at all massive as in living pocket gophers. The mental foramen is -immediately anterior, and slightly ventral, to the anterior extension -of the crest. The basitemporal fossa is absent as such, but its -position is marked by a slight depression. - -_Specimens examined._--Two rami; nos. 29147 (holotype) and 33446; -several isolated teeth 30194 and 30195, including an upper incisor and -a dp4 (deciduous lower premolar), all from Univ. Michigan Mus. Paleo. - -_Referred species._--One. - - *_Pliogeomys buisi_ Hibbard, 1954. Papers Michigan Acad. Sci., - Arts, and Letters, 39:353. Type from Buis local fauna, latest - middle Pliocene, Beaver County, Oklahoma. - - -Genus =Zygogeomys= Merriam - - 1895. _Zygogeomys_ Merriam, N. Amer. Fauna, 8:195, January 31. - - 1942. _Nerterogeomys_ Gazin, Proc. U. S. Nat. Mus., 92:507 - (type, _Geomys persimilis_ Hay, 1927). - -_Type._--_Zygogeomys trichopus_ Merriam, 1895, from Nahuatzen, -Michoacán. - -_Chronologic range._--Late Pliocene (Benson and Curtis Ranch local -faunas, Arizona, and ?Rexroad Formation, Kansas) to Recent. - -_Description and discussion._--The size is small to medium for the -subfamily Geomyinae. This genus is distinguished principally by the -retention of primitive features. In the living species, the skull is -generalized, rather than specialized toward either extreme -dolichocephaly or platycephaly. The angular process is short, barely -exceeding the lateral extensions of the mastoid process of the -squamosal. The rostrum is remarkably narrow in relation to its length. -The jugal is reduced and displaced ventrally, causing the maxillary -arm of the zygomata to articulate with the squamosal arm of the -zygomata along the dorsal border of the zygomatic arch (a feature -observed also in _Orthogeomys cherriei costaricensis_). - -The upper incisor, recovered in material from the late Pliocene and -middle Pleistocene, is bisulcate as in the genus _Geomys_ and the -primitive genus _Pliogeomys_. The enamel plate across the posterior -wall of P4 is either complete (late Pliocene to late Pleistocene) or -restricted to the lingual half of the tooth (always restricted in -living species). The Pliocene specimens of the Rexroad local fauna -referred to _Nerterogeomys_ cf. _minor_ by Hibbard (1950:138-139) are -exceptional. In these specimens the length and position of the -posterior enamel plate is variable; however, all but one specimen had -persistant enamel. Evidently, in approximately 43 per cent of the -specimens, a complete enamel blade was present (see Paulson, -1961:139), and in the others (except the one without any enamel) the -plate was restricted to a small area of the ventral surface, usually -on the lingual side of the loph. Hibbard suggested that the decrease -in size of the plate, and its restriction to the lingual side, may be -a function of age. Hence, most adults would be characterized by the -reduced posterior plate on the upper premolar. Although age may be the -important factor, intragroup variation cannot be ruled out. It is of -interest to note that in all specimens from the Benson (type series of -_P. minor_) and Curtis Ranch local faunas, the former of late Pliocene -age and the latter of middle Pleistocene age, the enamel plates are -complete on the posterior face of the upper premolar. As mentioned -before, the specimens from Kansas may actually represent the -transitional stages of the early evolution of _Geomys_ in which the -posterior plate of P4 is entirely lost. The enamel pattern of p4 is -like that in other members of the tribe (excepting the genus -_Pliogeomys_). The re-entrant angles of P4 and p4 are widely open -(obtuse) in the examples recovered from late Pliocene and middle -Pleistocene deposits, representing retention of a trait that is -primitive in the Geomyini (see account of phylogeny). - -M1 and M2 are elliptical in cross-section and each has an enamel plate -on both the anterior and posterior surface. In the living species (_Z. -trichopus_), the posterior enamel plate fails to reach the labial -margin of the tooth and is restricted to the lingual two-thirds of the -posterior surface; however, the enamel plates are complete in the late -Pliocene species (_Z. minor_) and the middle Pleistocene species (_Z. -persimilis_), being only slightly separated from the anterior plate by -narrow tracts of dentine on the ends of the tooth. M3 is partly -biprismatic in the living species, the two incompletely divided lophs -being separated by a distinct outer sulcus. The posterior loph is -elongated and forms a conspicuous heel paralleling the evolution of -this trait in the genus _Orthogeomys_; therefore, the crown is longer -than wide. The posterior part of the tooth is protected by two lateral -enamel plates; of the two, the lingual plate is especially long and -extends to the end of the heel. M3 has not been recovered in the -Pliocene species, but in the middle Pleistocene species (_Z. -persimilis_) M3 is subtriangular, no longer than wide, and the lateral -inflections are weakly developed. The trend towards elongation of M3 -evidently occurred in late Pleistocene evolution of the genus. All -three of the inferior molars are elliptical, and only the posterior -enamel plate is present (as in all other genera of the tribe except -_Pliogeomys_). - -The masseteric ridge of the mandible is well developed. In the late -Pliocene species _Z. persimilis_ and _Z. minor_ the mental foramen is -directly beneath the anterior extension of the masseteric ridge, but -in the living species, _Z. trichopus_, the foramen lies well anterior -to the ridge. The basitemporal fossa in the living species is well -developed and deep; in the Pliocene species it is usually distinct but -shallow (late Pliocene specimens of _Z. minor_). - -_Referred species._--Three (two extinct and one living; the last has -two subspecies): - - *_Zygogeomys minor_ (Gidley), 1922. U. S. Geol. Surv. Prof. Paper, - 131:123, December 26. Type from Benson local fauna (late - Pliocene), Cochise County, Arizona; also known from the - Rexroad local fauna, Meade County, Kansas. - - *_Zygogeomys persimilis_ Hay, 1927. Carnegie Inst. Washington - Publ., 136. Originally described by Gidley, 1922 (U. S. Geol. - Surv. Prof. Papers, 131:123, December 26) as _Geomys - parvidens_ which was preoccupied by _G. parvidens_ Brown, - 1908. Type from Curtis Ranch local fauna (middle - Pleistocene), Cochise County, Arizona. - - _Zygogeomys trichopus trichopus_ Merriam, 1895. N. Amer. Fauna, - 8:196, January 31. Type from Nahuatzen, Michoacán. - - _Zygogeomys trichopus tarascensis_ Goldman, 1938. Proc. Biol. Soc. - Washington, 51:211, December 23. Type from 6 mi. SE - Pátzcuaro, 8,000 ft., Michoacán. - - -Genus =Geomys= Rafinesque - - 1817. _Geomys_ Rafinesque, Amer. Monthly Mag., 2(1):45, November. - - 1817. _Diplostoma_ Rafinesque, Amer. Monthly Mag., 2(1):44-45, - November. Included species: _Diplostoma fusca_ Rafinesque - [= _Mus bursarius_ Shaw] and _Diplostoma alba_ Rafinesque - [= _Mus bursarius_ Shaw] from the Missouri River region. - - 1820. _Saccophorus_ Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66. - Type: _Mus bursarius_ Shaw, from upper Mississippi Valley. - - 1823. _Pseudostoma_ Say, Long's Expd. Rocky Mts., I, pp. 406. Type: - _Pseudostoma bursaria_ [= _Mus bursarius_ Shaw], from upper - Mississippi Valley. - - 1825. _Ascomys_ Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822), - p. 20., fig. 2. Type: _Ascomys canadensis_ Lichtenstein - [= _Mus bursarius_ Say], probably from upper Mississippi Valley. - - 1944. _Parageomys_ Hibbard, Bull. Geol. Soc. Amer., 55:735, June. - Type: _Parageomys tobinensis_ Hibbard, from Pleistocene, Cudahy - (Tobin) local fauna, Russell Co., Kansas. - -_Type._--_Geomys pinetis_ Rafinesque, 1817, restricted to Screven -County, Georgia, in region of the pines. - -_Chronologic range._--Late Pliocene faunas of Blancan age (Rexroad, -Kansas, and Sand Draw, Nebraska, local faunas) to Recent. Reported -from numerous Pleistocene deposits of all stratigraphic levels, -especially from the Great Plains, where common today. - -_Description and discussion._--Pocket gophers of this genus are -medium-sized geomyids; none is so small as the average-sized -_Thomomys_. The skull is generalized and lacks the dolichocephalic and -platycephalic specializations seen in the genera _Orthogeomys_ and -_Pappogeomys_, respectively. _Geomys_ closely resembles _Zygogeomys_, -but retains fewer of the primitive characters of the ancestral stock. -At the same time, _Geomys_ has several specializations. Even so, a -considerable amount of parallelism is evident in the phyletic trends -of the two genera. - -The upper incisor of _Geomys_ is bisulcate as in _Pliogeomys_ and -_Zygogeomys_; the deeper grove is medial and the shallower grove lies -near the inner border of the tooth. The premolar, above and below, is -bicolumnar; and two columns are joined at their mid-points (deep -re-entrant angles separate the columns at the sides). A permanent -enamel plate protects the anterior face of the anterior loph, and -enamel bands outline each of the re-entrant folds. In p4 a complete -enamel plate covers the posterior surface of the posterior loph. All -of the enamel bands are interrupted by tracts of dentine, except in -the initial stages of wear of the occlusal surface of the newly -erupted tooth. For a short time in living _Geomys_, the enamel bands -are continuous as observed in juveniles of _Geomys bursarius major_ -(KU 5628, 8531, and 41540). But, the enamel cap is thin and the -dentine tracts, which are high on the sides of the tooth, are soon -revealed by a minimum of wear on the crown. Therefore, the adult, or -final, pattern characterized by interrupted enamel plates emerges -early in life and remains throughout the life of the individual. -Evidence from fossil _Geomys_, especially from specimens from early -and late Pleistocene deposits, suggests that the final adult pattern -appears later, ontogenetically, than in Recent specimens. Some of the -fossil premolars in initial stages of wear have continuous and -uninterrupted bands of enamel. _Geomys quinni_ of the late Pliocene -and early Pleistocene has the interrupted pattern seen in late -Pleistocene and Recent _Geomys_. Also, in late Pliocene and early -Pleistocene species, the re-entrant folds diverge laterally and form -"open" angles. In later taxa (middle Pleistocene to Recent) the folds -are compressed and parallel-sided, and the "open" folds are found only -in the early stages of wear. - -The posterior enamel plate of P4 disappears in the final stages of -wear as the interrupted enamel pattern is formed. In the late -Pleistocene and Recent _Geomys_, the loss of the posterior plate -occurs early in life, usually in the first phases of wear on the -occlusal surface of the newly erupted tooth, but in fossils of -_Geomys_ of corresponding ontogenetic age from the early and middle -Pleistocene, the posterior plate is retained in some individuals until -a later phase of wear, thereby delaying the appearance of the final -pattern. Indeed, in five or fewer per cent of the individuals (see -Paulson, 1961:138-139; and White and Downs, 1961:18) a vestige of -enamel is retained throughout life or at least until late in -adulthood. In _Geomys tobinensis_, for example, a thin, but -transversely complete, plate of enamel occurs all the way down to the -base of the loph (Paulson, _loc. cit._) and would persist throughout -life. In _Geomys garbanii_, a vestige on the lingual side of the -posterior surface of a fully adult specimen was noted by White and -Downs (_loc. cit._). Vestiges of the posterior plate occur less -frequently in living geomyids. Paulson (_loc. cit._) found a posterior -plate in one of 75 specimens of _Geomys bursarius dutcheri_. A young -(suture present between exoccipitals and supraoccipital) female of -_Geomys pinetis austrinus_ (KU 23358) has a vestige of the posterior -plate on the lingual side of the tooth as White and Downs (_loc. -cit._) observed in a specimen of _Geomys garbanii_. The enamel, I -suspect, tends to be thicker on the lingual than on the labial side of -the loph and extends farther down the lingual surface in some -individuals; therefore, wear on the occlusal surface erodes it down to -the dentine more rapidly on the labial than on the lingual side. The -tendency of enamel to be retained is a primitive feature. - -A lower molar of _Geomys_ is a single elliptical column, and enamel is -restricted to the posterior surface as in _Zygogeomys_, _Orthogeomys_, -and _Pappogeomys_. Paulson (_loc. cit._) found a thin enamel plate on -the anterior surfaces of the lower molars in about five per cent of -the individuals of _Geomys tobinensis_ from the Cudahy local fauna -(middle Pleistocene, deposits of the late Kansan glaciation). An -anterior plate is unknown in other members of the tribe Geomyini, -except in the primitive genus _Pliogeomys_ of the middle Pliocene. -Occurrence of the plate in _Geomys tobinensis_ is an atavistic trait. -Primitive dental patterns occur occasionally in geomyids, as pointed -out above, but the frequency of occurrence in _G. tobinensis_ is -higher than would be expected. - -M1 and M2, like the lower molars, are elliptical in cross-section. -Complete enamel plates on the anterior and posterior surfaces are -separated by tracts of dentine on the sides of each tooth. M3 is -usually suborbicular (sometimes subtriangular) in cross-section. The -tooth is not especially elongated posteriorly and usually has no -definite heel; therefore, it is not significantly longer than wide. -Living species of _Geomys_ rarely have a well defined outer re-entrant -fold on M3; less than 10 per cent of the individuals (and usually only -one side in each individual in which it occurs) have it, although a -shallow inconspicuous groove occurs more frequently. The biprismatic -molar characteristic of the ancestral morphotype is less often found -in _Geomys_ than in any other living member of the tribe Geomyini. The -outer re-entrant fold and biprismatic pattern are more often present -in the extinct species _Geomys garbanii_ of the Middle Pleistocene -than in other species. Less than 24 per cent of the third upper molars -in _Geomys garbanii_ lack a tract of the re-entrant fold and more than -38 per cent have a well developed outer fold (see White and Downs, -1961:13, 18). The bicolumnar pattern, although incomplete, would be -clearly evident in those teeth having a well marked re-entrant fold; -the pattern occurs less frequently in those teeth with no fold or only -a slight one. M3 of geomyids is not usually recovered and, therefore, -the occlusal pattern of M3 is unknown in most extinct kinds of -_Geomys_. In Recent _Geomys_ the fold is more common in the eastern -_pinetis_ species-group than in the western _bursarius_ species-group. - -The masseteric ridge on the outer side of the mandible is well -developed in all species of the genus. The position of the mental -foramen relative to the anterior part of the ridge varies with -individuals and according to species. The basitemporal fossa is always -present, but is shallower in the late Pliocene and Pleistocene species -than in Recent species. The angular process is short. - -_Referred species._--The twelve species, five of which are extinct, -are as follows: - -_quinni_ species-group - - *_Geomys quinni_ McGrew, 1944. Geol. Ser., Field Mus. Nat. Hist., - 9 (546):49, January 20. Type from Sand Draw local fauna (late - Pliocene), Brown County, Nebraska; also known from - Broadwater-Lisco local faunas (early Pleistocene), Morrill and - Garden counties, Nebraska, Deer Park local fauna (early - Pleistocene), Meade County, Kansas. - - *_Geomys paenebursarius_ Strain, 1966. Bull. Texas Memorial Mus., - 10:36. Type from Hudspeth local fauna (early Pleistocene), - Hudspeth County, Texas. - - *_Geomys tobinensis_ Hibbard, 1944. Bull. Geol. Soc. Amer., - 55:736. Type from Tobin local fauna (middle Pleistocene), - Russell County, Kansas; also known from Cudahy local fauna - (middle Pleistocene), Meade County, Kansas. - - *_Geomys garbanii_ White and Downs, 1961. Contrib. Sci., Los - Angeles Co. Mus., 42:1-34, June 30. Type from Vallecito Creek - local fauna (middle Pleistocene), San Diego County, California. - - *_Geomys bisulcatus_ Marsh, 1871. Amer. Jour. Sci., 3:121. Type - from Loup River fossil beds, near Camp Thomas, Nebraska - (probably late Pleistocene). - - -_bursarius_ species-group - - *_Geomys parvidens_ Brown, 1908. Mem. Amer. Mus. Nat. Hist., - 9:194. (An extinct subspecies of _Geomys bursarius_ according to - White and Downs, 1961:6). Type from Conard Fissure local fauna - (late Pleistocene), northern Arkansas. - - _Geomys bursarius_ (Shaw, 1800). Trans. Linn. Soc. London, 5:227. - Type from somewhere in Upper Mississippi Valley, North America. - - _Geomys arenarius_ Merriam, 1895. N. Amer. Fauna, 8:139, January 31. - Type from El Paso, El Paso County, Texas. - - _Geomys personatus_ True, 1889. Proc. U. S. Nat. Mus., 11:159, - January 5. Type from Padre Island, Cameron County, Texas. - - -_pinetis_ species-group - - _Geomys pinetis_ Rafinesque, 1806. Amer. Monthly Mag., 2 (1):45, - November. Type locality restricted to Screven County, Georgia. - - _Geomys colonus_ Bangs, 1898. Proc. Boston Soc. Nat. Hist., 28:178, - March. Type from Arnot Plantation, about 4 mi. W St. Marys, - Camden County, Georgia. - - _Geomys cumberlandius_ Bangs, 1898. Proc. Boston Soc. Nat. Hist., - 28:180, March. Type from Stafford Place, Cumberland Island, - Camden County, Georgia. - - _Geomys fontanelus_ Sherman, 1940. Jour. Mamm., 21:341, August 13. - Type from 7 mi. NW Savannah, Chatham County, Georgia. - - -Genus =Orthogeomys= Merriam - - 1895. _Orthogeomys_ Merriam, N. Amer. Fauna 8:172, January 31. - - 1895. _Heterogeomys_ Merriam, N. Amer. Fauna 8:179, January 31 - (type, _Geomys hispidus_ Le Conte, 1862). - - 1895. _Macrogeomys_ Merriam, N. Amer. Fauna 8:185, January 31 (type, - _Geomys heterodus_ Peters, 1865). - -_Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca, -México. - -_Chronologic range._--Late Pleistocene Wisconsin deposits (San -Josecito Cave local fauna, Nuevo León, México) to Recent. - -_Description and discussion._--Species of this genus are of medium to -large size. The skull is strongly dolichocephalic in most species; -the posterior part of the skull is especially narrow. The angular -processes are remarkably short, especially in relation to the length -of the mandible. The nasals and rostrum are relatively broad and -heavy. The pelage is coarse, and often hispid. In some species the -hairs are so sparsely distributed that the body appears almost naked, -and none has so dense a covering of hair as do other genera. The genus -occurs entirely within the tropical life-zones, and most of the -external features seem to be associated with adaptation to tropical -conditions. - -The upper incisor is unisulcate; the sulcus is usually near the inner -border of the tooth, but in some species (subgenus _Orthogeomys_) it -is more medial, and in a few individuals with an extremely wide groove -the outer lip of the sulcus may actually reach the middle of the -tooth. The groove is compressed or open. The premolar is a double -column united at the mid-point. The two prisms are of approximately -equal size, and the lateral re-entrant folds are so compressed that -their sides are parallel. Enamel plates cover the anterior surface and -border the re-entrant angles in both upper and lower premolars. As in -other members of the tribe, the lower premolar has a fourth enamel -plate on the posterior surface of the posterior lophid. In the upper -premolar, the enamel plate is reduced to a narrow blade on the lingual -side of the loph as in the living species of the genus _Zygogeomys_. -In the subgenus _Orthogeomys_ the posterior plate is usually absent, -and otherwise is narrow and near the lingual border of the tooth. - -Each lower molar, in the final stage of wear, consists of a single -elliptical column having an enamel plate only on the posterior -surface. The first and second upper molars are single elliptical -columns having one enamel plate on the anterior surface and another -on the posterior surface. The plates are separated by a tract of -dentine on each side of the tooth. The third upper molar is partly -bilophodont, and the two lophs are separated by a deep outer -re-entrant fold. In many of the species an inner re-entrant fold also -is retained, but in the adult tooth it is less distinct than the -outer. In all of the species the posterior loph is long and forms a -conspicuous heel; consequently the crown is significantly longer then -wide. Moreover, the posterior loph has an enamel plate on each side. -The labial plate always borders the outer re-entrant fold, and in the -subgenus _Orthogeomys_ is infrequently separated into two small -plates. - -The mandible is relatively long. Its masseteric ridge is well -developed and massive. The basitemporal fossa is usually deep and well -defined; it tends to be shallow in the subgenus _Orthogeomys_, and in -young individuals is hardly more than a slight depression. - - -Key to the Subgenera of _Orthogeomys_ - - A Frontal wide and greatly inflated; no interorbital - constriction; enamel plate on posterior wall of P4 usually - absent, although sometimes having small plate, restricted - to lingual end of wall. Subgenus _Orthogeomys_ p. 529 - - A´ Frontal narrow and not greatly inflated; interorbital - region decidedly constricted; enamel plate on posterior wall - of P4 always present but short and restricted to lingual end - of wall. - - B Anterior margin of mesopterygoid fossa even with plane of - posterior wall of M3; postorbital bar weakly developed; - anteroposterior occlusal length of M3 equal to, or less - than, combined length of M1 and M2. - Subgenus _Heterogeomys_ p. 530 - - B´ Anterior margin of mesopterygoid fossa decidedly behind - plane of posterior wall of M3; postorbital bar strongly - developed; anteroposterior occlusal length of M3 more - than combined length of M1 and M2. - Subgenus _Macrogeomys_ p. 531 - - -Subgenus =Orthogeomys= Merriam - - 1895. _Orthogeomys_ Merriam, N. Amer. Fauna, 8:172, January 31. - -_Type._--_Geomys scalops_ Thomas, 1894, from Tehuantepec, Oaxaca, -México. - -_Chronologic range._--Known only from the Recent. - -_Description._--Skull elongated and narrow (many skulls of nearly -uniform breadth throughout), being extreme in dolichocephalic -specializations; mandibles long and narrow, rami not spreading -laterally, being more nearly parallel-sided than in other subgenera; -angular processes short; breadth across zygomata not significantly -exceeding breadth across mastoid processes (in many skulls -considerably less); interorbital area remarkably broad, lacking deep -constriction; frontals between orbits greatly inflated laterally, -postorbital prominence inconspicuous; mesopterygoid fossa extending to -level of posterior margin of M3; I having sulcus broader than in other -subgenera, mostly on inner half of anterior surface but sometimes -overlapping mid-line; enamel plate lacking from posterior wall of P4, -rarely retaining narrow vestige near lingual border of posterior loph; -M3 having distinct heel, bicolumnar pattern with inner re-entrant fold -usually minute, occlusal length less than in other subgenera, length -less than combined lengths of M1-2; hair generally coarse, sometimes -hispid, sparse, in lowland forms, so sparse as to impart appearance of -nakedness. - -_Referred species and subspecies._--Fourteen taxa: - - _Orthogeomys grandis alleni_ Nelson and Goldman, 1930. Jour. Mamm., - 11:156, May 9. Type from near Acapulco, 2000 ft., Guerrero. - - _Orthogeomys grandis annexus_ Nelson and Goldman, 1933. Proc. Biol. - Soc. Washington, 46:195, October 26. Type from Tuxtla - Gutierrez, 2600 ft., Chiapas. - - _Orthogeomys grandis carbo_ Goodwin, 1956. Amer. Mus. Novit., - 1757:5, March 8. Type from Excurano, 2500 ft., Cerro de San - Pedro, 20 km. W Mixtequilla, Oaxaca. - - _Orthogeomys grandis felipensis_ Nelson and Goldman, 1930. Jour. - Mamm., 11:157, May 9. Type from Cerro San Felipe, 10 mi. N - Oaxaca, Oaxaca. - - _Orthogeomys grandis huixtlae_ Villa, 1944. Anal. Inst. Biol. Univ. - Nac. México, 15:319. Type from Finca Lubeca, 12 km. NE - Huixtla, 850 m., Chiapas. - - _Orthogeomys grandis grandis_ (Thomas, 1893). Ann. Mag. Nat. Hist., - ser. 6, 12:270, October. Type from Dueñas, Guatemala. - - _Orthogeomys grandis latifrons_ Merriam, 1895. N. Amer. Fauna, - 8:178, January 31. Type from Guatemala, exact locality - unknown. - - _Orthogeomys grandis nelsoni_ Merriam, 1895. N. Amer. Fauna, 8:176, - January 31. Type from Mt. Zempoaltepec, 8000 ft., Oaxaca. - - _Orthogeomys grandis pluto_ Lawrence, 1933. Proc. New England Zool. - Club, 13:66, May 8. Type from Cerro Cantoral, north of - Tegucigalpa, Honduras. - - _Orthogeomys grandis scalops_ (Thomas, 1894). Ann. Mag. Nat. Hist., - ser. 6, 13:437, May. Type from Tehuantepec, Oaxaca. - - _Orthogeomys grandis soconuscensis_ Villa, 1949. Anal. Inst. Biol. - Univ. Nac. México, 19:267, April 8. Type from Finca - Experanza, 710 m., 45 km. (by road) NW Huixtla, Chiapas. - - _Orthogeomys grandis guerrerensis_ Nelson and Goldman, 1930. Jour. - Mamm., 11:158, May 9. Type from El Limón, in valley of Río de - las Balsas approximately 20 mi. NW La Unión, Guerrero. - - _Orthogeomys cuniculus_ Elliot, 1905. Proc. Biol. Soc. Washington, - 18:234, December 9. Type from Zanatepec, Oaxaca. - - _Orthogeomys pygacanthus_ Dickey, 1928. Proc. Biol. Soc. Washington, - 41:9, February 1. Type from Cacaguatique, 3500 ft., Dept. San - Miguel, El Salvador. - - -Subgenus =Heterogeomys= Merriam - - 1895. _Heterogeomys_ Merriam, N. Amer. Fauna, 8:179, January 21. - -_Type._--_Geomys hispidus_ Le Conte, 1852, from near Jalapa, Veracruz. - -_Chronologic range._--Late Pleistocene, Wisconsin deposits (San -Josecito Cave local fauna, Nuevo León) to the Recent. - -_Description._--Skull dolichocephalic (less so than in the other -subgenera); zygomata more widely spreading than in _Orthogeomys_; -ramus and angular process short; interorbital area noticeably -constricted; frontals between orbits neither exceptionally broad or -inflated; mesopterygoid fossa extending to level of posterior margin -of M3; I having sulcus on inner third of anterior surface usually -narrower than in subgenus _Orthogeomys_; enamel plate on posterior -wall of P4 restricted to lingual half of loph; M3 distinctly -biprismatic, posterior loph usually circumscribed by shallow inner -re-entrant fold and outer deep fold well developed in all members of -genus; posterior loph forming conspicuous heel longer than in subgenus -_Orthogeomys_; occlusal length equal to or slightly less than combined -lengths of M1-2; hair coarse and hispid but never so sparse as to -impart appearance of nakedness. - -_Referred species and subspecies._--Eleven taxa: - - *_Orthogeomys onerosus_ (Russell, 1960). Univ. Kansas Publ., Mus. - Nat. Hist., 9 (21):544, January 14. Type from San Josecito Cave - local fauna, Upper Pleistocene, Nuevo León. - - _Orthogeomys hispidus cayoensis_ (Burt, 1937). Occ. Papers Mus. - Zool., Univ. Michigan, 365:1, December 16. Type from Mountain - Pine Ridge, 12 mi. S El Cayo, British Honduras. - - _Orthogeomys hispidus chiapensis_ (Nelson and Goldman, 1929). Proc. - Bio. Soc. Washington, 42:151, March 30. Type from Tenejapa, 16 - mi. NE San Cristobal, Chiapas. - - _Orthogeomys hispidus concavas_ (Nelson and Goldman, 1929). Proc. - Biol. Soc. Washington, 42:148, March 30. Type from Pinal de - Amoles, Querétaro. - - _Orthogeomys hispidus hispidus_ (Le Conte, 1852). Proc. Acad. Nat. - Sci. Philadelphia, 6:158. Type from near Jalapa, Veracruz. - - _Orthogeomys hispidus latirostris_ (Hall and Alvarez, 1961). Anal. - Escuela Nac. Ciencias Biol., 10:121, December 20. Type from - Hacienda Tamiahua, Cabo Rojo, Veracruz. - - _Orthogeomys hispidus negatus_ (Goodwin, 1953). Amer. Mus. Novit., - 1620:1, May 4. Type from Gomez Ferias, 1300 ft., about 45 mi. S - Ciudad Victoria, 10 km. W Pan American Highway, Tamaulipas. - - _Orthogeomys hispidus tehuantepecus_ (Goldman, 1939). Jour. - Washington Acad. Sci., 29:174, April 15. Type from mountains 12 - mi. NW Santo Domingo and about 60 mi. N Tehuantepec, 1600 ft., - Oaxaca. - - _Orthogeomys hispidus torridas_ (Merriam, 1895). N. Amer. Fauna, - 8:183, January 31. Type from Chichicaxtle, Veracruz. - - _Orthogeomys hispidus yucatanensis_ (Nelson and Goldman, 1929). - Proc. Biol. Soc. Washington, 42:150, March 30. Type from - Campeche, Campeche. - - _Orthogeomys lanius_ (Elliot, 1905). Proc. Biol. Soc. Washington, - 18:235, December 9. Type from Xuchil, Veracruz. - - -Subgenus =Macrogeomys= Merriam - - 1895. _Macrogeomys_ Merriam, N. Amer. Fauna, 8:185, January 31. - -_Type._--_Geomys heterodus_ Peters, 1865, from Costa Rica, exact -locality unknown. - -_Chronologic range._--Known only from the Recent. - -_Description._--Skull dolichocephalic in varying degree (overlapping -subgenera _Orthogeomys_ and _Heterogeomys_ in this respect); mandibles -elongated, not spreading far laterally; angular processes decidedly -short; breadth across zygomata in no instance significantly exceeding -mastoid breadth; interorbital area strongly constricted; frontals -between orbits slightly inflated laterally (especially in forms having -more strongly dolichocephalic skulls); postorbital prominence -conspicuous; anterior margin of mesopterygoid fossa terminating well -behind M3; I having narrow and deep sulcus entirely on inner third of -anterior surface; enamel plate on posterior wall of P4 restricted to -inner half of loph; M3 bilophodont (outer and inner re-entrant folds -each circumscribing a loph), posterior loph remarkably elongated and -forming pronounced heel, length of crown more than combined lengths of -M1-2; hair wooly in some individuals, harsh in others but seldom -hispid, never so sparse as in subgenus _Orthogeomys_; some species -having white markings, especially on lumbar region and head. - -_Referred species and subspecies._--Eleven taxa: - - _Orthogeomys heterodus cartagoensis_ (Goodwin, 1943). Amer. Mus. - Novit., 1227:2, April 22. Type from Paso Ancho, Province - Cartago, Costa Rica. - - _Orthogeomys heterodus dolichocephalus_ (Merriam, 1895). N. Amer. - Fauna, 8:189, January 31. Type from San José, Costa Rica. - - _Orthogeomys heterodus heterodus_ (Peters, 1865). Monatsb. preuss. - Acad. Wiss., Berlin, 1865:177. Type from Costa Rica, exact - locality unknown. - - _Orthogeomys cavator nigrescens_ (Goodwin, 1943). Amer. Mus. Novit., - 1227:3, April 22. Type from El Muneco (Río Navarro), 10 mi. S - Cartago, 4000 ft., Province Cartago, Costa Rica. - - _Orthogeomys cavator pansa_ (Bangs, 1902). Bull. Mus. Comp. Zool., - 39:44, April. Type from Bogava (= Bugaba), 600 ft., Chiriquí, - Panamá. - - _Orthogeomys dariensis_ (Goldman, 1912). Smithsonian Misc. Coll., - 60(2):8, September 20. Type from Cana, 2000 ft., mountains of - eastern Panamá. - - _Orthogeomys underwoodi_ (Osgood, 1931). Field Mus. Nat. Hist., - Publ. 295, Zool. Ser., 185:143, Aug. 3. Type from Alto de - Jabillo Pirris, between San Geronimo and Pozo Azul, western - Costa Rica. - - _Orthogeomys cherriei carlosensis_ (Goodwin, 1943). Amer. Mus. - Novit., 1227:3, April 22. Type from Cataratos, San Carlos, - Alajuela, Costa Rica. - - _Orthogeomys cherriei cherriei_ (J. A. Allen, 1893). Bull. Amer. - Mus. Nat. Hist., 5:337, December 16. Type from Santa Clara, - Costa Rica. - - _Orthogeomys cherriei costaricensis_ (Merriam, 1895). N. Amer. - Fauna, 8:192, January 31. Type from Pacuare, Costa Rica. - - _Orthogeomys matagalpae_ (J. A. Allen, 1910). Bull. Amer. Mus. Nat. - Hist., 28:97, April 30. Type from Peña Blanca, Matagalpa, - Nicaragua. - - -Genus =Pappogeomys= Merriam - - 1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31. - - 1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31. - Type: _Geomys merriami_ Thomas. - - 1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31. - Type: _Geomys gymnurus_ Merriam; Hooper, Jour. Mamm., - 27:397, November 25, 1946. - -_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope -Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco. - -_Chronologic range._--Late Pliocene, from deposits of early Blancan -age (Benson local fauna, Arizona) to the Recent. However in the -Pleistocene, only late Pleistocene records are known, and -_Pappogeomys_ has not been found in early (late Blancan) or middle -(Irvingtonian) Pleistocene local faunas. Presumably the genus was -restricted to México during the Pleistocene until post-Wisconsin time. - -_Description and discussion._--The size ranges from as little as in -the smaller kinds of _Thomomys_ to the maximum attained in the -subfamily and matched elsewhere perhaps in only a few of the larger -subspecies of _Orthogeomys grandis_. Depending on the species and -subgenus, the form of the skull varies from generalized to -specialized. The generalized skulls are short and not especially -narrow; the zygomatic arches are spread laterally so far that the -breadth across them exceeds the breadth across the mastoid processes. -The most specialized skulls are platycephalic and the breadth across -the mastoid processes equals or exceeds the breadth across the -zygomatic arches (even so, the zygomatic arches are still relatively -widespread). In correlation with the great breadth of the posterior -part of the cranium, the rami of the mandibles diverge widely -posteriolaterally and the angular processes are remarkably elongated. -The rostrum is moderately broad in most species, but not nearly so -broad and heavy as in _Orthogeomys_. - -The single deep, median sulcus on the outer surface of the upper -incisor is slightly displaced to the inner side of the tooth. The -posterior surface of P4 lacks enamel (small vestige found on lingual -end of posterior wall in only two adult individuals--UA 3260 and KU -100442, of the subgenus _Pappogeomys_); the other three plates are -fully developed as usual. The p4 is provided with four fully developed -enamel plates, in the pattern characteristic of the tribe Geomyini. In -the p4 of the late Pliocene species (_P. bensoni_) the re-entrant -angles are open (obtuse), a trait that is evidently primitive in the -Geomyini. - -All three lower molars are single, compressed, elliptical columns with -enamel on only the posterior surfaces. M1 and M2 are also elliptical -in cross-section and decidedly anteroposteriorly compressed, like the -lower molars. Nevertheless, the enamel pattern is variable; enamel -plates may be retained completely across both the anterior and -posterior walls of M1 and M2 or only the anterior plate may be -retained without reduction and the posterior plate may be reduced so -that only a vestige is retained on the lingual fourth of the tooth or -the posterior plate may be completely lost. - -M3 tends to remain at least incompletely bilophodont by reason of -retaining a permanent labial re-entrant fold in most species (with -exceptions in _Pappogeomys bulleri_ and some old adults of _P. -castanops_). Primitively the occlusal surface of M3 is subtriangular -(subgenus _Pappogeomys_), but in the _castanops_ species-group of the -advanced subgenus _Cratogeomys_, the posterior loph usually is reduced -and the occlusal surface is quadriform or obcordate. Curiously, the -trend towards reduction of the posterior loph is reversed in one -subspecies (_P. merriami fulvescens_) and, the loph has elongated into -a pronounced heel in some specimens, resembling the condition in -_Orthogeomys_. The entire range of variation occurs in _P. m. -fulvescens_. The subtriangular pattern is retained in the most -specialized species of _Cratogeomys_ where that pattern is associated -with extreme platycephaly in the _gymnurus_ species-group. In most -species the posterior loph supports two lateral plates, the outer one -always bordering the labial re-entrant fold. In _Pappogeomys bulleri_ -and in the _castanops_ species-group, the outer re-entrant fold of M3 -tends to be obsolete, and the tooth becomes quadriform or -suborbiculate in some individuals and loses the bilophodont pattern -that characterizes other species. The lingual enamel plate is -displaced to the posterior surface of the tooth, and one or both -plates may disappear with advancing age. Consequently, only the -anterior enamel plate remains in some adults, and constitutes the -maximum degree of reduction of enamel on M3 in the Geomyinae. In many -adults of _Pappogeomys bulleri_, the enamel investment of the -posterior loph is complete and the two lateral plates are connected, -without interruption around the posterior apex of the tooth, evidently -representing the retention of a primitive character of the ancestral -lineage. - -The m3 of _P. bensoni_ from the late Pliocene is distinguished by -minute lateral inflections suggesting the primitive biprismatic -pattern. Also the posterior enamel plates of m1 and m2 are remarkably -long, extending around the ends of the tooth. The associated upper -incisor was unisulcate as in the modern species, and the basitemporal -fossa of the mandible is well developed and deep. - -The lower jaw is stout and relatively short. The masseteric ridge is -well developed and has an especially thick crest. The basitemporal -fossa is deep. In most living species, the pelage is soft and dense, -but in one species, _Pappogeomys fumosus_, the hairs are coarse and -hispid somewhat as in _Orthogeomys_. - - -Key to the Subgenera of _Pappogeomys_ - - A Enamel plates completely developed across posterior walls of - M1 and M2, except in one species (_P. alcorni_) having enamel - restricted to lingual fourth in M1; sagittal crest lacking - owing to impressions of temporal muscles remaining separated - (even in old adults); zygomata slender, and without platelike - expansion at lateral angle. Subgenus _Pappogeomys_ p. 534 - - A´ Enamel lacking on posterior walls of M1 and M2; pronounced - sagittal crest developed in adults of both sexes by union - of temporal impressions at middorsal line; zygomata stout - and wide, with lateral angle expanded into broad plate. - Subgenus _Cratogeomys_ p. 535 - - -Subgenus =Pappogeomys= Merriam - - 1895. _Pappogeomys_ Merriam, N. Amer. Fauna, 8:145, January 31. - -_Type._--_Geomys bulleri_ Thomas, 1892, from near Talpa, west slope -Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco. - -_Chronologic range._--Late Pliocene (Benson local fauna, Arizona) to -Recent, but no specimens known from Pleistocene. - -_Description._--Small, approximately same size as small subspecies of -_Thomomys umbrinus_ but forefeet larger and claws longer; skull of -generalized shape, broad, relatively short, smoothly rounded, not -especially compressed dorso-ventrally; zygomatic breadth great but not -exceeding mastoid breadth; zygomata relatively slender for geomyid and -lacking platelike expansions at lateral angles; rostrum relatively -narrow; sagittal crest lacking, owing to impressions of temporal -muscles remaining separated; angular process of mandible not -especially elongated; enamel plates extending completely across -posterior wall of M1 and M2, except in one species, _P. alcorni_, -where posterior plate of M1 remains only on lingual fourth of -posterior wall (remainder of plate lacking); with wear, plates -sometimes exceptionally thin completely across posterior face of M2 -and especially M1 in a few individuals of _P. bulleri_ much as Paulson -(1961:138-139) describes in extinct _Geomys tobinensis_; one or both -plates rarely disappear in final stages of attrition in old -individuals resulting in same dental pattern found in _Cratogeomys_; -M1 and M2 retaining enamel plate on anterior wall throughout life; M3 -usually subtriangular in cross-section but sometimes suborbiculate or -ovoid, crown slightly bilophodont owing to shallowness of labial -re-entrant angle in modern species; posterior loph of M3 not -especially elongated and crown not significantly longer than wide; -both lateral enamel plates of M3 usually well developed and -approximately equal in length, occasionally plates reduced in length -and rarely one or both plates are lost with wear in old individuals; -patch of whitish or buffy hairs surrounding nose of most individuals. - -The primitive character of the lower dentition, as described in the -species account above, suggest that _Cratogeomys_ [= _Pappogeomys_] -_bensoni_ Gidley should be referred to the subgenus _Pappogeomys_ -rather than _Cratogeomys_. Only the upper dentition would make -positive identification possible; however, reference to the subgenus -_Pappogeomys_ seems to be the best arrangement at this time. - -_Referred species._--Three (one extinct): - - *_Pappogeomys bensoni_ (Gidley), 1922. U. S. Geol. Surv. Prof. - Papers, 131:123. Type from Benson local fauna (late Pliocene), - Cochise County, Arizona. - - _Pappogeomys alcorni_ Russell, 1957. Univ. Kansas Publ. Mus. Nat. - Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco. - - _Pappogeomys bulleri_ Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6, - vol. 10:196, August. Type from "near Talpa," west slope of - Sierra Madre de Mascota, Jalisco. - - -Subgenus =Cratogeomys= Merriam - - 1895. _Cratogeomys_ Merriam, N. Amer. Fauna, 8:150, January 31. - - 1895. _Platygeomys_ Merriam, N. Amer. Fauna, 8:162, January 31. - Type: _Geomys gymnurus_ Merriam, 1892. - -_Type._--_Geomys merriami_ Thomas, 1893, from "Southern México," -probably in Valley of México. - -_Chronologic range._--Late Pleistocene, from Wisconsin deposits (San -Josecito Cave, Nuevo León, Upper Bercerra, México, and Burnet Cave, -New Mexico, local faunas) to the Recent. - -_Description._--Size medium to large; skull becoming angular and -rugose with age, and tending towards platycephaly and dorso-ventral -compression; zygomata stout, each bearing platelike expansion at -anterolateral angle into which anterior end of jugal becomes morticed; -breadth across zygomata great relative to length of skull; rostrum -relatively broad; squamosals expanding medially with age eventually -growing over lateral parts of parietals, and sometimes also expanding -laterally displacing postglenoid notch; sagittal crest well developed -in adults of both sexes, but especially high and bladelike in males; -lambdoidal crest prominent in all but young animals, having dorsal -outline broadly convex posteriorly in most species but strongly -sinuous in _gymnurus_-group; enamel plate on posterior wall of P4 -absent; enamel plates present only on anterior walls of M1 and M2; M3 -variform in occlusal shape (as described in species account), either -subtriangular (_gymnurus_-group), quadriform or obcordate -(_castanops_-group, with exceptions as noted before); lateral plates -of M3 usually present in all species, labial plate approximately as -long as lingual plate in _gymnurus_-group (like that in subgenus -_Pappogeomys_) or distinctly shorter in _castanops_-group (labial -plate scarcely extending beyond border of labial re-entrant fold); one -or both lateral plates tending to disappear with wear in -_castanops_-group, with lingual plate usually disappearing first; -breadth across angular processes clearly more than breadth across -zygomatic processes, especially in _gymnurus_-group. - -_Remarks._--In the species of the _castanops_-group the skulls can be -spoken of as generalized and the least platycephalic of the subgenus. -Indeed, the species of the _castanops_-group are hardly more -specialized in this respect than is the subgenus _Pappogeomys_. -In these skulls the breadth across the squamosal processes is less -than that across the zygomatic arches, although the two dimensions -are almost equal in some examples of _P. merriami_ of the -_castanops_-group (where squamosal breadth varies from 85 to 98% of -zygomatic breadth). In the species having marked platycephalic skulls -(_gymnurus_ species-group) the breadth across the squamosal processes -equals or exceeds the breadth across the zygomatic arches (squamosal -breadth rarely 97 to 99% of zygomatic breadth), except in _P. zinseri_ -and _P. tylorhinus zodius_. - -The variable character of the third upper molar as between species -suggests that this tooth is presently undergoing active evolution. The -structure of this tooth, although differing between taxa, is -remarkably stable in other kinds of Geomyini. The most remarkable -modification of M3 in _Cratogeomys_ is the obcordate pattern -developed in _P. merriami_ of the _castanops_-group. The posterior -loph and entire tooth is shortened somewhat resembling in shape that -of _Thomomys_. Moreover, the posterior loph is twisted labially; -consequently, its posterior surface now forms the labial border of the -weakly defined posterior loph. Owing to the torsion, the lingual -enamel plate has been rotated to the posterior surface of the tooth. -Therefore, the tooth is provided with two transverse enamel plates, -including the plate on the anterior wall of the tooth. The labial -plate is greatly reduced, its total surface being restricted to the -small labial inflection. The highly specialized obcordate M3 is not -found in the most specialized platycephalic skulls characteristic of -the _gymnurus_ species-group. Instead the _gymnurus_-group retains the -primitive subtriangular pattern without significant modification. - -_Referred species._--Seven: - - -_castanops_ species-group - - _Pappogeomys castanops_ (Baird, 1852). Report Stanbury's Exp'd. to - Great Salt Lake, p. 313, June. Type from "Prairie road to Bent's - Fort," near present town of Las Animas, Colorado. - - _Pappogeomys merriami_ (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6, - 12:271, October. Type from "southern Mexico," probably Valley of - México (see Merriam, 1895:152). - - -_gymnurus_ species-group - - _Pappogeomys fumosus_ (Merriam, 1892). Proc. Biol. Soc. Washington, - 7:165, September 29. Type from 3 mi. W Colima, Colima. - - _Pappogeomys gymnurus_ (Merriam, 1892). Proc. Biol. Soc. Washington, - 7:166, September 29. Type from Zapotlan (Ciudad Guzman), - Jalisco. - - _Pappogeomys neglectus_ (Merriam, 1902). Proc. Biol. Soc. - Washington, 15:68, March 22. Type from Cerro de la Calentura, - about 8 mi. NW Pinal de Amoles, Querétaro. - - _Pappogeomys tylorhinus_ (Merriam, 1895). N. Amer. Fauna, 8:167, - January 31. Type from Tula, Hidalgo. - - _Pappogeomys zinseri_ (Goldman, 1939). Jour. Mamm., 20:91, February - 15. Type from Lagos, Jalisco. - - - - -PHYLOGENY OF THE GEOMYIDAE - - -The fossil record of the Geomyidae provides a sequence of morphotypes, -each representing a stage in the phyletic development of the family. -Most of the preserved specimens probably represent the stufenreihe -rather than the ahnenreihe, as Simpson (1953:219-220) points out. Even -so, the stufenreihe closely approximates the general trend of -evolution, and the level of structural organization in the different -stages of phyletic development may be ascertained. The actual -ancestral series of most lineages probably will remain unknown, but -hopefully some of the existing gaps will be filled by future -discoveries. From the established record, several clearly defined -lineages can be distinguished; in fact the sequence of origin, pattern -of evolution, and specializations, of the principal lineages are -reasonably well expressed. - - -Primitive Morphotype - -In the earliest known geomyids from the Upper Oligocene and Lower -Miocene, the premolars and molars are biprismatic and bilophodont. In -rodents, this is itself a specialized pattern, and is thought to have -evolved from a more primitive sextituberculate prototype by the union -of individual cusps, and probably also cuspules, forming the two -transverse enamel lophs. The primitive, common ancestor of the -Geomyidae and Heteromyidae with sextituberculate teeth in the early -Tertiary is unknown. - -As soon as geomyids attained the early bilophodont stage of evolution, -the basic morphological structure of the family was established. The -family probably first became clearly distinguished from other -Geomyoidea at this stage. In the early bilophodont stages of -evolution, owing to the relatively deep valley between them, the two -columns probably failed to unite in the normal cycle of wear, as they -do in all later geomyids. _Griphomys_ described by Wilson (1940:93) -from the late Eocene of California, has a bilophate pattern in which -the anterior and posterior lophs are separated by a persistent -transverse valley. The occlusal pattern of _Griphomys_ closely -resembles a stage through which the ancestors of the early Miocene -geomyids must have passed in their pre-Miocene evolution, as Wilson -suggests (1949:115-116). Although he (1940:95; 1949:110-118) -tentatively referred _Griphomys_ to the superfamily Geomyoidea and -Simpson (1945:80) went so far as to refer it to the family Geomyidae, -with a notation of _incertae sedis_, its exact relationship to the -pocket gophers is uncertain. However, the structure of the molariform -dentition of _Griphomys_ does not exclude it from the phyletic -ancestry of the Geomyidae. In subsequent stages of evolution the -anterior and posterior columns become united. Thereby part of the -valley floor between the transverse prisms was progressively elevated, -to the stage where attrition on the occlusal surface would unite the -two columns. On the unworn enamel cap of living geomyids the two -transverse enamel folds are separated by a shallow but well defined -valley, briefly reflecting the ancient ancestral pattern. - -Union of the lophs may have been either at the mid-points of the two -columns or at the edge of their protomeres. [A protomere is the half -of a tooth containing the protocone or protoconid--lingual side of -upper tooth and labial side of lower tooth. The paramere is the -opposite half of a given tooth--labial side of upper tooth and -lingual side in lower tooth. See Miller and Gidley, 1918:434.] Union -of the columns at the mid-points would have produced the figure-8 -occlusal pattern (or H-pattern), which is characteristic of the early -Miocene Geomyinae (_Dikkomys_). Union of the two columns at the -protomeres would have produced the U-shaped pattern of the -Entoptychinae, which also occurred in the early Miocene and were -contemporary with the earliest Geomyinae. Since pre-Miocene geomyids -are unknown, the actual phyletic development of the dentition is a -matter of speculation. Probably the development of the two divergent -lineages, one leading to the Entoptychinae and the other to the -subfamily Geomyinae, occurred in the Oligocene (as depicted in Fig. -3). Of the two lineages, the subfamily Geomyinae, in my view, is the -more primitive and less specialized. Support for this view is -furnished by a reconstruction of the pattern of occlusal wear in -_Dikkomys_ and _Pliosaccomys_, especially on the first and second -molars. - -In _Dikkomys_, the anterior and posterior column first unite near -their mid-points in the first stages of wear thus producing a figure-8 -shaped (H-shaped) occlusal pattern in the premolar and all three -molars. Evidently in the first two upper molars, the columns unite -closer to their lingual margins than their mid-points, but at any rate -both outer and inner re-entrant folds are evident at this stage of -wear. With continued attrition on m1 and m2 of _Dikkomys_, the -anterior and posterior columns secondarily unite at the edge of their -labial margins thus enclosing a fossette of enamel in the labial half -of the tooth. The lateral coalescence at the ends of the protomeres -occurs because of the shallow vertical depth of the labial re-entrant -fold, and the fossette itself does not reach the base of the crown and -with continued wear it too would disappear, but not until the last -stages of wear, at least in _Dikkomys matthewi_. The lingual -re-entrant fold is deep, and therefore, persistent through all stages -of wear. Although the amount of wear required for its effacement would -be great, the occlusal configuration of the first and second lower -molars in _Dikkomys_ could be eventually ground down to a U-pattern as -in the entoptychids. Only one upper molar of _Dikkomys_, the first, -has been recovered (see Wood, 1936:23, fig. 32B). Although the tooth -is in an early stage of wear, the lingual valley is minute. Less -attrition than required in m1 and m2 would progressively reduce the -lingual fold until it too would essentially form a U-pattern, perhaps -retaining a slight lingual inflection. Hence, the first upper molar -becomes a mirror image of the first lower molar, and the second upper -molar probably had the same pattern as the first (at least it does so -in _Pliosaccomys_). Both of the lateral re-entrant folds of the -premolar are deep vertically, and consequently would not disappear -with occlusal wear. Therefore, the H-pattern of the premolars is -retained throughout life. - -The m3 (M3 unknown for _Dikkomys_ or _Pliosaccomys_) also has deep -lateral folds; hence, it too retains the H-pattern in all stages of -attrition, although the isthmus between the two prisms may become -wider in the final phases of wear (as it does in _Pliosaccomys_). - -In _Pliosaccomys_, the stages of wear are essentially the same as -those described for _Dikkomys_, except that the anterior and posterior -loph of the first and second molars tend to unite closer to one side -of the tooth, lingual side in upper molars and labial in lower. Only a -slight inflection of the re-entrant fold is evident on the side of -union, and the inflection disappears in the first phases of wear as -the columns unite. Concomitant with the lateral shift in the initial -point of coalescence of the transverse lophs, the occlusal penetration -of the re-entrant fold from the opposite side increases in horizontal -depth, and the fold extends medially more than half way across the -occlusal surface, thus forming a pattern essentially like that of the -entoptychids. The U-pattern in _Pliosaccomys_ appears in the initial -stages of wear without going through an earlier H-pattern as is the -case in its Miocene ancestors of the genus _Dikkomys_, unless the -minute inflection is considered as indicative of that stage. The two -columns of the premolar and m3 are joined near their mid-points as in -_Dikkomys_; therefore, they retain their primitive H-pattern, a -feature unique to the Geomyinae. - -The evolutionary trend toward an ontogenetically earlier U-pattern in -the first two molars in the primitive lineage of the Geomyinae -suggests that the U-pattern characteristic of the Entoptychinae was -simply an earlier tendency toward the same specialization that -occurred later in the subfamily Geomyinae. If so, early entoptychines -would have been characterized by an H-pattern in the first stages of -attrition, like _Dikkomys_, and later developed union at the edge of -the protomeres. However, in the entoptychines, all the molariform -dentition, and not merely the first and second molar, became -specialized; consequently the U-pattern was produced on the occlusal -surfaces of each of the cheek teeth. As in _Pliosaccomys_, the -transitional phase, in which the two columns were united at their -mid-points, was eventually eliminated from the pattern of wear and -only the U-pattern, that now appeared in the initial stages of wear, -was retained. In the entoptychines of the early Miocene there is no -suggestion of the H-pattern that characterizes the Geomyinae, except -in the position of the cusps before wear in the lower molars of -_Pleurolicus sulcifrons_, which, according to Wood (1936:6), suggests -the H-pattern. In earlier unknown Oligocene stages of evolution, the -prisms possibly united first at their mid-points, and the columns may -have joined at the side of the tooth only in the terminal stages of -wear. The U-pattern of pre-Miocene entoptychines, therefore, may have -become the dominant occlusal pattern only in the later stages of -phyletic development. - -According to the recently expressed views of several paleontologists, -the Entoptychinae constitute the primitive lineage of the family and -the early Geomyinae constitute a specialized offshoot of the -entoptychine ancestral assemblage. The structure of the Entoptychinae, -especially of the less advanced genera, closely approximates that of -the hypothetical primitive morphotype. But, according to my view, the -subfamily Geomyinae constitutes the ancestral assemblage and its -structure is essentially that of the primitive morphotype of the -family. At any rate the structure of the early geomyines more closely -approximates the structure of the ancestral stock than the more -divergent entoptychines. Therefore, the genus _Dikkomys_ of the early -Miocene, the first known geomyine, is considered to be a generalized -geomyid, and, although it is a contemporary of the more specialized -entoptychid assemblage, is considered to be more closely allied to the -ancestral stock. - -The entoptychines were the dominant and most highly differentiated -geomyids of the early and middle Miocene. Nevertheless, they became -extinct in the middle Miocene, and the geomyines of that time survived -and later gave rise to the modern pocket gophers. Therefore, the early -history of the family Geomyidae is characterized by an early radiation -and trend toward specialization, followed by survival of the less -specialized Geomyinae and extinction of the more specialized -Entoptychinae. - - -Entoptychid Radiation - -The most abundant geomyids of the early and middle Miocene, the -Entoptychinae, consisted of at least 24 species (see Wood, 1936:4-25) -classified in four genera: _Pleurolicus_, _Gregorymys_, _Grangerimus_, -and _Entoptychus_. The genera were essentially contemporaneous (see -Figure 3). Even so, the subfamily was morphologically varied, pointing -to an earlier origin in the Oligocene (actually a part of the John Day -Fauna, including _Pleurolicus_ may be correlated with late Oligocene -Whitneyian age) followed by a relatively rapid radiation including all -four genera in the early Miocene. Two genera, _Pleurolicus_ and -_Gregorymys_, continued into the Middle Miocene (Hemingfordian). This -divergence, specialization, and subsequent radiation suggest that the -entoptychines evolved into a new major adaptive zone, in the sense -described by Simpson (1945:199-206). - -The radiation is correlated geographically and temporally with the -southward retreat of the Neotropical flora of the Tertiary from the -western United States and southward movement of the Arctic flora of -the Tertiary (see Axlerod, 1950; Berry, 1937:31-46; Chaney, -1947:139-148; and Kendeigh, 1961:280-283). In the early Tertiary the -Neotropical-tertiary geoflora occurred northward to at least 49° -latitude in western North America, and the boreal Arctic-tertiary -flora was restricted to a circumpolar zone. The southward and eastward -shift of the Neotropical-tertiary flora, associated with the drying -and chilling of the continent, began in the middle or late Oligocene -and was concurrent with the divergence and radiation of the -Entoptychinae. Beginning in late Oligocene and continuing at least -into middle Miocene, most of the region in which the entoptychines -occurred was occupied by the Arcto-tertiary geoflora of which the -temperate forest division contributed the dominate plant associations. -The maples, chestnuts, dogwoods, beeches, walnuts, oaks, elms, -birches, and sycamores of that flora were the forerunners of today's -eastern deciduous forest. It is my view that the entoptychines became -adapted to the conditions of this paleoecological environment and -radiated rapidly in the Arikareean when the major change occurred in -climax vegetation. The ancestral stock of the Geomyinae was not so -successful in the Arcto-tertiary climax, and most of it probably was -displaced southward along with the tropical flora. - -The skeleton in the entoptychines is not so strongly fossorial as in -the modern geomyids (Wilson, 1949:117), and these early geomyids -probably were semi-fossorial with somewhat the same burrowing habits -as those of the living mountain beaver (_Aplodontia_). Inasmuch as the -morphology and taxonomy of the entoptychines were discussed in detail -by Cope (1884) and reviewed later by Wood (_loc. cit._), there is no -need to recount the details here. According to Wood (_op. cit._, -27-28), _Pleurolicus_ occupied a central position in the entoptychid -radiation and perhaps appeared slightly earlier than the other genera. -Wilson (1949) suggested that the lower part of the John Day may -actually be Upper Oligocene rather than Lower Miocene, and this -arrangement is followed here. Also, _Pleurolicus_ is less specialized -than the other genera and occurs in deposits of both the Great Plains -and the Pacific Coast. _Gregorymys_, also little specialized, occurred -only on the Great Plains. The more specialized genera, _Grangerimus_ -and _Entoptycus_, evidently appeared somewhat later than _Pleurolicus_ -and evolved from it. Except for a record from southern Texas reported -recently by Hibbard and Wilson (1950:621-623) and the new species -described by MacDonald (1963:182) from the Sharps Formation of South -Dakota (early Arikareean), _Grangerimus_ is known only from the -Pacific coast. _Entoptycus_ was restricted to the Pacific Coast (John -Day fauna). - -_Entoptycus_ is the most specialized of the known genera; it has -pronounced fossorial adaptations, especially in the skull. Its -molariform teeth are rootless and ever-growing as in the modern -geomyines. Moreover, the continuous enamel bands on only moderately -worn teeth become separated in the final stages of wear into anterior -and posterior enamel plates by tracts of dentine that extend toward -the crown on the sides of each tooth. This extension was made possible -by the union of the two columns at both the lingual and labial margins -of the tooth forming an O-pattern, and the crown is essentially -monoprismatic save for the isolated enamel fossette in the center of -the tooth. The fossette is all that remains of the lateral re-entrant -fold that characterized the preceding U-pattern of the earlier stages -of wear. Late in the sequence of wear, the anterior enamel plate is -lost in the lower molars and the posterior plate in the upper molars. -The U-pattern characterizes the final stages of attrition in the other -genera of the Entoptychinae; none developed the dental specializations -seen in _Entoptycus_. Rootless, ever-growing cheek teeth, -discontinuous enamel patterns, and monoprismatic molars were not -evolved in the subfamily Geomyinae until the late Pliocene. - - -Phyletic Trends in Subfamily Geomyinae - -The subfamily Geomyinae is made up of three groups, recognized -taxonomically for the first time in this account as -tribes--Dikkomyini, Thomomyini, and Geomyini (for full discussion of -classification, see previous account). The phylogeny proposed by me is -illustrated in Figure 3. The tribe Dikkomyini is characterized by -generalized and primitive features that together form the basic -structural foundation of the subfamily. Evolution within the -Dikkomyini resulted in the acquisition and perfection of fossorial -adaptations. The Thomomyini and Geomyini are considerably more -specialized than the ancestral Dikkomyini from which they evolved. -The Geomyini are clearly more specialized than the Thomomyini, -suggesting closer affinity between the Thomomyini and the Dikkomyini -than between the Geomyini and the Dikkomyini. The specializations in -the dentition and the associated changes in the skull of the -Thomomyini and Geomyini permit more efficient mastication of fibrous -vegetation. Along with these specializations, fossorial adaptations -inherited from the Dikkomyini are retained without noteworthy -modification. - -_Dikkomys_, the earliest known genus of the tribe Dikkomyini, can be -taken as a starting point of evolution for the subfamily Geomyinae. -The Pliocene genus _Pliosaccomys_ is the only other known geomyine -having primitive features closely resembling those of _Dikkomys_. The -relatively close but previously unrecognized relationship between -_Dikkomys_ and _Pliosaccomys_ can be understood when patterns of wear -on the occlusal surfaces of the cheek teeth are taken into account. It -appears that _Pliosaccomys_ descended from _Dikkomys_-like stock, if -not _Dikkomys_ itself. Although _Dikkomys_ is towards the beginning of -this phyletic sequence and _Pliosaccomys_ towards the end of the -sequence, the primitive features shared by the two provide a -generalized morphotype for the subfamily Geomyinae. - -In the molariform dentition, an almost complete series of stages of -wear in _Pliosaccomys_ has been preserved, and those of _Dikkomys_ can -be reconstructed with reasonable accuracy from those that are known -(see Fig. 4): - -(1) In the initial stage of wear in _Dikkomys_ the anterior and -posterior columns are separated by an intervening valley (Fig. 4A), -and the occlusal surface of each column bears a loph of dentine -surrounded by a ring of enamel: protoloph on the anterior column and -metaloph on the posterior column of the upper teeth (protolophid and -hypolophid in corresponding positions in the lower teeth). Actually -this stage is not preserved in the known material of _Dikkomys_, but -does occur in both geomyines and entoptychines in all stages of -evolution, and it must have also occurred in _Dikkomys_ in order for -the next two stages, which are preserved, to have developed. - -(2) The occlusal surfaces are ground down to a level where the enamel -loops of the two columns join at their mid-points, thus forming an -H-shaped pattern (Fig. 4B), or more exactly a pattern resembling a -figure 8. Probably this was the primitive pattern in the final stage -of wear in the geomyid ancestor of the Oligocene. - -[Illustration: FIG. 3. Diagram depicting geologic range and probable -phyletic relationships of the family Geomyidae. Dashed lines represent -parts of lineages that are not represented by fossil records, and -solid lines represent parts of lineages verified by actual specimens. -Question marks indicate uncertainty of suggested ancestry of known -taxa. The relationships within the subfamily Entoptychinae are -modified after Wood (1936), and the temporal range of the Miocene -geomyids have been adjusted to agree with current stratigraphic -correlations. Hence, _Pleurolicus_, _Gregorymys_ and _Dikkomys_ are -illustrated as ranging into the Hemingfordian, rather than being -confined to the Arikareean (see MacDonald, 1963, and Black, 1961).] - -(3) In the pre-final stage of wear, the anterior and posterior lophs -of the first and second molars unite secondarily at the edge of their -protomeres (labial side in the lower and lingual in the upper), thus -enclosing an isolated enamel fossette (Fig. 4C). Lateral union occurs -in the lower teeth because the vertical depth of the labial re-entrant -angle is less than the depth of the lingual re-entrant fold. In the -upper teeth the reverse is true. The re-entrant angle on one side of -the premolar is as deep vertically as the angle on the other side of -that tooth, and both reach the base of the crown; therefore, they do -not disappear at any stage of attrition. The same pertains in the -third lower molar. - -(4) In the final stage of wear (Fig. 4D), the enamel fossette -disappears as a result of continued attrition on the occlusal surface -in the upper series. The fossette may vary somewhat in vertical depth -in m1 and m2, but the amount of wear required for its effacement would -be greater than in the upper teeth. Therefore, upon wear, the -U-pattern would become characteristic of the final stage in M1 (and -probably also M2), but the modified H-pattern described in Fig. 4C -would prevail in m1 and m2. Perhaps, in extremely worn teeth, the -labial fossette of m1 and m2 would disappear. If this advanced stage -of effacement is obtained, then the two columns would be united across -the entire surface of their protomeres from the center of the crown to -its labial edge, and the occlusal pattern would be in the shape of a -U. - -The occlusal pattern, at least in M1 and M2, in the final stages of -wear in _Dikkomys_ resembles that in the subfamily Entoptychinae, but -the U-pattern develops on only the first and probably the second molar -in _Dikkomys_ and not on all of the cheek teeth as it does in the -entoptychines. Judging from the material that has been described, the -U-pattern did not develop in the lower teeth of _Dikkomys_ until the -Hemingfordian (_D. woodi_), upper Rosebud, and specimens of _D. -matthewi_ from the earlier Arikareean, lower Harrison, suggest that -the modified H-pattern, with secondary coalescence at the edge of the -protomeres, persisted throughout life, without developing the -U-pattern in the final stages of wear. - -Essentially the same patterns of wear characterize the genus -_Pliosaccomys_, except that the earlier stages were telescoped and the -second stage was omitted while another (final) stage was added. The -stages are reconstructed in sequence in figure 4, and all are based on -preserved dentitions, as follows: - -(1) The first phases of wear produced the pattern (Fig. 4E and I) -described for _Dikkomys_ in the previous account (Fig 4A). - -(2) A small additional amount of wear produced the 2nd stage (Fig. 4F -and J) characterized by a U-pattern, formed by union of the anterior -and posterior columns at the edge of the protomeres of the first and -second molars, both above and below, without first forming an H-shaped -pattern. Union at the mid-points thus was omitted from the sequence of -wear in these two teeth. In the premolars and third molars the -primitive H-pattern did form, as in _Dikkomys_. The pattern of wear in -the first two molars is the same as in the entoptychines of the early -Miocene. The trend of evolution through which the _Pliosaccomys_ -lineage passed must have featured a progressively earlier union at the -edge of the tooth until the lateral coalescence occurred -simultaneously with the median union. At that stage, emphasis was -shifted to the union at the edge of the tooth, and eventually the -teeth failed to unite at their mid-points and the U-pattern developed -directly. Therefore, the horizontally deep re-entrant fold that -separates the two lophs of the U-pattern is equivalent to one fold -plus the apex of the opposite fold. - -(3) The horizontal re-entrant fold of the U-pattern was remarkably -shallow vertically and disappeared with little additional wear. Thus -the two parts of M1, and also of M2, are united into a single column -except for a slight inflection on the labial side and this is true -also of m1 and m2 except for a slight inflection on the lingual side -(Fig. 4G and K). The inflection appears to have persisted in the upper -teeth (Fig. 4H), but evidently with slight wear, disappeared in the -lower teeth (Fig. 4L). The final monocolumnar pattern was attained -early ontogenetically, evidently before the permanent premolar had -fully erupted; hence, the earlier stages occurred only in transition, -persisted for only a brief interval in the teeth of juveniles, and the -final stage developed in the young animal and lasted throughout the -rest of its life in _Pliosaccomys_. In _Dikkomys_ the two columns -never united into a single column, and a bilophodont occlusal pattern -persisted throughout life. - -The early phyletic development of the subfamily Geomyinae took place -in the tribe Dikkomyini from the early Miocene into the early -Pliocene. Compared with the rapid evolution of the specializations -that distinguish the Entoptychinae, the structural changes in the -early Geomyinae occurred at a remarkably slow rate. In fact the -lineage changed but little from _Dikkomys_ to _Pliosaccomys_, in parts -of the animal that can be compared, as illustrated by the low-crowned -and rooted cheek teeth, the continuous enamel bands, the lack of -grooving of the upper incisor, the retention of the primitive -H-pattern, both above and below, in the premolar and third lower -molar, and the ridges and fossae of the mandible to which the muscles -of mastication attach. The only major changes detected in the known -fragments are in the pattern of wear and the final configuration of -the first and second molars, as described above. The unification of -the two lophs in each of these two teeth into a single column was a -significant step in the evolution of the Geomyinae, and is a stage -between the primitive bilophodont pattern of the early and middle -Miocene geomyines having continuously bicolumnar teeth and the -monolophodont pattern in the modern pocket gophers of both lineages in -which these teeth consist of a single column in all but the initial -stages of wear. The monocolumnar structure of the first and second -molars in the final stages of wear, therefore, is closer to that in -the lineage of _Thomomys_ than it is to that of _Dikkomys_. Other -specializations in the dentition of _Pliosaccomys_, especially in m1 -and m2 where the H-pattern has been completely eliminated from the -sequence of wear, are too far advanced for _Pliosaccomys_ to have -given rise to the tribe Geomyini. The teeth in the immediate ancestor -of the Geomyini must have been less specialized in m1 and m2, perhaps -about as in _Dikkomys_. In the m1 and m2 of the tribe Geomyini, the -H-pattern is formed in the initial stages of wear; therefore, in the -early Pliocene ancestor, presently unknown in the fossil record, the -H-pattern probably was present. Even so, the ancestor of the Geomyini -and that of _Pliosaccomys_ probably were closely allied otherwise, and -both probably had attained the highly specialized fossorial -adaptations characterizing all modern pocket gophers, before the -divergence of _Pliosaccomys_ and the Geomyini took place. - -The evidence points to a major divergence of the geomyines that lived -in the latest Miocene or the early Pliocene (probably the latter) and -that gave rise to the two modern lineages, Thomomyini and Geomyini -(see Fig. 3). One, the most primitive of the two, gave rise to the -Thomomyini lineage that eventually evolved into _Thomomys_. -_Pliosaccomys_ is closely allied to the ancestry of this lineage, -although it is probably not the actual ancestor, as mentioned -previously. Aside from the aforementioned specializations of the first -and second molars, the features of the Thomomyini are less advanced -than in the other specialized lineage (tribe Geomyini). Primitive -traits retained in the tribe Thomomyini (and also characteristic of -the ancestral tribe Dikkomyini) are: (1) Small size, in general no -larger than the ancestral morphotype; (2) lack of grooving on the -upper incisor (although a slight rudimentary groove is developed -rarely in some living species); (3) retention of anterior and -posterior enamel plates in lower and upper cheek teeth; (4) premolars -having widely open re-entrant folds; (5) smooth and generalized skull -lacking marked angularity, regosity or cresting (neither the sagittal -nor the lambdoidal crest are ordinarily well developed except in -_Thomomys bulbivorus_); (6) forefoot small, less modified for digging -than in the Geomyini. - - - [Illustration: FIG. 4. Drawings of the molariform dentitions of - _Dikkomys_ and _Pliosaccomys_ (Tribe Dikkomyini) depicting the - patterns of wear on the occlusal surfaces. Ontogenetically, the - stages of wear are arranged from left to right in each row. Stages - not represented by actual specimens have been carefully - reconstructed from information provided by known stages in the - sequence of wear and the dentitions of other geomyines. × 5. - - A-D. _Dikkomys woodi_, right lower tooth-row, including p4-m3. - Patterns based on No. P26284 (FMNH) from Upper Rosebud (Middle - Miocene), Shannon Co., South Dakota (B above). - - E-H. _Pliosaccomys dubius_, left upper tooth-row, including P4-M2 - (M3 unknown). Patterns based on Nos. 1798 and 1799 (LAM) from - Smiths Valley (Middle Pliocene), Lyon Co., Nevada. - - I-L. _Pliosaccomys dubius_, right lower tooth-row, including p4-m3. - Patterns based on Nos. 1796 (holotype), 1804, and 1806 (LAM) - from Smiths Valley (Middle Pliocene), Lyon Co., Nevada. - ] - -The lineage of the Thomomyini is essentially rectilinear and without -the major branching seen in the tribe Geomyini. The one genus, -_Thomomys_, appears first in the Upper Pliocene (early Blancan time), -and the specializations characterizing the lineage had already -developed by that time. Evidently, the early stages of divergence from -the ancestral stock resulted in the development of rootless, -ever-growing, more hypsodont cheek teeth, simplification of M3, and -enlargement of the masseteric ridge on the mandible. The enamel -investment on the sides of the molariform teeth is interrupted owing -to intrusion of tracts of dentine on the sides of each column. Even -so, complete anterior and posterior plates are retained on all of the -cheek teeth (Fig. 5, K and L) and there is no trend toward additional -loss of enamel as in the Geomyini. The enamel on the sides of the -column has little functional value, and its elimination probably -reduces friction during the anteroposterior movements of the lower -jaw, thereby increasing the efficiency of the cutting blades on the -anterior and posterior wall of the tooth. The simplification of M3 was -achieved by union of the two columns of the primitive pattern into a -single column and obliteration of both the labial and lingual -re-entrant folds in the first stages of wear. The adult tooth (see -Fig. 5L) is without trace of the bilophate pattern and is not -elongated; therefore, its structure is essentially the same as that of -the first and second upper molars. - -In the Thomomyini, the two lophs of the unworn molars unite entirely -across the width of their surfaces with the first traces of wear (see -Fig. 5, I and J), owing to the shallow and uniform depth of the -transverse valley. In the molars, the final pattern is acquired, -therefore, before the deciduous premolar has been replaced by the -permanent tooth. A relatively shallow re-entrant inflection between -the ends of the parameres sometimes is retained, although it also will -disappear with slight additional wear. Therefore, both lophs tend to -unite completely with the first stages of wear in the Thomomyini, thus -omitting both U and H patterns from the sequence of wear. This is the -highest degree of specialization attained in the Geomyidae in regard -to the patterns of wear, since a sequence of bilophodont patterns -appear in both the Dikkomyini and Geomyini before the monoprismatic -pattern is developed. - - [Illustration: FIG. 5. Drawings of molariform dentitions - representative of the tribes Geomyini and Thomomyini depicting - patterns of wear on the occlusal surface. A-D represent, in - ontogenetic sequence from left to right, upper tooth-rows of the - tribe Geomyini. E-H represent, in the same sequence of stages, - lower tooth-rows of the tribe Geomyini. I-L represents both upper - and lower tooth-rows of both pre-final and final stages of wear in - the tribe Thomomyini. All × 5. - - A and E. _Geomys bursarius majusculus_, No. 2948 (KU), Douglas Co., - Kansas. Right upper (A) including DP4-M3; lower left (E) - including dp4-m3. - - B and F. _Pappogeomys bulleri burti_, No. 100444 (KU), 10 mi. NNW - Barra de Navidad, Jalisco. Right upper (B) including P4-M3; - right lower (F) including p4-m3 (both P4 and p4 with unworn - enamel caps). - - C and G. _Pappogeomys bulleri albinasus_, No. 31044 (KU), 10 mi. S - and 8 mi. W Guadalajara, Jalisco. Right upper (C) including - P4-M3; right lower (G) including p4-m3. - - D and H. _Pappogeomys bulleri albinasus_, No. 31002 (KU), W side La - Venta, 13 mi. W and 4 mi. N Guadalajara, Jalisco. Right - upper (D) including P4-M3; right lower (H) including p4-m3. - - I and J. _Thomomys talpoides bridgeri_, No. 6865 (KU), 2 mi. up Mink - Creek, Pocatella, Bannock Co., Idaho. Left upper (I), - DP4-M3; left lower (J), dp4-m3. - - K and L. _Thomomys talpoides fossor_, No. 13205 (KU), Wasson Ranch, - 3 mi. E Creede, Mineral Co., Colorado. Right lower (K), - p4-m3; left upper (L), P4-M3. - ] - -Relationship of the Geomyini with the ancestral Dikkomyini is most -clearly demonstrated in the sequence of wear on the occlusal surfaces -of the molars. As in all geomyids, the upper part of the crown is -biprismatic in the newly erupted tooth, and the two columns are -separated by an intervening valley. With slight attrition on the -unworn enamel cap, the weakly developed cusps merge and form a -transverse enamel loop on each of the two columns (see third molar in -Fig. 5, A and E), each loop enclosing a core of dentine that had -become exposed. The valley between the two columns is shallow, and -upon further wear of the tooth, the two loops unite. The two columns -become joined at different points in the upper and lower molars -depending on the varying depth of the valley in different teeth. -Therefore, upper and lower molars develop distinctly different -occlusal configurations. - -In the lower molars, the pattern characteristic of _Dikkomys_ (Fig. -4C) is preserved without significant modification, as illustrated in -an immature specimen of _Geomys_ (see Fig. 5E). The H-pattern and -modified H-pattern are developed in the same sequence of wear in the -Geomyini. A juvenal female (not illustrated), KU 2931, provides an -example of the intermediate H-pattern. In this specimen, the -protolophid and hypolophid of the left m2 are united only at their -mid-points, indicating that the pattern of wear occurs in the same -sequence in the Geomyini as it did in the Miocene genus _Dikkomys_. -After the two columns have become united at their mid-points, a -secondary union is formed at the edge of their protomeres, thus -enclosing the enamel fossette as illustrated in Figure 5E (this is the -modified H-pattern mentioned above). However, the fossette itself is -shallow and soon disappears with slight wear. At this stage, the -occlusal configuration would be in a U-pattern (m1 in Fig. 5E). The -lingual re-entrant fold is also shallow in vertical depth; therefore, -it is obliterated by wear following the eradication of the labial -fossette. Consequently, the two columns are united into one. In m3 -(see Figs. 5E, F, and G), the two columns merge by progressive lateral -expansion of the medial isthmus. - -In the first and second upper molars, the two columns unite across the -entire surface of their protomeres from near the lingual edge of the -crown to near its center. A minute inner inflection may be temporarily -retained in some teeth. At this stage (see Fig. 5B), the parameres are -still separated by the labial fissure, and the occlusal pattern is in -the shape of a U, resembling, but not exactly duplicating, the -pre-final pattern of Ml and M2 in the genus _Pliosaccomys_ (see Fig. -4H). The labial fissure is shallow, and, with further wear, the -inflection is worn away and the parameres also unite, thereby forming -a monoprimatic crown in the final stage. In M3, the two lophs first -become united near the edge of their protomeres (see Fig. 5B), -therefore forming a U-pattern similar to that developed in Ml and M2 -of _Pliosaccomys_. The connection of the two lophs is not directly at -the end of the protomere; consequently a shallow lingual inflection -remains. The lingual edge of the valley is also shallow, and, with -continued wear a second union of the two lophs takes place near the -ends of their parameres, and the deeper, interior part of the valley -remains as an isolated enamel fossette (see Fig. 5C). The two primary -lophs of the tooth are now joined near both sides, having shallow -lingual and labial re-entrant angles on the sides and the enamel -island in the center. With continued effacement of the occlusal -surface, the fossette will be eradicated, and the pattern of the -occlusal surface will become the partially biprismatic pattern of the -final stages (adult) of wear (see Fig. 5D). M3's of _Dikkomys_ and -_Pliosaccomys_ are not known; however, it seems reasonable to assume -that the pattern of wear in the M3 of Dikkomyini was not essentially -different from that of the Geomyini, except that it is likely that the -U-pattern of the second stage of wear in the Geomyini was probably the -final stage in the genus _Dikkomys_. - -Judging from the pre-final stages of wear, the dentition of the -Geomyini provides a curious combination of patterns that resemble in -part the Miocene genus _Dikkomys_ and in part the early and middle -Pliocene genus _Pliosaccomys_. There is no significant variation in -the premolars or third molars (at least in the lower teeth) of the -Geomyinae from the early Miocene to late Pliocene; therefore, -deviations of major significance are in the character of the first and -second molars. In the Geomyini, the patterns of wear of m1 and m2 are -the same as those of _Dikkomys_, and are distinctly different from -those of _Pliosaccomys_ where the two columns first unite at the edge -of their protomeres to form a U-pattern, rather than at their -mid-points to form an H-pattern. Even though the intermediate stages -of ontogeny in m1 and m2 of _Pliosaccomys_ and the Geomyini are -entirely different, the bicolumnar crowns of both eventually unite, -upon wear, into a single column. On the other hand, the patterns of M1 -and M2 in the Geomyini most closely resemble those of _Pliosaccomys_, -rather than _Dikkomys_. In this regard it should be pointed out that -the upper molars of _Dikkomys_ are presently represented by only one -tooth, an M1 in an early stage of wear. As described already, the -patterns of M1-2 evidently would be mirror images of m1-2 in -corresponding stages of wear. However, the initial union of the two -columns, in the M1 that is known, is somewhat to the lingual side of -center and the relatively small lingual valley does not reach the base -of the crown, indicating, that eventually with wear, the two columns -of _Dikkomys_ might have become united across the entire surface of -their protomeres as in _Pliosaccomys_. Even so, the two columns of M1 -do initially join closer to their mid-points than they do in -_Pliosaccomys_, and, if they did actually unite across their -protomeres, the union would have occurred with subsequent wear. That -is, the first occlusal pattern would be H-shaped (but with the -connection closer to the lingual than the labial side), as in m1 and -m2, and it would become U-shaped only after additional wear. This -sequence of patterns of M1 and M2, as already pointed out, does not -pertain in _Pliosaccomys_ or the Geomyini, since the U-pattern is -formed with the first union of the two columns at the edge of their -protomeres, and the primitive H-pattern is never developed, unless one -counts the slight lingual inflection, that occasionally is formed just -after the two columns unite, as being indicative of the primitive -pattern. As in the lower teeth, the bicolumnar crowns of early -ontogeny in both _Pliosaccomys_ and the Geomyini become eventually -united, with wear, into a single column. - -Based upon the foregoing evidence, it would seem likely that the -Geomyini evolved from an early Pliocene (perhaps late Miocene) -Dikkomyini ancestor that had evolved the specializations of M1 and M2 -that characterize its relative, _Pliosaccomys_, but had not also -evolved the specializations of m1 and m2 that distinguish -_Pliosaccomys_. Therefore, the ancestor of the Geomyini differed from -the _Pliosaccomys_-Thomomyini lineage in its retention, unmodified, of -the primitive patterns in m1 and m2 that characterized the earliest -known Geomyines (_Dikkomys_). The same patterns are preserved in m1 -and m2 of its modern descendents, the living Geomyini. In the -_Pliosaccomys_-Thomomyini lineage the pattern of m1 and m2 are -entirely different, as described above. - -The earliest record of the Geomyini is the extinct genus _Pliogeomys_ -(see Fig. 6) in the latest Hemphillian (middle Pliocene) and earliest -Blancan (late Pliocene). _Pliogeomys_ is more primitive than any -modern genus of the Geomyini, seems to have been a late survivor of -the primitive stock, but was itself probably a collateral lineage and -not on the direct line of descent. The cheek teeth in _Pliogeomys_ are -rooted and less hypsodont than in the late Pliocene examples of the -modern genera, and the anterior enamel plate of the lower molars shows -no indication of reduction, as would be expected if _Pliogeomys_ were -in the direct line of evolution. Separation of _Pliogeomys_ from the -main stem of the Geomyini probably occurred after several -specializations had already been achieved by the Geomyini. Two -inheritances might have been grooving on the upper incisors and some -reduction in amount of enamel on the sides of the cheek teeth. The -dentine tracts on the sides of the cheek teeth of _Pliogeomys_ are -narrow (see Fig. 7A) and barely separate the enamel blades and there -is no discernible reduction in the anterior enamel blades on its lower -molars. Those blades evidently were lost in the main lineage before -the Pleistocene radiation of the living genera took place. -_Pliogeomys_ is in an intermediate stage in evolution, and was not so -advanced as was the main lineage at the time _Pliogeomys_ died out. -Its structure does provide clues as to phyletic development that took -place in the main lineage. - -Specialized trends in the early phylogeny of the Geomyini included: -development of rootless, ever-growing cheek teeth and an increase in -hypsodonty; loss of the bicolumnar structure of the first and second -molars, and, consequently, the formation of a single elliptical column -in the final stage of wear; interruption of the enamel investment of -the molariform teeth and formation of anterior and posterior enamel -plates; and enlargement of the masseteric ridge and fossa. Each of -these trends occurred independently in the Thomomyini, and each is an -example of parallelism in the phyletic evolution of the two lineages. -Three additional specializations lacking in the Thomomyini are the -grooving on upper incisors, loss of anterior enamel plate in lower -molars, and development of a basitemporal fossa on the mandible. -Evidently, two grooves evolved in the ancestral incisors in the same -bisculcate pattern preserved in _Pliogeomys_, _Zygogeomys_ and -_Geomys_. The innermost groove is weakly developed in _Pliogeomys_, -suggesting that this character was in an intermediate stage of -evolution in the ancestral lineage at the time that _Pliogeomys_ split -off. Numerous other specializations in the Geomyini appeared later, -but evolved in the different genera that diverged from the ancestral -lineage and are discussed separately in the next account. Only two of -the major features characterizing the Dikkomyini are retained in the -Geomyini: the H-pattern on the occlusal surface of the m1 and m2 -developed during the initial stages of wear, and the bicolumnar -pattern of M3. Adaptive radiation produced the living genera of the -Geomyini in the late Pliocene and early Pleistocene (see Fig. 6) and -subsequent specialization of the ancestral morphology followed. - -Parallelism in the molars of later geomyines and the Entoptychinae is -illustrated by the lateral interruption of the enamel investment and -loss of enamel plates and by the omission of the H-pattern stage in -the first and second molars (in _Pliosaccomys_). Resemblance of -dentitions in certain stages of wear in _Pliosaccomys_ and in -entoptychines led some investigators, for instance, Hibbard -(1953:357), to suggest that _Pliosaccomys_ descended from one of the -less specialized entoptychines, possibly _Grangerimus_ but probably -_Gregorymys_. Actually, the highly specialized upper and lower -premolars and third molars of the entoptychines rule them out as -ancestors of the later geomyines. The evolution of entoptychine-like -features in _Pliosaccomys_ is regarded as an example of iteration, a -pattern of parallelism (see Simpson, 1953:248-253) where an -allochronic and independent lineage undergoes the same evolutionary -trend that phyletically characterized an earlier lineage, usually -after the latter has become extinct. In this case, the lineage giving -rise to _Pliosaccomys_ passed through the same phyletic stages in its -evolution in the early Pliocene (and possibly the late Miocene) as did -the entoptychines in the late Oligocene and early Miocene. - -Another parallelism by iteration, occurring in the middle and late -Pliocene in both the Thomomyini and Geomyini, is the loss of enamel -from the lateral surfaces of the cheek teeth, and, in the Geomyini -only, the eventual loss of the anterior plate in the lower teeth and -the posterior plate in the upper teeth. Both features were evolved -more than an epoch earlier in the specialized entoptychid genus -_Entoptychus_ of the lower Miocene. In _Entoptychus_, only the -posterior plate of the lower molars and the anterior plate of the -upper molars remained in the final stages of attrition, although a -central enamel fossette, a remnant of the re-entrant fold, remained -throughout life. Iteration is also expressed in the subfamily -Geomyinae by the development of grooving on the upper incisor and the -formation of the basitemporal fossa. A shallow but distinct -basitemporal fossa occurs between the coronoid process and the third -lower molar in the genus _Entoptychus_ and a sulcated upper incisor, a -single shallow groove usually near the median border of the tooth, is -found in the genus _Gregorymys_ of the subfamily Entoptychinae. Both -features are regarded as advanced specializations in the tribe -Geomyini, even though each was evolved in the entoptychines of the -Lower Miocene. - -The postcranial skeleton of living genera of pocket gophers, as befits -animals that spend most of their life within underground burrows, are -highly specialized for a fossorial life. Elements of the postcranial -skeleton recovered from Lower Miocene deposits indicate that the -entoptychines were only semi-fossorial (see Cope, 1884:857; Wood, -1936:4-5; Wilson, 1949:117-118). One of the basic trends of the -entoptychines was towards greater fossorial adaptation; the skeleton -of _Entoptychus_ shows a greater degree of fossorial adaptation than -earlier genera of the subfamily. There is no reason to suppose that -the geomyine genus _Dikkomys_, which lived at the same times as the -entoptychines, had acquired any more advanced fossorial adaptations -than had the entoptychines. - -The most pronounced fossorial adaptations seem to have evolved only in -the ancestral lineage of the modern geomyines, probably in the latter -part of the Miocene and in the early Pliocene, before the modern -Thomomyini and Geomyini diverged. Extreme fossorial adaptations in -herbivorous rodents, such as those characteristic of the modern pocket -gophers and their immediate ancestors, are thought to have evolved -only in response to pronounced arid conditions. The Entoptychinae and -evidently the early geomyines lived in environments that were either -tropical or temperate, and under conditions more mesic than I would -consider necessary to bring about selection pressure resulting in -fossorial specializations. In late Oligocene and early Miocene, -according to Axelroad (1958:433-509), arid conditions did not exist in -the United States, and the only xerophytic environments in North -America occurred on the Central Plateau of México. Moreover (Axelroad, -_loc. cit._), arid conditions did not develop in the western United -States until the early Pliocene. Geomyids evidently became extinct in -this region at the close of the Middle Miocene, and none appear in -fossil deposits in the western United States until the latest Lower -Pliocene (Clarendonian). The reappearance of geomyids, _Pliosaccomys_, -in the western United States coincides with a trend toward aridity and -the northward movement of the Madro-tertiary geoflora into the Great -Basin and Great Plains from its place of origin on the Central Plateau -of México (Axelroad, _loc. cit._). Later, in the middle and later -Pliocene, the Madro-tertiary geoflora gave rise to the modern -xerophytic plants that now characterize the desert vegetation of North -America. - -The Madro-tertiary climax does not appear as a major flora until the -Miocene, but probably originated earlier. According to Axelroad (_loc. -cit._), this xerophytic flora evolved from elements of the -Neotropical-tertiary geoflora that became adapted to arid conditions -that developed in the rain shadow of the high mountains flanking the -Central Plateau of México. Originally, the Madro-tertiary flora -consisted of small trees, shrubs, and grasses. Although some elements -of this flora moved northward in the late Miocene, the major part of -it remained in México until the early Pliocene. In the western United -States, mountain formation increased in intensity in the Pliocene and -continued on into the early Pleistocene. As the mountains became more -elevated, especially the Sierra Nevada and Cascade ranges, they -blocked the prevailing winds from the Pacific Ocean and extensive -aridity developed on their leeward side. As xeric conditions became -widespread, the Madro-tertiary flora successfully occupied the drier -regions of southern California, the Great Basin, and the western parts -of the Great Plains. - -While the Entoptychinae probably evolved in response to the -Arcto-tertiary flora, the late Tertiary geomyines probably evolved in -response to the Madro-tertiary geoflora on the Central Plateau of -México. Some of these early geomyines, especially ancestors of the -modern lineages, probably were pushed southward by competition with -the more specialized entoptychines. Most geomyines were pushed out of -the northern area of distribution, except for _Dikkomys_ that survived -in association with the entoptychids throughout the early and middle -Miocene. During this time, and probably continuing on into the late -Miocene, the geomyines occurring to the south in México became adapted -to the arid environments of the Madro-tertiary geoflora. - -Of course, information is lacking about climates in several parts of -the late Miocene and early Pliocene. When such information becomes -available it conceivably could modify the hypothesis outlined -immediately above. - -The principal trend of evolution in these semi-fossorial rodents was -toward more complete fossorial adaptation, and the pronounced -fossorial features characteristic of the modern pocket gophers were -perfected. This trend continued in response to the intense selection -pressures in this arid environment. The principal structural -characters effected were in the postcranial anatomy, especially in the -skeletal and muscular systems. Consequently, it is not surprising that -in skull and dentition, _Pliosaccomys_ differs but little from -_Dikkomys_. Therefore, most of the basic structural specializations so -far developed for subterranean existence probably had evolved by the -time geomyines moved back north in the early Pliocene. Both modern -lineages, the tribes Thomomyini and Geomyini, have essentially the -same fossorial features, and it seems unlikely that these features -were acquired independently in the relatively short period of time -available to them after their divergence; probably they were inherited -from a common ancestor. These probabilities indicate that the -evolution of the fossorial specialization was in the later phyletic -development of the tribe Dikkomyini. - - -Plio-Pleistocene radiation of Geomyini - -Unlike the lineage of the Thomomyini that remained essentially -rectilinear through out its history, the Geomyini in the late Pliocene -and the early Pleistocene underwent adaptive radiation in a degree -comparable to the earlier radiation of the Entoptychinae, and all of -the later history of the tribe is dominated by the radiation--the -resulting structural diversity. At least four lineages were produced -by the Plio-Pleistocene radiation (see Fig. 6); each originated at -essentially the same time (late Pliocene) presumably from the same -ancestral stock. Each of these lineages within the Geomyini has given -rise to one of the four modern genera: _Zygogeomys_, _Geomys_, -_Orthogeomys_, and _Pappogeomys_. - - [Illustration: FIG. 6. Plio-Pleistocene radiation of the Tribe - Geomyini.] - - -_Morphotype_ - -The immediate, unknown, ancestor probably lived on the Central Plateau -of México. After the radiation began the ancestors of _Geomys_ and -_Zygogeomys_ extended their ranges northward. - -Features of the hypothetical morphotype, that would permit derivation -of the modern genera would include the following: (1) Skull -generalized, neither excessively long and narrow or short and broad; -(2) skull smoothly rounded, without pronounced angularity, rugosity or -cresting (sagittal crest probably lacking, even in old individuals); -(3) zygomata slender, without lateral platelike expansions; (4) -rostrum moderately broad; (5) upper incisors bisulcate, two grooves in -pattern found in _Pliogeomys_, _Zygogeomys_ and _Geomys_; (6) lateral -re-entrant angles of premolars obtuse; (7) p4 having four enamel -plates (one on anterior wall, one on posterior wall, and two lateral -plates) and lower molars having one enamel plate on the posterior wall -of tooth (anterior plate is lacking); (8) P4 having four enamel -plates, in same pattern as described for p4, M1 having two enamel -plates (one anterior and one posterior), M2 same as M1, M3 having -three plates (one anterior, two lateral on sides of posterior loph, -none posterior); (9) M3 subtriangular in cross-section, distinctly -bicolumnar, two columns marked by shallow re-entrant folds and -connected by broad isthmus; (10) masseteric ridge large, forming high -crest bordering masseteric fossa; (11) basitemporal fossa shallow; -(12) angular process of mandible short, its lateral projection barely -exceeding that of zygomatic arch. - - -_Specializations in Genera_ - -In relation to the primitive morphotype, increase in size, -simplification of dentition, and changes in shape of skull are -regarded as specializations. Considerable parallelism between the four -lineages is seen. But each lineage is distinguished by a combination -of specialized features, and three by a few unique specializations. - -Among trends resulting in simplification of the dentition, reduction -of enamel on the posterior wall of the upper cheek teeth has occurred -in various degrees in all lineages of the Geomyini even to loss of all -enamel on the posterior wall of the premolars and molars in two -genera. Loss of some enamel is more common on P4 than on M1-2, and has -occurred in all genera (see Figs. 7 and 9.) - -In evolutionary sequence loss of enamel from M1 and M2 usually occurs -after, but never preceding, the reduction of enamel on P4. Loss of -enamel plates from the posterior face of M1 and M2 is associated with -the evolution of an efficient anterotransverse shearing action of the -teeth. - -On the anterior wall of those teeth no reduction of the cutting blade -has been observed; a complete anterior plate is retained in all living -Geomyini. - -Presence of both the posterior and anterior plates decreases the -efficiency of transverse shearing, by providing two upper plates -(anterior plate of one tooth and posterior plate of the preceding -tooth) over which the lower cutting blade _simultaneously_ must pass -with each movement. The advantages of shearing over the more common -mechanics of planing are largely lost unless the posterior plates are -eliminated. Also, none of the living Geomyini have retained a -definitive posterior enamel plate on M3, the last upper molar; but two -well-developed lateral plates, that extend almost all of the way back -to the posterior apex of M3, have been retained, and, together -function as a posterior plate. Loss of either or both of the lateral -plates of M3 is rare, and occurs only in old individuals. Their loss -in the final stages of wear may represent the beginning of a new trend -in those species where it occurs (the _castanops_-group of the -subgenus _Cratogeomys_). In any case, reduction of enamel takes place -by transverse shortening of the plate through the complete loss of -enamel on one end, the diminution beginning first on the labial end -and proceeding by progressive atrophy to the lingual end of the plate. -Evidently, when enamel has been eliminated from the labial end of a -plate, the rate of loss decreases markedly, and the last stages of -evolution, terminating in complete loss of an enamel plate, occurs -more slowly. Evolution may be arrested before complete loss has -occurred, and that part of the enamel that remains forms a short, -vestigial plate restricted to the lingual one-fourth or one-third of -the wall. The enamel pattern of the lower dentition is the same in all -of the diverging lineages, with no evidence of additional loss of -enamel from that which had already occurred in their common ancestor -(see Figs. 7 and 9). Reduction and loss of enamel plates began and was -terminated in the lower dentition before reduction began in the upper -dentition. - -Other dental specializations have occurred in the shape of the third -upper molar and in the pattern of grooving in the upper incisor. -Unlike M3 of the Thomomyini, that of the Geomyini differs in shape -from M2, and its enamel investment differs from that of M2. -Primitively, M3 was probably subtriangular in cross-section, and the -posterior loph evidently projected posteriorly as a short, rudimentary -heel that formed the apex of the triangle. Other shapes of M3 are -considered to be specializations that have been derived from the -primitive form. In addition to the primitive subtriangular pattern, -the M3 of living Geomyini may be suborbicular, quadriform, elongate, -or obcordate in shape. Usually each lineage is characterized by only -one pattern, but in one genus (_Pappogeomys_) all patterns occur. Of -the different forms, the elongate and obcordate seem to be the most -highly specialized deviations from the triangular-shaped tooth. The -bicolumnar pattern is accentuated in the elongate type (Fig. 7D, F, H) -by deep lateral re-entrant folds, on both the lingual and labial -sides, and by the elongation of the posterior loph into a pronounced -heel. Teeth having this pattern have been illustrated by Merriam -(1895:76-82) in Figures 27 (6 and 7), 28 (c and d), 34 (7 through 15), -and 35 (8). - - [Illustration: FIG. 7. Molariform dentitions of the Tribe Geomyini. - Drawings illustrating enamel patterns characteristic of - _Pliogeomys_, _Zygogeomys_, and the subgenera of _Orthogeomys_ - (_Orthogeomys_, _Heterogeomys_ and _Macrogeomys_). × 5. - - A. _Pliogeomys buisi_, No. 29157 (UMMP), holotype, Buis Ranch - (Upper Middle Pliocene), Beaver Co., Oklahoma. Right lower, - p4-m2 (m3 unknown). - - B and C. _Zygogeomys trichopus trichopus_, adult female, No. 51971 - (FMNH), Mt. Tancítaro, 10,500 ft., Michoacán. Left upper - (B), P4-M3; right lower (C), p4-m3. - - D and E. Subgenus _Orthogeomys_. _Orthogeomys grandis guerrerensis_, - adult female, No. 39807 (KU), 1/2 mi. E La Mira, 300 ft., - Michoacán. Left upper (D), P4-M3; right lower (E), p4-m3. - - F and G. Subgenus _Heterogeomys_. _Orthogeomys hispidus hispidus_, - adult female, No. 23975 (KU), 4 km. W Tlapacoyan, 700 ft., - Veracruz. Left upper (F), P4-M3; right lower (G), p4-m3. - - H and I. Subgenus _Macrogeomys_. _Orthogeomys heterodus - cartagoensis_, adult female, No. 60664 (KU), Rancho - Redando, Volcán Lrozá, Prov. San José, Costa Rica. Left - upper (H), P4-M3; right lower (I), p4-m3. - ] - - - -The subcordate form is characterized by pronounced anteroposterior -compression, and retention of a distinct labial re-entrant fold. The -posterior loph apparently has been rotated in such a way that what was -previously its posterior border now lies on the outer margin of the -tooth; therefore, the axis of the posterior loph is strongly oblique -in relation to the anteroposterior bearing of the maxillary tooth-row, -and the median enamel plate also has been rotated and so lies -transversely across the posterior wall of the tooth. Owing to the -rotation of the posterior loph, the apex of the obcordate tooth is at -its lingual side. The subcordate type is illustrated by Merriam (_loc. -cit._) in Figures 27 (3 and 4), 28 (a and b), 34 (3 and 4), and 35 (5, -6, and 7). The suborbicular and quadriform types are less specialized -than the two described above. Both are characterized by reduction, -often obliteration, of the bicolumnar pattern of the subtriangular -ancestral form, especially marked by the decrease in depth of the -lateral re-entrant folds and the decrease in length of the posterior -projection of the posterior loph. With these changes, the tooth -becomes essentially monocolumnar, its occlusal surface oval in outline -in one and squarish in shape in the other. Occlusal views of the -suborbicular form are presented by Merriam (_loc. cit._) in Figure 33 -(1, 5, 6, 7, 11, and 12) and the quadriform tooth is depicted in -Figure 29. Grooved upper incisors are characteristic of the living -Geomyini, but variation occurs in the number of grooves, and, if only -one groove is present, its position on the anterior face of the tooth -varies. Except for the previously mentioned (p. 480) abnormal tooth -having three grooves, incisors with no more than two grooves are found -in these pocket gophers, and this number of grooves is taken to be -primitive. Loss of one or the other of the two grooves of the -bisulcate pattern, therefore, is regarded as specialization. However, -complete loss of both grooves never occurs in the Geomyini. Each of -the four major lineages is characterized by one of the three patterns -of grooving, and the particular groove-pattern is remarkably stable in -each group. - -Shape of skull varies from dolichocephalic to platycephalic. The -morphology of each has been described in foregoing accounts. The -dolichocephalic skull is highly specialized for planing, a grinding -action of the teeth; whereas, the platycephalic skull is highly -specialized for shearing, a slicing action of the teeth. Of course, -concomitant specializations of the dentition, as described above, are -closely associated with both specialized trends in the skull. Most -kinds of living Geomyini have generalized skulls that show no tendency -toward either of the specialized conditions. - -Increase in size of body and skull is seen in most Pleistocene -lineages of the Geomyini. Judging from the smallness of the skull in -late Pliocene species, representing the base of three of these -lineages, the ancestral species of the living assemblage were no -larger than the living species of the subgenus _Pappogeomys_ or the -smaller subspecies of _Geomys bursarius_. The recorded range of -variation in condylobasal length is 36.1 to 45.5 in _Pappogeomys -bulleri_, including both adult males and females. Probably the skulls -of the ancestral species were not significantly larger. Maximum -dimensions of males in living species are 74.5 (subgenus -_Cratogeomys_) and 75.0 (subgenus _Orthogeomys_). These are more than -twice the minima observed in _Pappogeomys bulleri_. - - -Zygogeomys - -This is the least specialized and most primitive of the four lineages, -has a generalized type of skull, two grooves on the anterior face of -each upper incisor, an enamel plate on the posterior wall of P4, open -or divergent lateral re-entrant angles on the premolars, and a -bicolumnar and elongated M3. All of these features are primitive and -essentially as in the ancestral morphotype. No other modern genus -retains so much of the primitive structure. Phyletic trends in -_Zygogeomys_ are not well documented in the fossil record; and only a -few fossils are known and they are fragmentary as discussed before. -The genus is represented in the late Pliocene (_Z. minor_), middle -Pleistocene (_Z. persimilis_), and Recent (_Z. trichopus_). The living -species is a relict population in the mountains of Central