Part 4

The numerous records of Thomomys and especially Geomys reported from supposed Miocene or Pliocene deposits are without foundation (see Matthew, 1899:66; 1909:114, 116, 119; 1910:67, 72; 1923a:369; 1924:66; Matthew and Cook, 1909:382; Cook and Cook, 1933:49; and Simpson, 1945:80). Most of the records of Geomys date back to the description of Geomys bisculcatus Marsh (1871:121) from the Loup Fork beds of Nebraska (near Camp Thomas on the Middle Loup River). At first Marsh and other investigators thought these beds were of the late Miocene age. Subsequently the Loup Fork fauna was determined by Matthew (1923b) to be mostly early Pliocene (Clarendonian), but with a later Pleistocene element. Recently, Schultz and Stout (1948:560) have shown that the various Loup River faunas and also those from along the Niobrara River (Hay Springs, Rushville, Gordon local faunas) are of middle Pleistocene age, the fossil-bearing beds occurring just below the Pearlette Ash. These beds are those termed the Loup Fork or North Prong of Middle Loup by the earlier workers who supposed them to be of Miocene or Pliocene age. Both Geomys and Thomomys have been recovered from most of these deposits, but they are no older than middle Pleistocene. This is not surprising in view of the primitive structure of the geomyids known from Miocene and Pliocene beds, but the supposed early appearance of Geomys and Thomomys led to much confusion concerning geomyid evolution in the late Tertiary.

The dearth of geomyines in the Miocene is counterbalanced by the relatively abundant and highly differentiated gophers of the subfamily Entoptychinae. They reached the zenith of their development in this period. Four genera and a number of species are known from the western part of the United States, mostly from beds along the Pacific Coast and in the northern part of the Great Plains. The great diversification of the group in a relatively short period suggests prior movement into a new adaptive zone and subsequent specialization in different subzones and therefore an episode of radial adaptation. The radiation of the entoptychines is discussed elsewhere in the account of geomyid phylogeny, but it should be noted here that both the Geomyinae and the Entoptychinae appear in the fossil record at about the same time in the early Miocene. The principal distinguishing features of each of the two lineages were well developed at the time of their first occurrence, and the entoptychines were the more successful in early Miocene. The Entoptychinae are known only from the early and middle Miocene, unless the earlier deposits of the John Day Formation of Oregon from which mammals have been recovered are considered to be latest Whitneyian (latest Oligocene); for correlations, see Wilson (1949:75). Both lineages likely had an earlier history extending back to their divergence in the Oligocene.

Pliocene

The oldest and most primitive Pliocene geomyine is Pliosaccomys dubius Wilson (1936:20) from the Smith Valley local fauna of middle Pliocene (Hemphillian) age in Nevada. According to Wilson (op. cit.:15) the beds probably were deposited near the middle of Hemphillian time. Shotwell (1956:730) recorded Pliosaccomys dubius from the McKay Reservoir and from the Otis Basin (1963:73) local faunas of the middle Pliocene (Hemphillian) of Oregon, and Green (1956:155) has recovered remains of Pliosaccomys (cf. dubius) from the Wolf Creek local fauna, uppermost part of the lower Pliocene (late Clarendonian in age), of Shannon County, South Dakota. Recently, James (1963:101) has described a second species, Pliosaccomys wilsoni, of this primitive genus. The new species was found in early Pliocene deposits (late Clarendonian) from the Nettle Spring local fauna (Apache Canyon), in the Cuyama Valley, Ventura County, California. Pliosaccomys wilsoni does not differ greatly from P. dubius; however, the few differences in dental characters seem to warrant specific recognition. The reduction of cusps on the metalophid of p4 from three (dubius) to two (wilsoni) and the lack of accessory cuspules on the protolophid of p4 in wilsoni are probably specializations, suggesting that P. dubius even though the more recent in age is the less advanced of the two. P. wilsoni is known only from a lower jaw of a young individual that had dp4 in place, along with m1 and m2. The permanent premolar was in the process of erupting, and the deciduous tooth was removed so that the unworn surface of p4 could be examined.

Pliosaccomys occurred geographically in the area that the Entoptychinae had occupied in the early Miocene. The Smith Valley material includes dentitions in almost all stages of wear and the chronological sequences in the development of the patterns of wear can be reconstructed. An understanding of the dental patterns of the primitive geomyines is based mostly on the interpretation of the stages of wear in Pliosaccomys.

No other pocket gopher is known from the area in which Pliosaccomys occurred, and it is unknown after middle Hemphillian age. Pliosaccomys has closer affinities with Dikkomys of the early Miocene than with any geomyid of the modern assemblage and gives no clue to the origin of the lineage culminating in the modern pocket gophers of the tribe Geomyini.

