1817. Geomys Rafinesque, Amer. Monthly Mag., 2(1):45, November.
1817. Diplostoma Rafinesque, Amer. Monthly Mag., 2(1):44-45, November. Included species: Diplostoma fusca Rafinesque [= Mus bursarius Shaw] and Diplostoma alba Rafinesque [= Mus bursarius Shaw] from the Missouri River region.
1820. Saccophorus Kuhl, Beitr. Zool. und Vergl. Anat., pp. 65, 66. Type: Mus bursarius Shaw, from upper Mississippi Valley.
1823. Pseudostoma Say, Long's Expd. Rocky Mts., I, pp. 406. Type: Pseudostoma bursaria [= Mus bursarius Shaw], from upper Mississippi Valley.
1825. Ascomys Lichtenstein, Abh. K. Akad. Wiss. Berlin (1822), p. 20., fig. 2. Type: Ascomys canadensis Lichtenstein [= Mus bursarius Say], probably from upper Mississippi Valley.
1944. Parageomys Hibbard, Bull. Geol. Soc. Amer., 55:735, June. Type: Parageomys tobinensis Hibbard, from Pleistocene, Cudahy (Tobin) local fauna, Russell Co., Kansas.
Type.—Geomys pinetis Rafinesque, 1817, restricted to Screven County, Georgia, in region of the pines.
Chronologic range.—Late Pliocene faunas of Blancan age (Rexroad, Kansas, and Sand Draw, Nebraska, local faunas) to Recent. Reported from numerous Pleistocene deposits of all stratigraphic levels, especially from the Great Plains, where common today.
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.
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.
Chronologic range.—Late Pleistocene Wisconsin deposits (San Josecito Cave local fauna, Nuevo León, México) to Recent.
Description and discussion.—Species of this genus are of medium to large size. The skull is strongly dolichocephalic in most species; the posterior part of the skull is especially narrow. The angular processes are remarkably short, especially in relation to the length of the mandible. The nasals and rostrum are relatively broad and heavy. The pelage is coarse, and often hispid. In some species the hairs are so sparsely distributed that the body appears almost naked, and none has so dense a covering of hair as do other genera. The genus occurs entirely within the tropical life-zones, and most of the external features seem to be associated with adaptation to tropical conditions.
The upper incisor is unisulcate; the sulcus is usually near the inner border of the tooth, but in some species (subgenus Orthogeomys) it is more medial, and in a few individuals with an extremely wide groove the outer lip of the sulcus may actually reach the middle of the tooth. The groove is compressed or open. The premolar is a double column united at the mid-point. The two prisms are of approximately equal size, and the lateral re-entrant folds are so compressed that their sides are parallel. Enamel plates cover the anterior surface and border the re-entrant angles in both upper and lower premolars. As in other members of the tribe, the lower premolar has a fourth enamel plate on the posterior surface of the posterior lophid. In the upper premolar, the enamel plate is reduced to a narrow blade on the lingual side of the loph as in the living species of the genus Zygogeomys. In the subgenus Orthogeomys the posterior plate is usually absent, and otherwise is narrow and near the lingual border of the tooth.
Each lower molar, in the final stage of wear, consists of a single elliptical column having an enamel plate only on the posterior surface. The first and second upper molars are single elliptical columns having one enamel plate on the anterior surface and another on the posterior surface. The plates are separated by a tract of dentine on each side of the tooth. The third upper molar is partly bilophodont, and the two lophs are separated by a deep outer re-entrant fold. In many of the species an inner re-entrant fold also is retained, but in the adult tooth it is less distinct than the outer. In all of the species the posterior loph is long and forms a conspicuous heel; consequently the crown is significantly longer then wide. Moreover, the posterior loph has an enamel plate on each side. The labial plate always borders the outer re-entrant fold, and in the subgenus Orthogeomys is infrequently separated into two small plates.
The mandible is relatively long. Its masseteric ridge is well developed and massive. The basitemporal fossa is usually deep and well defined; it tends to be shallow in the subgenus Orthogeomys, and in young individuals is hardly more than a slight depression.
Key to the Subgenera of Orthogeomys
1895. Orthogeomys Merriam, N. Amer. Fauna, 8:172, January 31.
Type.—Geomys scalops Thomas, 1894, from Tehuantepec, Oaxaca, México.
Chronologic range.—Known only from the Recent.
