Part 2

The fossil record of the subfamily Geomyinae begins in the early Miocene of western North America. No geomyids have been recovered from beds of the late Miocene age. Beginning with the early Pliocene the fossil record becomes progressively more complete, and geomyines are relatively abundant in deposits of late Pliocene and Pleistocene age. Although pocket gophers of the subfamily Geomyinae are rare in lower Miocene deposits, members of the subfamily Entoptychinae are relatively common and highly diversified. Four genera and a number of species have been described (see Wood, 1936:4-25), and the subfamily ranged widely in western North America. I interpret this to mean that the geomyines were indeed uncommon in the early Miocene and their distribution restricted since so few of their remains have been recovered in comparison with entoptychines and the known records are only from the northern part of the Great Plains. On the other hand, entoptychines enjoyed a widespread distribution in western North America (see discussion beyond). Probably the geographic range of the geomyines was largely allopatric to that of the more specialized entoptychines. The zone of fossoral adaptation for herbivorous rodents is ecologically narrow, and as a result competition is severe. As a rule, the outcome of episodes of intergroup competition is geographic exclusion. If these rodents were fossorial in the early Miocene--their morphology suggests they were at least semi-fossorial--mutually exclusive patterns of distribution are to be expected.

Miocene

_Dikkomys_ is the only genus of the Geomyinae known from the early and middle Miocene. _Dikkomys matthewi_ was described by Wood (1936) on the basis of isolated teeth from lower Harrison deposits (Arikareean in age) near Agate, Sioux County, Nebraska. Later, Galbreath (1948:316-317) described the features of an almost complete mandible recovered from the younger upper Rosebud deposits, now considered by MacDonald (1963:149-150) to be middle Miocene, near Wounded Knee, Shannon County, South Dakota. More recently Black (1961:13) has described a new species, _Dikkomys woodi_, from the Deep River Formation, Meagher County, Montana. The Deep River Formation is late Hemingfordian (middle Miocene) in age. No remains of _Dikkomys_ have been identified in the extensive rodent fauna of the John Day beds of the lower Miocene of Oregon, although entoptychines are abundant in these deposits.

In the present account, _Dikkomys_ is regarded as the ancestor from which the Pliocene and modern geomyines were derived. These probably did not evolve from the subfamily Entoptychinae because the dentition of entoptychines, especially the premolars and third molars, was already highly specialized by Miocene time.

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

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

Pliocene

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

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

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

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

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

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

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

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

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

Pleistocene

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

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

_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.