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.