Description.—Skull elongated and narrow (many skulls of nearly uniform breadth throughout), being extreme in dolichocephalic specializations; mandibles long and narrow, rami not spreading laterally, being more nearly parallel-sided than in other subgenera; angular processes short; breadth across zygomata not significantly exceeding breadth across mastoid processes (in many skulls considerably less); interorbital area remarkably broad, lacking deep constriction; frontals between orbits greatly inflated laterally, postorbital prominence inconspicuous; mesopterygoid fossa extending to level of posterior margin of M3; I having sulcus broader than in other subgenera, mostly on inner half of anterior surface but sometimes overlapping mid-line; enamel plate lacking from posterior wall of P4, rarely retaining narrow vestige near lingual border of posterior loph; M3 having distinct heel, bicolumnar pattern with inner re-entrant fold usually minute, occlusal length less than in other subgenera, length less than combined lengths of M1-2; hair generally coarse, sometimes hispid, sparse, in lowland forms, so sparse as to impart appearance of nakedness.
Referred species and subspecies.—Fourteen taxa:
Orthogeomys grandis alleni Nelson and Goldman, 1930. Jour. Mamm., 11:156, May 9. Type from near Acapulco, 2000 ft., Guerrero.
Orthogeomys grandis annexus Nelson and Goldman, 1933. Proc. Biol. Soc. Washington, 46:195, October 26. Type from Tuxtla Gutierrez, 2600 ft., Chiapas.
Orthogeomys grandis carbo Goodwin, 1956. Amer. Mus. Novit., 1757:5, March 8. Type from Excurano, 2500 ft., Cerro de San Pedro, 20 km. W Mixtequilla, Oaxaca.
Orthogeomys grandis felipensis Nelson and Goldman, 1930. Jour. Mamm., 11:157, May 9. Type from Cerro San Felipe, 10 mi. N Oaxaca, Oaxaca.
Orthogeomys grandis huixtlae Villa, 1944. Anal. Inst. Biol. Univ. Nac. México, 15:319. Type from Finca Lubeca, 12 km. NE Huixtla, 850 m., Chiapas.
Orthogeomys grandis grandis (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6, 12:270, October. Type from Dueñas, Guatemala.
Orthogeomys grandis latifrons Merriam, 1895. N. Amer. Fauna, 8:178, January 31. Type from Guatemala, exact locality unknown.
Orthogeomys grandis nelsoni Merriam, 1895. N. Amer. Fauna, 8:176, January 31. Type from Mt. Zempoaltepec, 8000 ft., Oaxaca.
Orthogeomys grandis pluto Lawrence, 1933. Proc. New England Zool. Club, 13:66, May 8. Type from Cerro Cantoral, north of Tegucigalpa, Honduras.
Orthogeomys grandis scalops (Thomas, 1894). Ann. Mag. Nat. Hist., ser. 6, 13:437, May. Type from Tehuantepec, Oaxaca.
Orthogeomys grandis soconuscensis Villa, 1949. Anal. Inst. Biol. Univ. Nac. México, 19:267, April 8. Type from Finca Experanza, 710 m., 45 km. (by road) NW Huixtla, Chiapas.
Orthogeomys grandis guerrerensis Nelson and Goldman, 1930. Jour. Mamm., 11:158, May 9. Type from El Limón, in valley of Río de las Balsas approximately 20 mi. NW La Unión, Guerrero.
Orthogeomys cuniculus Elliot, 1905. Proc. Biol. Soc. Washington, 18:234, December 9. Type from Zanatepec, Oaxaca.
Orthogeomys pygacanthus Dickey, 1928. Proc. Biol. Soc. Washington, 41:9, February 1. Type from Cacaguatique, 3500 ft., Dept. San Miguel, El Salvador.
1895. Heterogeomys Merriam, N. Amer. Fauna, 8:179, January 21.
Type.—Geomys hispidus Le Conte, 1852, from near Jalapa, Veracruz.
Chronologic range.—Late Pleistocene, Wisconsin deposits (San Josecito Cave local fauna, Nuevo León) to the Recent.