Pliogeomys buisi Hibbard (1954:353) was found in the Buis Ranch local fauna, of latest middle Pliocene, on the west side of Buckshot Arroyo, Beaver County, Oklahoma. The original material included a right ramus bearing the premolar and first two molars (the holotype) and five isolated premolars and molars. One of the molars is slightly worn and from an immature individual. One premolar is a deciduous tooth. Hibbard (op. cit.:342) identified the beds from which he obtained the Buis Ranch local fauna as from the lowermost part of the Upper Pliocene. Moreover, he judged the Buis Ranch local fauna to be only slightly older than the Saw Rock Canyon local fauna of Seward County in southwestern Kansas. Previously (Hibbard, 1953:408-410), the Saw Rock Canyon local fauna had been assessed as older than the Rexroad local faunas (latest late Pliocene) and, therefore, representative of the early part of the late Pliocene. More recently, Hibbard (1956:164) identified the Buis Ranch beds as part of the Ogallala Formation, which here occurs unconformably just beneath the Rexroad Formation (composed of strata nearly all of late Pliocene age). Therefore, he regarded the Buis Ranch beds as latest middle Pliocene in age. Hibbard (1954:356) suggested that pocket gopher remains from the Saw Rock Canyon local fauna were referable to Pliogeomys buisi, and, in effect, tentatively assigned them to Pliogeomys (in his description of the genus Hibbard remarked that the upper incisor is bisulcate as in Geomys, and the only upper incisor that he mentions was one of the Saw Rock Canyon fossils and not part of the Buis Ranch material). Pliogeomys has closer affinities with modern pocket gophers of the tribe Geomyini than it does with the middle Pliocene genus Pliosaccomys.

The pocket gopher fauna known from the late Pliocene was more varied than the faunas known from any earlier time. In addition to the extinct Pliogeomys, which occurs in early late Pliocene (see discussion above), the living genera Zygogeomys, Geomys, Pappogeomys (in the sense used on p. 534), and Thomomys first appear in the late Pliocene. The only other living genus, Orthogeomys, makes its first appearance in the late Pleistocene.

The earliest record of the genus Thomomys is based on a fragment of a left mandibular ramus bearing p4 and m1, Thomomys gidleyi Wilson (1933b:122), from the Hagerman local fauna of Twin Falls County, Idaho. Wilson (loc. cit.) was uncertain as to age (late Pliocene or early Pleistocene) but subsequently (1937:38 and 67-70) settled on the middle part of the late Pliocene. Hibbard (1958:11) later considered the age as early Pleistocene (suggesting that the deposits accumulated in the Aftonian interglacial interval) but subsequently (Hibbard et al., 1965:512), on the basis of potassium argon age determinations, also settled on late Pliocene.

Remains of Nerterogeomys [=Zygogeomys] have been found in the Benson local fauna, Cochise County, Arizona, and the Rexroad local fauna of Kansas. This early Blancan gopher first was described as Geomys minor by Gidley (1922:123), and was later referred by Gazin (1942:487) to his new genus Nerterogeomys. Hibbard (1950:138) identified specimens from the Fox Canyon locality, one of the localities of Meade County, Kansas, where the Rexroad local fauna is preserved, as Nerterogeomys, and tentatively referred them to the species N. minor. Nerterogeomys cf. minor has been recovered also from Locality 3 of the Rexroad local fauna (Hibbard, 1950:171) of Meade County, Kansas. Apparently these are also the small gophers about which Franzen (1947:58) wrote. She assigned them to the genus Geomys, and they may actually be a primitive form of Geomys that represents an intermediate stage in the development of the enamel pattern from the uninterrupted loops of the ancestor to the discontinuous pattern of modern Geomys. I favor this interpretation; the evidence, however, is inconclusive, and I have, therefore, reluctantly allocated them, along with the other specimens of Nerterogeomys, to the genus Zygogeomys. In an early paper, Hibbard (1938:244) erroneously referred the same specimens, two upper premolars of a young individual, to the genus Thomomys, and the same material was identified with the genus Geomys, also without specific assignment, in a later paper (Hibbard, 1941b:278). Thomomys is unknown from the late Pliocene of the Great Plains. The specimens previously referred to Nerterogeomys are assigned to the genus Zygogeomys for the first time in this report; for a discussion of the systematic arrangement see the accounts beyond. The type and paratype of Nerterogeomys from the Benson local fauna of Arizona have no indication of enamel reduction.

