Transcriber's note:
Throughout, an asterisk before the name (i.e., *Entoptychinae) represents an extinct family/genus/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
31-4628
Evolution and Classification
of the Pocket Gophers of the
Subfamily Geomyinae
BY
ROBERT J. RUSSELL
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.
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.
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.
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.
In the primitive genera the enamel pattern is bilophate and the enamel loop (see [p. 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.
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.
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.
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.
The skull in most geomyines is generalized, being neither extremely long and narrow nor short, broad and flat as in specialized skulls (see [p. 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 [p. 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.
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.
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.
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.
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 [p. 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.
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
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.
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 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.
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.
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.
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
* 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.
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
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
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 [p. 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
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
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
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
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
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.
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.
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.
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
A´ Molars pear-shaped, not sub-crescent or ovate, in cross-section, crown becoming abruptly narrow at one end of tooth. Subgenus Thomomys
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.
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
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
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
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