Description.—Skull dolichocephalic (less so than in the other subgenera); zygomata more widely spreading than in Orthogeomys; ramus and angular process short; interorbital area noticeably constricted; frontals between orbits neither exceptionally broad or inflated; mesopterygoid fossa extending to level of posterior margin of M3; I having sulcus on inner third of anterior surface usually narrower than in subgenus Orthogeomys; enamel plate on posterior wall of P4 restricted to lingual half of loph; M3 distinctly biprismatic, posterior loph usually circumscribed by shallow inner re-entrant fold and outer deep fold well developed in all members of genus; posterior loph forming conspicuous heel longer than in subgenus Orthogeomys; occlusal length equal to or slightly less than combined lengths of M1-2; hair coarse and hispid but never so sparse as to impart appearance of nakedness.
Referred species and subspecies.—Eleven taxa:
*Orthogeomys onerosus (Russell, 1960). Univ. Kansas Publ., Mus. Nat. Hist., 9 (21):544, January 14. Type from San Josecito Cave local fauna, Upper Pleistocene, Nuevo León.
Orthogeomys hispidus cayoensis (Burt, 1937). Occ. Papers Mus. Zool., Univ. Michigan, 365:1, December 16. Type from Mountain Pine Ridge, 12 mi. S El Cayo, British Honduras.
Orthogeomys hispidus chiapensis (Nelson and Goldman, 1929). Proc. Bio. Soc. Washington, 42:151, March 30. Type from Tenejapa, 16 mi. NE San Cristobal, Chiapas.
Orthogeomys hispidus concavas (Nelson and Goldman, 1929). Proc. Biol. Soc. Washington, 42:148, March 30. Type from Pinal de Amoles, Querétaro.
Orthogeomys hispidus hispidus (Le Conte, 1852). Proc. Acad. Nat. Sci. Philadelphia, 6:158. Type from near Jalapa, Veracruz.
Orthogeomys hispidus latirostris (Hall and Alvarez, 1961). Anal. Escuela Nac. Ciencias Biol., 10:121, December 20. Type from Hacienda Tamiahua, Cabo Rojo, Veracruz.
Orthogeomys hispidus negatus (Goodwin, 1953). Amer. Mus. Novit., 1620:1, May 4. Type from Gomez Ferias, 1300 ft., about 45 mi. S Ciudad Victoria, 10 km. W Pan American Highway, Tamaulipas.
Orthogeomys hispidus tehuantepecus (Goldman, 1939). Jour. Washington Acad. Sci., 29:174, April 15. Type from mountains 12 mi. NW Santo Domingo and about 60 mi. N Tehuantepec, 1600 ft., Oaxaca.
Orthogeomys hispidus torridas (Merriam, 1895). N. Amer. Fauna, 8:183, January 31. Type from Chichicaxtle, Veracruz.
Orthogeomys hispidus yucatanensis (Nelson and Goldman, 1929). Proc. Biol. Soc. Washington, 42:150, March 30. Type from Campeche, Campeche.
Orthogeomys lanius (Elliot, 1905). Proc. Biol. Soc. Washington, 18:235, December 9. Type from Xuchil, Veracruz.
1895. Macrogeomys Merriam, N. Amer. Fauna, 8:185, January 31.
Type.—Geomys heterodus Peters, 1865, from Costa Rica, exact locality unknown.
Chronologic range.—Known only from the Recent.
Description.—Skull dolichocephalic in varying degree (overlapping subgenera Orthogeomys and Heterogeomys in this respect); mandibles elongated, not spreading far laterally; angular processes decidedly short; breadth across zygomata in no instance significantly exceeding mastoid breadth; interorbital area strongly constricted; frontals between orbits slightly inflated laterally (especially in forms having more strongly dolichocephalic skulls); postorbital prominence conspicuous; anterior margin of mesopterygoid fossa terminating well behind M3; I having narrow and deep sulcus entirely on inner third of anterior surface; enamel plate on posterior wall of P4 restricted to inner half of loph; M3 bilophodont (outer and inner re-entrant folds each circumscribing a loph), posterior loph remarkably elongated and forming pronounced heel, length of crown more than combined lengths of M1-2; hair wooly in some individuals, harsh in others but seldom hispid, never so sparse as in subgenus Orthogeomys; some species having white markings, especially on lumbar region and head.
Referred species and subspecies.—Eleven taxa:
Orthogeomys heterodus cartagoensis (Goodwin, 1943). Amer. Mus. Novit., 1227:2, April 22. Type from Paso Ancho, Province Cartago, Costa Rica.
Orthogeomys heterodus dolichocephalus (Merriam, 1895). N. Amer. Fauna, 8:189, January 31. Type from San José, Costa Rica.
