The Project Gutenberg eBook ofEvolution and Classification of the Pocket Gophers of the Subfamily Geomyinae

The Project Gutenberg eBook ofEvolution and Classification of the Pocket Gophers of the Subfamily GeomyinaeThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: Evolution and Classification of the Pocket Gophers of the Subfamily GeomyinaeAuthor: Robert J. RussellRelease date: July 20, 2012 [eBook #40282]Most recently updated: October 23, 2024Language: EnglishCredits: Produced by Chris Curnow, Joseph Cooper, Tom Cosmas andthe Online Distributed Proofreading Team athttp://www.pgdp.net*** START OF THE PROJECT GUTENBERG EBOOK EVOLUTION AND CLASSIFICATION OF THE POCKET GOPHERS OF THE SUBFAMILY GEOMYINAE ***

This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.

Title: Evolution and Classification of the Pocket Gophers of the Subfamily GeomyinaeAuthor: Robert J. RussellRelease date: July 20, 2012 [eBook #40282]Most recently updated: October 23, 2024Language: EnglishCredits: Produced by Chris Curnow, Joseph Cooper, Tom Cosmas andthe Online Distributed Proofreading Team athttp://www.pgdp.net

Title: Evolution and Classification of the Pocket Gophers of the Subfamily Geomyinae

Author: Robert J. Russell

Release date: July 20, 2012 [eBook #40282]Most recently updated: October 23, 2024

Language: English

Credits: Produced by Chris Curnow, Joseph Cooper, Tom Cosmas andthe Online Distributed Proofreading Team athttp://www.pgdp.net

*** START OF THE PROJECT GUTENBERG EBOOK EVOLUTION AND CLASSIFICATION OF THE POCKET GOPHERS OF THE SUBFAMILY GEOMYINAE ***

Transcriber's note:Throughout, an asterisk before the name (i.e., *Entoptychinae) represents an extinct family/genus/species.

Throughout, an asterisk before the name (i.e., *Entoptychinae) represents an extinct family/genus/species.

University of Kansas PublicationsMuseum of Natural History

Vol. 16, No. 6, pp. 473-579, 9 figures in text

August 5, 1968

Evolution and Classificationof the Pocket Gophers of theSubfamily Geomyinae

ROBERT J. RUSSELL

University of KansasLawrence1968

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 KansasLawrence, Kansas

PRINTED BYROBERT R. (BOB) SANDERS, STATE PRINTERTOPEKA, KANSAS1968Look for the Union Label31-4628

Evolution and Classificationof the Pocket Gophers of theSubfamily Geomyinae

ROBERT J. RUSSELL

CONTENTS

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, andG. cumberlandius,Orthogeomys cuniculusandO. pygacanthusof the subgenusOrthogeomys, andO. dariensisandO. matagalpaeof the subgenusMacrogeomys. 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 generaDikkomys,Pliosaccomys,Pliogeomys,Nerterogeomys, andParageomysalso were studied, as were examples of the extinct speciesGeomys quinni,Geomys tobinensis, andOrthogeomys onerosus. Considerable fossil material of living species, especially of the generaGeomysandPappogeomys, 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 inOrthogeomysand Pleistocene taxa ofGeomyswhen 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 ofthe 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.

Specimens of the extinct generaDikkomys,Pliosaccomys,Pliogeomys,Nerterogeomys, andParageomysalso were studied, as were examples of the extinct speciesGeomys quinni,Geomys tobinensis, andOrthogeomys onerosus. Considerable fossil material of living species, especially of the generaGeomysandPappogeomys, 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 inOrthogeomysand Pleistocene taxa ofGeomyswhen 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 ofthe 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 toothis 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 (seep. 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 ofGeomysfrom 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 genusGeomys, 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 whichGeomysfeeds. 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 superficialdivision 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.

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.

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 (seep. 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 ofOrthogeomys(seep. 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 compressionof 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 subgenusCratogeomys(seeFig. 1, E and F) has a platycephalic skull. The trend toward platycephalic specialization has been the major feature of evolution inCratogeomys.

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 herbivorousrodents 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

Dikkomysis the only genus of the Geomyinae known from the early and middle Miocene.Dikkomys matthewiwas 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 ofDikkomyshave 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,Dikkomysis 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 ofThomomysand especiallyGeomysreported 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 ofGeomysdate back to the description ofGeomys bisculcatusMarsh (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. BothGeomysandThomomyshave been recovered from most of these deposits, but they are no older than middle Pleistocene. This is not surprising in view of the primitivestructure of the geomyids known from Miocene and Pliocene beds, but the supposed early appearance ofGeomysandThomomysled 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 isPliosaccomys dubiusWilson (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) recordedPliosaccomys dubiusfrom 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 ofPliosaccomys(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 wilsonidoes not differ greatly fromP. dubius; however, the few differencesin 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 inwilsoniare probably specializations, suggesting thatP. dubiuseven though the more recent in age is the less advanced of the two.P. wilsoniis 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.

