Part 12

Description and discussion.--Pocket gophers of this genus are medium-sized geomyids; none is so small as the average-sized Thomomys. The skull is generalized and lacks the dolichocephalic and platycephalic specializations seen in the genera Orthogeomys and Pappogeomys, respectively. Geomys closely resembles Zygogeomys, but retains fewer of the primitive characters of the ancestral stock. At the same time, Geomys has several specializations. Even so, a considerable amount of parallelism is evident in the phyletic trends of the two genera.

The upper incisor of Geomys is bisulcate as in Pliogeomys and Zygogeomys; the deeper grove is medial and the shallower grove lies near the inner border of the tooth. The premolar, above and below, is bicolumnar; and two columns are joined at their mid-points (deep re-entrant angles separate the columns at the sides). A permanent enamel plate protects the anterior face of the anterior loph, and enamel bands outline each of the re-entrant folds. In p4 a complete enamel plate covers the posterior surface of the posterior loph. All of the enamel bands are interrupted by tracts of dentine, except in the initial stages of wear of the occlusal surface of the newly erupted tooth. For a short time in living Geomys, the enamel bands are continuous as observed in juveniles of Geomys bursarius major (KU 5628, 8531, and 41540). But, the enamel cap is thin and the dentine tracts, which are high on the sides of the tooth, are soon revealed by a minimum of wear on the crown. Therefore, the adult, or final, pattern characterized by interrupted enamel plates emerges early in life and remains throughout the life of the individual. Evidence from fossil Geomys, especially from specimens from early and late Pleistocene deposits, suggests that the final adult pattern appears later, ontogenetically, than in Recent specimens. Some of the fossil premolars in initial stages of wear have continuous and uninterrupted bands of enamel. Geomys quinni of the late Pliocene and early Pleistocene has the interrupted pattern seen in late Pleistocene and Recent Geomys. Also, in late Pliocene and early Pleistocene species, the re-entrant folds diverge laterally and form "open" angles. In later taxa (middle Pleistocene to Recent) the folds are compressed and parallel-sided, and the "open" folds are found only in the early stages of wear.

The posterior enamel plate of P4 disappears in the final stages of wear as the interrupted enamel pattern is formed. In the late Pleistocene and Recent Geomys, the loss of the posterior plate occurs early in life, usually in the first phases of wear on the occlusal surface of the newly erupted tooth, but in fossils of Geomys of corresponding ontogenetic age from the early and middle Pleistocene, the posterior plate is retained in some individuals until a later phase of wear, thereby delaying the appearance of the final pattern. Indeed, in five or fewer per cent of the individuals (see Paulson, 1961:138-139; and White and Downs, 1961:18) a vestige of enamel is retained throughout life or at least until late in adulthood. In Geomys tobinensis, for example, a thin, but transversely complete, plate of enamel occurs all the way down to the base of the loph (Paulson, loc. cit.) and would persist throughout life. In Geomys garbanii, a vestige on the lingual side of the posterior surface of a fully adult specimen was noted by White and Downs (loc. cit.). Vestiges of the posterior plate occur less frequently in living geomyids. Paulson (loc. cit.) found a posterior plate in one of 75 specimens of Geomys bursarius dutcheri. A young (suture present between exoccipitals and supraoccipital) female of Geomys pinetis austrinus (KU 23358) has a vestige of the posterior plate on the lingual side of the tooth as White and Downs (loc. cit.) observed in a specimen of Geomys garbanii. The enamel, I suspect, tends to be thicker on the lingual than on the labial side of the loph and extends farther down the lingual surface in some individuals; therefore, wear on the occlusal surface erodes it down to the dentine more rapidly on the labial than on the lingual side. The tendency of enamel to be retained is a primitive feature.

A lower molar of Geomys is a single elliptical column, and enamel is restricted to the posterior surface as in Zygogeomys, Orthogeomys, and Pappogeomys. Paulson (loc. cit.) found a thin enamel plate on the anterior surfaces of the lower molars in about five per cent of the individuals of Geomys tobinensis from the Cudahy local fauna (middle Pleistocene, deposits of the late Kansan glaciation). An anterior plate is unknown in other members of the tribe Geomyini, except in the primitive genus Pliogeomys of the middle Pliocene. Occurrence of the plate in Geomys tobinensis is an atavistic trait. Primitive dental patterns occur occasionally in geomyids, as pointed out above, but the frequency of occurrence in G. tobinensis is higher than would be expected.