Orthogeomys heterodus heterodus (Peters, 1865). Monatsb. preuss. Acad. Wiss., Berlin, 1865:177. Type from Costa Rica, exact locality unknown.
Orthogeomys cavator nigrescens (Goodwin, 1943). Amer. Mus. Novit., 1227:3, April 22. Type from El Muneco (Río Navarro), 10 mi. S Cartago, 4000 ft., Province Cartago, Costa Rica.
Orthogeomys cavator pansa (Bangs, 1902). Bull. Mus. Comp. Zool., 39:44, April. Type from Bogava (= Bugaba), 600 ft., Chiriquí, Panamá.
Orthogeomys dariensis (Goldman, 1912). Smithsonian Misc. Coll., 60(2):8, September 20. Type from Cana, 2000 ft., mountains of eastern Panamá.
Orthogeomys underwoodi (Osgood, 1931). Field Mus. Nat. Hist., Publ. 295, Zool. Ser., 185:143, Aug. 3. Type from Alto de Jabillo Pirris, between San Geronimo and Pozo Azul, western Costa Rica.
Orthogeomys cherriei carlosensis (Goodwin, 1943). Amer. Mus. Novit., 1227:3, April 22. Type from Cataratos, San Carlos, Alajuela, Costa Rica.
Orthogeomys cherriei cherriei (J. A. Allen, 1893). Bull. Amer. Mus. Nat. Hist., 5:337, December 16. Type from Santa Clara, Costa Rica.
Orthogeomys cherriei costaricensis (Merriam, 1895). N. Amer. Fauna, 8:192, January 31. Type from Pacuare, Costa Rica.
Orthogeomys matagalpae (J. A. Allen, 1910). Bull. Amer. Mus. Nat. Hist., 28:97, April 30. Type from Peña Blanca, Matagalpa, Nicaragua.
1895. Pappogeomys Merriam, N. Amer. Fauna, 8:145, January 31.
1895. Cratogeomys Merriam, N. Amer. Fauna, 8:150, January 31. Type: Geomys merriami Thomas.
1895. Platygeomys Merriam, N. Amer. Fauna, 8:162, January 31. Type: Geomys gymnurus Merriam; Hooper, Jour. Mamm., 27:397, November 25, 1946.
Type.—Geomys bulleri Thomas, 1892, from near Talpa, west slope Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.
Chronologic range.—Late Pliocene, from deposits of early Blancan age (Benson local fauna, Arizona) to the Recent. However in the Pleistocene, only late Pleistocene records are known, and Pappogeomys has not been found in early (late Blancan) or middle (Irvingtonian) Pleistocene local faunas. Presumably the genus was restricted to México during the Pleistocene until post-Wisconsin time.
Description and discussion.—The size ranges from as little as in the smaller kinds of Thomomys to the maximum attained in the subfamily and matched elsewhere perhaps in only a few of the larger subspecies of Orthogeomys grandis. Depending on the species and subgenus, the form of the skull varies from generalized to specialized. The generalized skulls are short and not especially narrow; the zygomatic arches are spread laterally so far that the breadth across them exceeds the breadth across the mastoid processes. The most specialized skulls are platycephalic and the breadth across the mastoid processes equals or exceeds the breadth across the zygomatic arches (even so, the zygomatic arches are still relatively widespread). In correlation with the great breadth of the posterior part of the cranium, the rami of the mandibles diverge widely posteriolaterally and the angular processes are remarkably elongated. The rostrum is moderately broad in most species, but not nearly so broad and heavy as in Orthogeomys.
The single deep, median sulcus on the outer surface of the upper incisor is slightly displaced to the inner side of the tooth. The posterior surface of P4 lacks enamel (small vestige found on lingual end of posterior wall in only two adult individuals—UA 3260 and KU 100442, of the subgenus Pappogeomys); the other three plates are fully developed as usual. The p4 is provided with four fully developed enamel plates, in the pattern characteristic of the tribe Geomyini. In the p4 of the late Pliocene species (P. bensoni) the re-entrant angles are open (obtuse), a trait that is evidently primitive in the Geomyini.
All three lower molars are single, compressed, elliptical columns with enamel on only the posterior surfaces. M1 and M2 are also elliptical in cross-section and decidedly anteroposteriorly compressed, like the lower molars. Nevertheless, the enamel pattern is variable; enamel plates may be retained completely across both the anterior and posterior walls of M1 and M2 or only the anterior plate may be retained without reduction and the posterior plate may be reduced so that only a vestige is retained on the lingual fourth of the tooth or the posterior plate may be completely lost.