Pliosaccomysoccurred 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 inPliosaccomys.

No other pocket gopher is known from the area in whichPliosaccomysoccurred, and it is unknown after middle Hemphillian age.Pliosaccomyshas closer affinities withDikkomysof 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 buisiHibbard (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 toPliogeomys buisi, and, in effect, tentatively assigned them toPliogeomys(in his description of the genus Hibbard remarked that the upper incisor is bisulcate as inGeomys, and the only upper incisor that he mentions was one of the Saw Rock Canyon fossils and not part of the Buis Ranch material).Pliogeomyshas closer affinities with modern pocket gophers of the tribe Geomyini than it does with the middle Pliocene genusPliosaccomys.

The pocket gopher fauna known from the late Pliocene was more varied than the faunas known from any earlier time. In addition to the extinctPliogeomys, which occurs in early late Pliocene (see discussion above), the living generaZygogeomys,Geomys,Pappogeomys(in the sense used onp. 534), andThomomysfirst appear in the late Pliocene. The only other living genus,Orthogeomys, makes its first appearance in the late Pleistocene.

The earliest record of the genusThomomysis based on a fragment of a left mandibular ramus bearing p4 and m1,Thomomys gidleyiWilson (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 (Hibbardet al., 1965:512), on the basis of potassium argon age determinations, also settled on late Pliocene.

Remains ofNerterogeomys[=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 asGeomys minorby Gidley (1922:123), and was later referred by Gazin (1942:487) to his new genusNerterogeomys. 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, asNerterogeomys, and tentatively referred them to the speciesN. minor.Nerterogeomyscf.minorhas 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 genusGeomys, and they may actually be a primitive form ofGeomysthat represents an intermediate stage in the development of the enamel pattern from the uninterrupted loops of the ancestorto the discontinuous pattern of modernGeomys. I favor this interpretation; the evidence, however, is inconclusive, and I have, therefore, reluctantly allocated them, along with the other specimens ofNerterogeomys, to the genusZygogeomys. In an early paper, Hibbard (1938:244) erroneously referred the same specimens, two upper premolars of a young individual, to the genusThomomys, and the same material was identified with the genusGeomys, also without specific assignment, in a later paper (Hibbard, 1941b:278).Thomomysis unknown from the late Pliocene of the Great Plains. The specimens previously referred toNerterogeomysare assigned to the genusZygogeomysfor the first time in this report; for a discussion of the systematic arrangement see the accounts beyond. The type and paratype ofNerterogeomysfrom the Benson local fauna of Arizona have no indication of enamel reduction.

Specimens of the genusGeomysfrom the late Pliocene were referred to the largeGeomys quinniMcGrew, first by Franzen (1947:55) and later by Hibbard and Riggs (1949:835) and Hibbard (1950:171).Geomys quinnihas been obtained from the Fox Canyon locality and Locality 3 of the Rexroad local fauna. At Locality 3, bothZygogeomys(cf.minor) andGeomys quinnihave been found together, butGeomys quinnican 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 ofGeomys quinni; therefore, it seems unlikely that the smaller gophers referred toZygogeomysare actually the young ofGeomys quinni. Hibbard (personal communication, May, 1966) informed me that specimens ofGeomysfrom the late Pliocene (Fox Canyon and Rexroad Locality 3) are erroneously referred toG. 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 ofGeomys quinnito other species will restrictquinnito the early Pleistocene.

Cratogeomys bensoniGidley (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 genusPappogeomyssince the lower dentitions of all the genera of the tribe Geomyini have the same enamel pattern, and the subgeneraPappogeomysandCratogeomyshave 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 subgenusPappogeomys. 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 generaGeomysandThomomysare especially common. Few specimens are known of the generaOrthogeomysandPappogeomys, 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 ofGeomysandThomomysfrom Pleistocene deposits farther north, and the marked absence of other genera, may mean thatOrthogeomysandPappogeomysdid not range northward from southern and central México in most of the Pleistocene. One species ofPappogeomyseventually 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 ofZygogeomys, 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 ofZygogeomyshave been found. Today a single species exists as a relic in the mountains of central México andZygogeomysmay have retreated southward to its present refugium in the late Pleistocene. Perhaps,Zygogeomysoccurred in northern México and the southwestern United States in the early and middle Pleistocene (seep. 2), occupying the area between the ranges ofPappogeomysto the south andGeomysto thenorth. Competition withPappogeomys, and especiallyGeomys, during Irvingtonian time may have extirpatedZygogeomysover most of this area, and by late Pleistocene (Sangamon) much of the former range ofZygogeomyscame to be occupied by one or the other of its competitors. The occurrence ofGeomys garbaniiin southern California (see White and Downs, 1961) and the unidentified species ofGeomysin Aguascalientes (Mooser, 1959; forfaunal correlation, see Hibbard and Mooser, 1963), both from deposits of Irvingtonian age, supports this suggestion.

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