Specimens of the genus Geomys from the late Pliocene were referred to the large Geomys quinni McGrew, first by Franzen (1947:55) and later by Hibbard and Riggs (1949:835) and Hibbard (1950:171). Geomys quinni has been obtained from the Fox Canyon locality and Locality 3 of the Rexroad local fauna. At Locality 3, both Zygogeomys (cf. minor) and Geomys quinni have been found together, but Geomys quinni can be distinguished by its much larger size and the advanced enamel pattern of the cheek teeth (see systematic accounts beyond). All age classes are represented among the specimens of Geomys quinni; therefore, it seems unlikely that the smaller gophers referred to Zygogeomys are actually the young of Geomys quinni. Hibbard (personal communication, May, 1966) informed me that specimens of Geomys from the late Pliocene (Fox Canyon and Rexroad Locality 3) are erroneously referred to G. quinni. According to Hibbard, this material represents instead two distinct undescribed species, descriptions of which have been submitted by him for publication. Allocation of late Pliocene specimens of Geomys quinni to other species will restrict quinni to the early Pleistocene.

Cratogeomys bensoni Gidley (1922:123) was of medium size. The name was based on an upper incisor bearing a single median sulcus and an associated lower jaw containing all of the cheek teeth from the Benson local fauna, Cochise County, Arizona. Additional lower jaws carrying various teeth also were recovered. The specimens might just as well have been assigned to the genus Pappogeomys since the lower dentitions of all the genera of the tribe Geomyini have the same enamel pattern, and the subgenera Pappogeomys and Cratogeomys have upper incisors with median grooves. The specimens are too fragmentary to warrant more than generic identification. Mainly because of their late Pliocene age and primitive traits the specimens are here regarded as early representatives of the subgenus Pappogeomys. Discovery of the upper molariform dentition would make a more precise assignment possible.

Pleistocene

Numerous specimens of geomyids from many localities and horizons are available from the Pleistocene of North America. Specimens of the genera Geomys and Thomomys are especially common. Few specimens are known of the genera Orthogeomys and Pappogeomys, especially from the early and middle Pleistocene, owing, probably, to slight knowledge of the early Pleistocene of México where these two genera are thought to have evolved (see map, Figure 2). This lack of knowledge about early Pleistocene deposits in México is a handicap in the present instance since the center of differentiation for several of the modern genera is judged to have been in México, probably on, and at the edge of, the Central Plateau. The relative abundance of the remains of Geomys and Thomomys from Pleistocene deposits farther north, and the marked absence of other genera, may mean that Orthogeomys and Pappogeomys did not range northward from southern and central México in most of the Pleistocene. One species of Pappogeomys eventually ranged into the southwestern United States in the late Pleistocene (toward the end of the Wisconsin) and it occurs there today, but the genus is essentially Mexican.

The fossil record of Zygogeomys, as the genus is here understood, evidently continued in the United States will into the Middle Pleistocene, depending upon the stratigraphic interpretation of the age of the Curtis Ranch local fauna from southeastern Arizona. Hibbard (1958:25) regarded the Curtis Ranch local fauna as Irvingtonian in age, a local fauna that lived either in the late Kansan glacial or the Yarmouthian interglacial, and his correlation is tentatively followed here. In deposits laid down later than those of Irvingtonian age no remains of Zygogeomys have been found. Today a single species exists as a relic in the mountains of central México and Zygogeomys may have retreated southward to its present refugium in the late Pleistocene. Perhaps, Zygogeomys occurred in northern México and the southwestern United States in the early and middle Pleistocene (see Fig. 2), occupying the area between the ranges of Pappogeomys to the south and Geomys to the north. Competition with Pappogeomys, and especially Geomys, during Irvingtonian time may have extirpated Zygogeomys over most of this area, and by late Pleistocene (Sangamon) much of the former range of Zygogeomys came to be occupied by one or the other of its competitors. The occurrence of Geomys garbanii in southern California (see White and Downs, 1961) and the unidentified species of Geomys in Aguascalientes (Mooser, 1959; for faunal correlation, see Hibbard and Mooser, 1963), both from deposits of Irvingtonian age, supports this suggestion.

1. Thomomys 2. Geomys 3. Zygogeomys 4. Pappogeomys 5. Orthogeomys ]

Thomomys

The earliest Pleistocene records of Thomomys are mostly isolated teeth. Although they can be identified as genus Thomomys, most of the materials are too fragmentary to be identified to species. In Thomomys two distinct patterns of occlusal surfaces of the molars can be recognized: the generalized elliptical pattern in the subgenus Pleisothomomys, not unlike the pattern in other geomyids, and the pear-shaped pattern in the subgenus Thomomys, which results from constriction of the upper molars on the labial side and constriction of the lower molars on the lingual side. Some fossils assigned to Thomomys were not examined with this distinction in mind by the persons who made the assignments. Consequently some of the identifications now in the literature may be subject to change.