M3 tends to remain at least incompletely bilophodont by reason of retaining a permanent labial re-entrant fold in most species (with exceptions in Pappogeomys bulleri and some old adults of P. castanops). Primitively the occlusal surface of M3 is subtriangular (subgenus Pappogeomys), but in the castanops species-group of the advanced subgenus Cratogeomys, the posterior loph usually is reduced and the occlusal surface is quadriform or obcordate. Curiously, the trend towards reduction of the posterior loph is reversed in one subspecies (P. merriami fulvescens) and, the loph has elongated into a pronounced heel in some specimens, resembling the condition in Orthogeomys. The entire range of variation occurs in P. m. fulvescens. The subtriangular pattern is retained in the most specialized species of Cratogeomys where that pattern is associated with extreme platycephaly in the gymnurus species-group. In most species the posterior loph supports two lateral plates, the outer one always bordering the labial re-entrant fold. In Pappogeomys bulleri and in the castanops species-group, the outer re-entrant fold of M3 tends to be obsolete, and the tooth becomes quadriform or suborbiculate in some individuals and loses the bilophodont pattern that characterizes other species. The lingual enamel plate is displaced to the posterior surface of the tooth, and one or both plates may disappear with advancing age. Consequently, only the anterior enamel plate remains in some adults, and constitutes the maximum degree of reduction of enamel on M3 in the Geomyinae. In many adults of Pappogeomys bulleri, the enamel investment of the posterior loph is complete and the two lateral plates are connected, without interruption around the posterior apex of the tooth, evidently representing the retention of a primitive character of the ancestral lineage.
The m3 of P. bensoni from the late Pliocene is distinguished by minute lateral inflections suggesting the primitive biprismatic pattern. Also the posterior enamel plates of m1 and m2 are remarkably long, extending around the ends of the tooth. The associated upper incisor was unisulcate as in the modern species, and the basitemporal fossa of the mandible is well developed and deep.
The lower jaw is stout and relatively short. The masseteric ridge is well developed and has an especially thick crest. The basitemporal fossa is deep. In most living species, the pelage is soft and dense, but in one species, Pappogeomys fumosus, the hairs are coarse and hispid somewhat as in Orthogeomys.
Key to the Subgenera of Pappogeomys
1895. Pappogeomys Merriam, N. Amer. Fauna, 8:145, January 31.
Type.—Geomys bulleri Thomas, 1892, from near Talpa, west slope Sierra de Mascota, 8500 ft. (actually about 5000 ft.), Jalisco.
Chronologic range.—Late Pliocene (Benson local fauna, Arizona) to Recent, but no specimens known from Pleistocene.
Description.—Small, approximately same size as small subspecies of Thomomys umbrinus but forefeet larger and claws longer; skull of generalized shape, broad, relatively short, smoothly rounded, not especially compressed dorso-ventrally; zygomatic breadth great but not exceeding mastoid breadth; zygomata relatively slender for geomyid and lacking platelike expansions at lateral angles; rostrum relatively narrow; sagittal crest lacking, owing to impressions of temporal muscles remaining separated; angular process of mandible not especially elongated; enamel plates extending completely across posterior wall of M1 and M2, except in one species, P. alcorni, where posterior plate of M1 remains only on lingual fourth of posterior wall (remainder of plate lacking); with wear, plates sometimes exceptionally thin completely across posterior face of M2 and especially M1 in a few individuals of P. bulleri much as Paulson (1961:138-139) describes in extinct Geomys tobinensis; one or both plates rarely disappear in final stages of attrition in old individuals resulting in same dental pattern found in Cratogeomys; M1 and M2 retaining enamel plate on anterior wall throughout life; M3 usually subtriangular in cross-section but sometimes suborbiculate or ovoid, crown slightly bilophodont owing to shallowness of labial re-entrant angle in modern species; posterior loph of M3 not especially elongated and crown not significantly longer than wide; both lateral enamel plates of M3 usually well developed and approximately equal in length, occasionally plates reduced in length and rarely one or both plates are lost with wear in old individuals; patch of whitish or buffy hairs surrounding nose of most individuals.