M1 and M2, like the lower molars, are elliptical in cross-section. Complete enamel plates on the anterior and posterior surfaces are separated by tracts of dentine on the sides of each tooth. M3 is usually suborbicular (sometimes subtriangular) in cross-section. The tooth is not especially elongated posteriorly and usually has no definite heel; therefore, it is not significantly longer than wide. Living species of Geomys rarely have a well defined outer re-entrant fold on M3; less than 10 per cent of the individuals (and usually only one side in each individual in which it occurs) have it, although a shallow inconspicuous groove occurs more frequently. The biprismatic molar characteristic of the ancestral morphotype is less often found in Geomys than in any other living member of the tribe Geomyini. The outer re-entrant fold and biprismatic pattern are more often present in the extinct species Geomys garbanii of the Middle Pleistocene than in other species. Less than 24 per cent of the third upper molars in Geomys garbanii lack a tract of the re-entrant fold and more than 38 per cent have a well developed outer fold (see White and Downs, 1961:13, 18). The bicolumnar pattern, although incomplete, would be clearly evident in those teeth having a well marked re-entrant fold; the pattern occurs less frequently in those teeth with no fold or only a slight one. M3 of geomyids is not usually recovered and, therefore, the occlusal pattern of M3 is unknown in most extinct kinds of Geomys. In Recent Geomys the fold is more common in the eastern pinetis species-group than in the western bursarius species-group.

The masseteric ridge on the outer side of the mandible is well developed in all species of the genus. The position of the mental foramen relative to the anterior part of the ridge varies with individuals and according to species. The basitemporal fossa is always present, but is shallower in the late Pliocene and Pleistocene species than in Recent species. The angular process is short.

Referred species.--The twelve species, five of which are extinct, are as follows:

quinni species-group

*Geomys quinni McGrew, 1944. Geol. Ser., Field Mus. Nat. Hist., 9 (546):49, January 20. Type from Sand Draw local fauna (late Pliocene), Brown County, Nebraska; also known from Broadwater-Lisco local faunas (early Pleistocene), Morrill and Garden counties, Nebraska, Deer Park local fauna (early Pleistocene), Meade County, Kansas.

*Geomys paenebursarius Strain, 1966. Bull. Texas Memorial Mus., 10:36. Type from Hudspeth local fauna (early Pleistocene), Hudspeth County, Texas.

*Geomys tobinensis Hibbard, 1944. Bull. Geol. Soc. Amer., 55:736. Type from Tobin local fauna (middle Pleistocene), Russell County, Kansas; also known from Cudahy local fauna (middle Pleistocene), Meade County, Kansas.

*Geomys garbanii White and Downs, 1961. Contrib. Sci., Los Angeles Co. Mus., 42:1-34, June 30. Type from Vallecito Creek local fauna (middle Pleistocene), San Diego County, California.

*Geomys bisulcatus Marsh, 1871. Amer. Jour. Sci., 3:121. Type from Loup River fossil beds, near Camp Thomas, Nebraska (probably late Pleistocene).

bursarius species-group

*Geomys parvidens Brown, 1908. Mem. Amer. Mus. Nat. Hist., 9:194. (An extinct subspecies of Geomys bursarius according to White and Downs, 1961:6). Type from Conard Fissure local fauna (late Pleistocene), northern Arkansas.

Geomys bursarius (Shaw, 1800). Trans. Linn. Soc. London, 5:227. Type from somewhere in Upper Mississippi Valley, North America.

Geomys arenarius Merriam, 1895. N. Amer. Fauna, 8:139, January 31. Type from El Paso, El Paso County, Texas.

Geomys personatus True, 1889. Proc. U. S. Nat. Mus., 11:159, January 5. Type from Padre Island, Cameron County, Texas.

pinetis species-group

Geomys pinetis Rafinesque, 1806. Amer. Monthly Mag., 2 (1):45, November. Type locality restricted to Screven County, Georgia.

Geomys colonus Bangs, 1898. Proc. Boston Soc. Nat. Hist., 28:178, March. Type from Arnot Plantation, about 4 mi. W St. Marys, Camden County, Georgia.

Geomys cumberlandius Bangs, 1898. Proc. Boston Soc. Nat. Hist., 28:180, March. Type from Stafford Place, Cumberland Island, Camden County, Georgia.

Geomys fontanelus Sherman, 1940. Jour. Mamm., 21:341, August 13. Type from 7 mi. NW Savannah, Chatham County, Georgia.

Genus =Orthogeomys= Merriam

1895. Orthogeomys Merriam, N. Amer. Fauna 8:172, January 31.

1895. Heterogeomys Merriam, N. Amer. Fauna 8:179, January 31 (type, Geomys hispidus Le Conte, 1862).

1895. Macrogeomys Merriam, N. Amer. Fauna 8:185, January 31 (type, Geomys heterodus Peters, 1865).

Type.--Geomys scalops Thomas, 1894, from Tehuantepec, Oaxaca, México.