Three occurrences of Thomomys are from the early and middle Pleistocene, with a possible fourth (depending upon the age of the Hay Springs local fauna of Nebraska). The earliest Pleistocene record is from the Broadwater-Lisco beds along the North Platte River in Morrill County, western Nebraska. Possibly the specimen from there was misidentified. Those beds are Lower Pleistocene, and are regarded by Schultz and Stout (1948:560-561, 573) and by Hibbard (1958:11), as having been deposited mostly during the Aftonian interglacial. There is also some indication that some of the strata were deposited late in the Nebraskan glaciation. There are no other early Pleistocene records of Thomomys. Savage (1951:228) reported the genus from the Irvington local fauna, Alameda County, California. The specimens were not identified to species, although they were described as indistinguishable from Thomomys bottae. Paulson (1961:137) recorded specimens from the Cudahy local fauna, Meade County, Kansas. These fragmentary specimens are referable to the subgenus Thomomys, owing to the strong constriction of the molars, but have not been identified to species. The Cudahy is an Irvingtonian local fauna, and is considered to have been deposited during the late Kansan glaciation. The stratum containing the Cudahy local fauna immediately underlies the Pearlette Ash. The Cudahy material includes five isolated molars and a fragmentary ramus bearing only the premolar. The genus Thomomys has been recovered also from the Hay Springs local fauna in Sheridan County, northwestern Nebraska, by Shultz and Tanner (1957:71). The Hay Springs local fauna is considered to have been deposited in late Kansan glaciation or in early Yarmouth interglacial by Shultz and Tanner (op. cit.:69), or of Irvingtonian age; however, Hibbard (1958:25) regarded the beds containing this fauna as Illinoian (thus post-Irvingtonian in age), and equivalent in age to the Berends local fauna of Oklahoma and the Butler Springs and Mt. Scott local faunas of Kansas. The Thomomys from Hay Springs local fauna has not been referred to species.

The relative abundance of Geomys, and rarity of Thomomys, in Great Plains fossil beds of early and middle Pleistocene is probably due to allopatric distributions of the two genera. The Great Plains area was evidently the center of distribution and differentiation of Geomys. Perhaps Thomomys evolved earlier to the west, in the Great Basin and Pacific Coastal regions, and not on the Great Plains.

Upper Pleistocene records of Thomomys are more common. The genus was widespread in beds identified with the Illinoian and Sangamon and extended its range eastward to the Atlantic Coast. Stephens (1960:1961) reported Thomomys from the Doby Springs local fauna, Harper County, northwestern Oklahoma. The material (34 isolated teeth) was too fragmentary to permit assignment to species. The molars are constricted on one side, indicative of the subgenus Thomomys, like the Cudahy specimens reported by Paulson (see discussion above). Stephens erroneously mentioned that the enamel plate on the posterior face of the upper premolar is unique in Thomomys; this plate occurs also in Zygogeomys. The Doby Springs local fauna was recovered from beds that have been identified as Illinoian deposits, and it is correlated with the Berends local fauna in Beaver County, Oklahoma, and the Butler Springs local fauna in Meade County, Kansas (see Stephens, op. cit.: 1700).

Local faunas in Maryland and Florida of Rancholabrean age include Thomomys, in every instance referable to the subgenus Pleisothomomys on the basis of unconstricted molars. Thomomys potomacensis (Gidley and Gazin, 1933), from Cumberland Cave local fauna, Allegany County in western Maryland, is the type of the genus Pleisothomomys Gidley and Gazin (1933:354). Pleisothomomys is here regarded as a subgenus. The material used in the original description included four lower jaws, one with a complete dentition. Hibbard (1958:25) pointed out that the Cumberland Cave assemblage is a composite fauna including both glacial and interglacial forms. He placed the stratigraphic position of the fauna as definitely Upper Pleistocene, probably deposited in both Illinoian glaciation and during the Sangamon interglacial. T. potomacensis is significantly larger than T. orientalis Simpson (1928:6), from the Saber-tooth Cave local fauna, Citrus County, Florida. Simpson's material included a rostral fragment with an incisor, premolar, and first molar. The Saber-tooth Cave local fauna is regarded by Kurten (1965:219) as having been recovered from Sangamon deposits. Thomomys is unknown from Wisconsin deposits in the eastern United States, and today the genus does not occur east of the Great Plains.