México. -Judging from the known material, the phyletic trends in the genus have -been increase in size, reduction of enamel on the posterior face of P4 -(occurring only in the living species) where a short enamel plate is -retained on the lingual side of the tooth (see Fig. 7B), loss of the -outer fourth of the enamel blade on the posterior wall of M1 and M2 -(also occurring only in the living species), development of a more -pronounced heel on the M3 by progressive elongation of the posterior -loph, reduction in size of the jugal and its displacement ventrally, -which allows the maxillary and squamosal bones to meet along the -dorsal border of the zygomatic arch. The last specialization is seen -in at least one taxon of _Orthogeomys_ (_Orthogeomys cherriei -costaricensis_). In my opinion, too much weight has been given to this -feature in past classifications. Reduction of enamel in the upper -dentition evidently occurred in the late Pleistocene, since the -posterior plates on the upper cheek teeth were complete in specimens -from the middle Pleistocene (_Z. persimilis_). - - -Geomys - -_Geomys_, slightly more specialized than _Zygogeomys_, must also be -regarded as one of the most primitive of the living genera. Primitive -features that have been retained are the generalized type of skull, -the bisulcate pattern of grooves on the upper incisor, and the -retention of enamel plates on both the anterior and posterior walls -of M1 and M2 (see Fig. 9A). All of these primitive features are -shared with _Zygogeomys_. In addition, three other trends, or -specializations, in evolution characterize the phyletic development of -_Geomys_. One major trend is toward loss of the enamel plate from the -posterior wall of P4. No trace of enamel remains on the posterior wall -of this tooth in late Pleistocene or Recent species of _Geomys_, and -at least one of the earlier species (_quinni_) was also characterized -by loss of this enamel plate. Secondly, M3 retains only a vestige of -the primitive bicolumnar pattern after the initial stages of wear. In -most Recent specimens, especially of the species _G. bursarius_, the -lateral re-entrant fold and the heel of M3 are small, and the -re-entrant inflection is hardly evident. The lateral fold is more -frequently well-developed in Irvingtonian species than in living -species (White and Downs, 1961:13), illustrating progressive loss of -the bicolumnar pattern in Pleistocene evolution. A third trend -involves the modification of the lateral folds of the premolars. -Primitively the angles of these folds are broadly open or divergently -V-shaped, and some of the earliest species of _Geomys_, for example -_G. quinni_, have retained this feature throughout life. Nevertheless, -the main trend is toward progressive compression of the folds -resulting in their walls being more nearly perpendicular, and -parallel, to the long axis of the tooth. Obtuse re-entrant angles -persist in premolars of young individuals of Irvingtonian species, but -the adults are characterized by well-compressed folds, as in Recent -species. - -Remains of _Geomys_ are abundant, especially from Pleistocene deposits -of the Great Plains, but in most instances specific assignment is -difficult or impossible since only isolated teeth or fragments of -skulls have been preserved. Estimates of phyletic relationships of the -known species of _Geomys_ are depicted in Figure 8; those estimates -are useful in discussing the phyletic development of the genus. One of -the earliest known species, _Geomys quinni_, ranges from Upper -Pliocene to the later stages of the Lower Pleistocene (Aftonian -interglacial deposits). The dentition of _G. quinni_ is essentially -the same as in the living species except that open lateral re-entrant -angles are retained in the premolars. _Geomys paenebursarius_, also of -the early Pleistocene, is a smaller species and seems to be more -directly in the line of evolution of the modern species. As yet -unnamed smaller species of _Geomys_ from the Rexroad fauna (late -Pliocene) and Saunders fauna (latest Aftonian) may also be on the main -line of evolution. Surprisingly, _Geomys tobinensis_ and _Geomys -garbanii_ of later Irvingtonian provincial age are less specialized -than either _Geomys quinni_ or _Geomys paenebursarius_. It is likely -that _G. tobinensis_ and the unnamed species from the Dixon are closer -to the main line of descent than _G. paenebursarius_ suggesting that -the direct ancestral lineage of the living species of _Geomys_ was -more conservative and less specialized than _Geomys paenebursarius_ of -the Lower Pleistocene. _Geomys quinni_ and _G. paenebursarius_ seem to -have acquired specialized dental features in the early Pleistocene. -_Geomys quinni_ was successful on the Great Plains, and persisted into -the late Blancan. The main line may be represented in the early -Pleistocene by _Geomys paenebursarius_ from the Hancock formation of -the Texas Trans-Pecos. The structure of _G. paenebursarius_ indicates -that it is in or close to the main line of descent, and probably -evolved from one of the more primitive late Pliocene species of -_Geomys_ from the Rexroad fauna. - - [Illustration: FIG. 8. Tentative arrangement of species of the genus - _Geomys_, depicting phylogenetic trends and probable relationships - within the genus.] - -Isolated teeth, to which the name _Geomys bisulcatus_ probably -applies, from Illinoian deposits on the Great Plains, show that the -dentition characteristic of the living _Geomys_ had been developed by -that time. Actually, the Illinoian material is too fragmentary to -show clearly its taxonomic or phyletic affinities with the species of -the later Pleistocene. Even so, the two main stocks of living -_Geomys_, _G. bursarius_ and _G. pinetis_, had certainly been -differentiated by Sangamon time. The other living species evidently -evolved from one or the other of these two stocks in a period of -isolation from the main population, probably in either the Wisconsin -or post-Wisconsin. For example, _Geomys arenarius_ clearly -differentiated from populations of _Geomys bursarius_ that were -isolated by the eastward retreat of the main population from the -southwestern United States as that region became more arid in the -post-Wisconsin. - -In review, it seems that the Recent species, represented basically by -_bursarius_ and _pinetis_, evolved from Illinoian species (_Geomys -bisulcatus?_), which descended in turn from the more primitive species -of the early Pleistocene, possibly _Geomys paenebursarius_ or possibly -from descendants of the Saunders species. Actually the Saunders -species may prove to be _Geomys paenebursarius_. At any rate, three -trends that took place during the Pleistocene stage of evolution, in -the direction of the modern species, were an increase in size, -progressive loss of the posterior enamel plate on P4, and a decrease -in the vertical depth of the enamel cap as a result of which the -dentine is reached in the initial phases of attrition on the tooth of -a juvenile. _Geomys garbanii_, occurring at the periphery of the range -of the genus, is regarded as a sterile offshoot of the primitive -_tobinensis_-line of evolution. - - -Orthogeomys - -This is one of the more specialized genera of the Geomyini. Save for -one record in the late Pleistocene (_Orthogeomys onerosus_), there is -no fossil history of the genus upon which to reconstruct its -phylogeny; therefore, its phyletic development must be estimated by -comparing it and the primitive morphotype of the tribe. Results of -that comparison suggest that _Orthogeomys_ has closer affinities with -_Zygogeomys_ than with any of the other genera, and that _Orthogeomys_ -may have originated in an early dichotomy of primitive _Zygogeomys_ -stock instead of descending from the ancestral stock of the tribe. -Except for the unisulcate incisors and the longer posterior loph on -the third upper molars, the teeth of the two genera do not differ -significantly. As in _Zygogeomys_, the enamel blade on the posterior -wall of P4 has been reduced to a short plate restricted to the lingual -third of the tooth (see Fig. 7F and H). In _Orthogeomys_, the trend in -reduction of enamel is carried to its extreme only in the subgenus -_Orthogeomys_, where this plate has been completely lost in most taxa -(see Fig. 7D). The most significant trends in _Orthogeomys_, and the -principal basis for recognizing the genus, are the dolichocephalic -specializations of the skull, as described elsewhere, and the adaptive -traits that have equipped the genus for living in tropical -environments. The dolichocephalic features are more sharply defined in -the subgenera _Orthogeomys_ and _Macrogeomys_, and are less developed -in the subgenus _Heterogeomys_. Aside from the general dolichocephalic -specializations, trends in _Orthogeomys_ include: Increase in size; -loss of the median one of the two grooves on the anterior face of the -upper incisor in the ancestral stock; increase in the anteroposterior -length of each of the cheek teeth, as well as the aforementioned -elongation of the posterior loph of M3; compression of the lateral -angles of the premolars; and the remarkable increase in the size of -the rostrum. - - -Pappogeomys - -The genus _Pappogeomys_, as it is conceived of in this study, is -comprised of two subgenera; one, _Pappogeomys_, is generalized and -primitive, and the other, _Cratogeomys_, is specialized, and includes -the most highly specialized of the modern pocket gophers. The subgenus -_Pappogeomys_ is regarded as the ancestral lineage, and the subgenus -_Cratogeomys_ is regarded as an early offshoot, probably in the early -Pleistocene, that became progressively more specialized in the course -of its subsequent evolution. In the same period of time, the subgenus -_Pappogeomys_ changed little. It is known only from late Pliocene -fragments and from the living species. The ancestral morphotype is -preserved in _Pappogeomys_. Primitive characters are: (1) Small size; -(2) skull generalized and smoothly rounded; (3) temporal ridges -separate (not uniting into a sagittal crest); (4) enamel plates -retained on both anterior and posterior walls of M1 and M2; (5) M3 -bilophate, its posterior loph short. Basic specializations are few and -include loss of the inner groove from the anterior face of the upper -incisor; anteroposterior compression of the lateral re-entrant folds -of the premolars; and loss of enamel from the posterior wall of P4. -All three features have been perpetuated in the advanced subgenus -_Cratogeomys_, suggesting that they were already developed in the -early evolution of the subgenus _Pappogeomys_ before _Cratogeomys_ -diverged. Agreement with _Geomys_ is demonstrated by the lack of -enamel on the posterior wall of P4 (see Fig. 9) and by retention of -the posterior enamel plate on M1 and M2. In _Pappogeomys (Pappogeomys) -alcorni_ the enamel from the posterior face of M1 has been lost from -all but the lingual fourth or so of the posterior wall (Fig. 9E). -Reduction of enamel in M1 provides an example of parallelism with the -more advanced subgenus _Cratogeomys_, discussed below. - -There is no record as yet of the early evolution of the subgenus -_Cratogeomys_. The features that characterize the subgenus were -already well developed in the first known fossils which are from -Wisconsin deposits of the late Pleistocene. _Cratogeomys_ is not a -homogenous assemblage; instead it is composed of two groups of living -species, the generalized _castanops_ group and the specialized -_gymnurus_ group. The _castanops_ group may be survivors of the -ancestral lineage that diverged in two different stages in the -phyletic development of the main line. Even so, the _castanops_ group -has acquired its peculiar specializations. Indeed, _P. merriami_ of -the _castanops_ group differs from the hypothetical stem more than -does _P. castanops_. Judging from the structure of the living species -of the subgenus _Cratogeomys_ and from the primitive subgenus -_Pappogeomys_, the subgenus _Cratogeomys_ featured five major trends: -(1) Increase in size; (2) formation of sagittal crest by union of the -temporal impressions; (3) increase in rugosity and angularity of the -skull; (4) progressive development of platycephalic specializations, -including the elongation of the angular process of the mandible; (5) -complete loss of enamel plates from the posterior wall of M1 and M2. -Each trend is thought to be adaptive. - - [Illustration: FIG. 9. Molariform dentitions of the Tribe Geomyini. - Drawings illustrating enamel patterns characteristic of _Geomys_ - and _Pappogeomys_ (including the subgenera _Pappogeomys_ and - _Cratogeomys_). × 5. - - A and B. _Geomys bursarius bursarius_, adult female, No. 46275 (KU), - Elk River, Sherborne Co., Minnesota. Left upper (A), P4-M3; - right lower (B), p4-m3. - - C and D. Subgenus _Pappogeomys_. _Pappogeomys bulleri albinasus_, - adult female, No. 31002 (KU), W side La Venta, 13 mi. W and - 4 mi. N Guadalajara, Jalisco. Left upper (C), P4-M3; right - lower (D), p4-m3. - - E and F. Subgenus _Pappogeomys_. _Pappogeomys alcorni_, adult - female, No. 31051 (KU), holotype, 4 mi. W Mazamitla, 6600 - ft., Jalisco. Left upper (E), P4-M3; right lower (F), - p4-m3. - - G and H. Subgenus _Cratogeomys_. _Pappogeomys gymnurus tellus_, - adult female, No. 31051 (KU), 1 mi. NE Tala, 4400 ft., - Jalisco. Left upper (G), P4-M3; right lower (H), p4-m3. - ] - -Loss of enamel is a trend common to all living genera of the tribe -Geomyini, but the greatest loss has occurred in _Cratogeomys_. It has -lost the plates on the posterior walls of M1 and M2 (Fig. 9G). If the -lateral plates of M3 are considered as one functional plate and the -lateral plates on either side of P4 together as two transverse plates, -then, the transverse cutting blades in _Cratogeomys_ number seven in -the upper and seven in the lower cheek teeth compared with 10 in the -upper and seven in the lower in the primitive morphotype. Indeed, in -some species of the subgenus, one or both of the lateral plates on M3 -is also lost, usually in old age, resulting in even greater reduction -of enamel. Loss of enamel from the posterior walls of the upper -molars may be associated with changes in the mechanics of mastication -from anteroposterior planing to anterotransverse shearing, as -discussed elsewhere. Merriam (1895:95-96) argues convincingly that the -posterior cutting blades of the upper molars would hinder efficient -shearing action of the teeth; hence, selection would favor their -reduction and eventual loss. Changes in the shape of the skull also -seem to be correlated with the shift from a planing to a shearing type -of mastication. More efficient shearing action, which depends upon -lateral movement of the jaw, can be developed if the functional -muscles insert farther laterally than is possible in the generalized -type of skull. Therefore, platycephalic specializations involved -lateral expansion of the braincase and mandible. Pronounced lateral -expansion has been developed only in the _gymnurus_ group of species, -suggesting that the dental specializations evolved earlier in the -evolution of the subgenus than did the platycephalic specializations -of the skull, and that the _castanops_ group separated from the -_gymnurus_ group before the common ancestor had developed the more -extreme trends in platycephaly. 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Paleo., 10:388-391, 2 figs., July. - - - -_Transmitted May 29, 1967._ - -[] - - - * * * * * - - -Transcriber's note: - -All obvious typographical errors were corrected. Minor changes were -made to standardize the text to match the most prevalent form used. - - -Typographical Corrections - - Page Correction - ==== ============ - 477 cumberlandicus => cumberlandius - 535 breath => breadth - - - - - - -End of the Project Gutenberg EBook of Evolution and Classification of the -Pocket Gophers of the Subfamily Geomyinae, by Robert J. 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