Thomomys of Rancholabrean provincial age from the western United States and México is known only from Wisconsin beds.

Three extinct species of Thomomys, all referable to the subgenus Thomomys, have been described. Thomomys microdon Sinclair (1905:146), based on the rostral portion of a skull without a mandible, is from the Potter Creek Cave local fauna, Shasta County, California, and has been recovered also from Samwel Cave, Shasta County, California. T. microdon closely resembles Thomomys monticola that lives in the area today. Thomomys scudderi Hay (1921:614) is from the Fossil Lake (or Christmas Lake) local fauna in central Oregon. Elftman (1931:10-11) referred these specimens to Thomomys townsendii, and he considered T. scudderi to be a synonym of T. townsendii. Davis (1937:156-158) disagreed with Elftman concerning the taxonomic status of T. scudderi, which he regarded as a valid species. According to Davis, T. scudderi is more closely allied to Thomomys bottae than to T. townsendii. Cope (1878:389; 1889:160-165) had referred the same specimens to Thomomys clusius (now Thomomys talpoides clusius). Cope considered the beds to be Pliocene in age. In all accounts of the Fossil Lake local fauna up to Hay (1921), the specimens of Thomomys were referred to the species clusius, talpoides, or bulbivorus (see Elftman, loc. cit.). The Fossil Lake local fauna is currently considered as being of Rancholabrean provincial age, probably dating from the Wisconsin glacial maximum when the lake reached its greatest size. The third extinct species described from the Wisconsin is Thomomys vetus Davis (1937:156), also from the Fossil Lake local fauna in Lake County, Oregon. Davis pointed out that T. vetus differs from T. scudderi Hay, of the same fauna, in larger size and other cranial details, and that it is closely allied to the living species Thomomys townsendii, and not to Thomomys talpoides, which is the only species of Thomomys living in the area today.

Thomomys townsendii was recovered by Gazin (1935:299) from the American Falls beds (probably Wisconsin deposits) in Idaho.

Thomomys talpoides is reported from the Howard Ranch local fauna in Hardeman County, western Texas, by Dalquest (1965:69-70), who referred the isolated teeth to T. talpoides on geographic grounds, apparently on the erroneous assumption that T. talpoides was the species of Thomomys nearest geographically to Hardeman County. Hay (1927:259) reported Thomomys fuscus [= Thomomys talpoides] from late Pleistocene beds near Wenatchee, Chelan County, Washington. Hibbard (1951:229) recorded Thomomys talpoides from late Pleistocene deposits in Greeley County, Kansas, and Walters (1957:540) reported the same species from late Pleistocene deposits in Clark County, Kansas. According to Hibbard (1958:14) other remains reported as T. talpoides have been recovered from numerous areas of Wisconsin glacial drift in western North America.

Thomomys bottae has been identified from Wisconsin age deposits in western North America, as follows: Burnet Cave, Gaudalupe Mt., New Mexico (Schultz and Howard, 1935:280); Carpinteria Asphalt, California (Wilson, 1933a:70); McKittrick Asphalt, Kern County, California (J. R. Schultz, 1938:206); Rancho La Brea, Los Angeles County, California (Dice, 1925:125--specimens described as a new subspecies, T. b. occipitalis); Papago Springs Cave, Santa Cruz County, Arizona (Skinner, 1942:150 and 158--probably bottae, but possibly umbrinus on the assumption that the two are specifically instead of subspecifically distinct); Isleta Cave, Bernalillo County, New Mexico (Harris and Findley, 1964:115--some of these fossils may be post-Wisconsin in age); Potter Creek Cave and Samwel Cave, Shasta County, California (Sinclair, 1905:146--identified as T. leucodon, now a subspecies of T. bottae; also see Hay, 1927:214-215).

Thomomys umbrinus has been reported from San Josecito Cave, Nuevo León, México (Russell, 1960:542); Upper Bercerra, México (Hibbard, 1955a:51--identified only as Thomomys sp., but undoubtedly referable to T. umbrinus). Post-Wisconsin remains of Thomomys umbrinus are reported by Alvarez (1964:6) from capa II and capa III of the Cueva La Nopalera, southwestern Hidalgo. Hay (1927:222-223) reported specimens of the genus Thomomys from Wisconsin deposits in Hawver Cave, Eldorado County, California, but did not assign them to species. Gilmore (1947:158) found the remains of Thomomys umbrinus in cave deposits near Quatro Ciénegas in central Coahuila. These cave deposits may have been laid down during the Wisconsin, but more likely accumulated in the post-Wisconsin.