The primitive character of the lower dentition, as described in the species account above, suggest that Cratogeomys [= Pappogeomys] bensoni Gidley should be referred to the subgenus Pappogeomys rather than Cratogeomys. Only the upper dentition would make positive identification possible; however, reference to the subgenus Pappogeomys seems to be the best arrangement at this time.
Referred species.—Three (one extinct):
*Pappogeomys bensoni (Gidley), 1922. U. S. Geol. Surv. Prof. Papers, 131:123. Type from Benson local fauna (late Pliocene), Cochise County, Arizona.
Pappogeomys alcorni Russell, 1957. Univ. Kansas Publ. Mus. Nat. Hist., 9(11):359. Type from 4 mi. W Mazamitla, Jalisco.
Pappogeomys bulleri Thomas, 1892. Ann. Mag. Nat. Hist., Ser. 6, vol. 10:196, August. Type from "near Talpa," west slope of Sierra Madre de Mascota, Jalisco.
1895. Cratogeomys Merriam, N. Amer. Fauna, 8:150, January 31.
1895. Platygeomys Merriam, N. Amer. Fauna, 8:162, January 31. Type: Geomys gymnurus Merriam, 1892.
Type.—Geomys merriami Thomas, 1893, from "Southern México," probably in Valley of México.
Chronologic range.—Late Pleistocene, from Wisconsin deposits (San Josecito Cave, Nuevo León, Upper Bercerra, México, and Burnet Cave, New Mexico, local faunas) to the Recent.
Description.—Size medium to large; skull becoming angular and rugose with age, and tending towards platycephaly and dorso-ventral compression; zygomata stout, each bearing platelike expansion at anterolateral angle into which anterior end of jugal becomes morticed; breadth across zygomata great relative to length of skull; rostrum relatively broad; squamosals expanding medially with age eventually growing over lateral parts of parietals, and sometimes also expanding laterally displacing postglenoid notch; sagittal crest well developed in adults of both sexes, but especially high and bladelike in males; lambdoidal crest prominent in all but young animals, having dorsal outline broadly convex posteriorly in most species but strongly sinuous in gymnurus-group; enamel plate on posterior wall of P4 absent; enamel plates present only on anterior walls of M1 and M2; M3 variform in occlusal shape (as described in species account), either subtriangular (gymnurus-group), quadriform or obcordate (castanops-group, with exceptions as noted before); lateral plates of M3 usually present in all species, labial plate approximately as long as lingual plate in gymnurus-group (like that in subgenus Pappogeomys) or distinctly shorter in castanops-group (labial plate scarcely extending beyond border of labial re-entrant fold); one or both lateral plates tending to disappear with wear in castanops-group, with lingual plate usually disappearing first; breadth across angular processes clearly more than breadth across zygomatic processes, especially in gymnurus-group.
Remarks.—In the species of the castanops-group the skulls can be spoken of as generalized and the least platycephalic of the subgenus. Indeed, the species of the castanops-group are hardly more specialized in this respect than is the subgenus Pappogeomys. In these skulls the breadth across the squamosal processes is less than that across the zygomatic arches, although the two dimensions are almost equal in some examples of P. merriami of the castanops-group (where squamosal breadth varies from 85 to 98% of zygomatic breadth). In the species having marked platycephalic skulls (gymnurus species-group) the breadth across the squamosal processes equals or exceeds the breadth across the zygomatic arches (squamosal breadth rarely 97 to 99% of zygomatic breadth), except in P. zinseri and P. tylorhinus zodius.
The variable character of the third upper molar as between species suggests that this tooth is presently undergoing active evolution. The structure of this tooth, although differing between taxa, is remarkably stable in other kinds of Geomyini. The most remarkable modification of M3 in Cratogeomys is the obcordate pattern developed in P. merriami of the castanops-group. The posterior loph and entire tooth is shortened somewhat resembling in shape that of Thomomys. Moreover, the posterior loph is twisted labially; consequently, its posterior surface now forms the labial border of the weakly defined posterior loph. Owing to the torsion, the lingual enamel plate has been rotated to the posterior surface of the tooth. Therefore, the tooth is provided with two transverse enamel plates, including the plate on the anterior wall of the tooth. The labial plate is greatly reduced, its total surface being restricted to the small labial inflection. The highly specialized obcordate M3 is not found in the most specialized platycephalic skulls characteristic of the gymnurus species-group. Instead the gymnurus-group retains the primitive subtriangular pattern without significant modification.
Referred species.—Seven:
castanops species-group
Pappogeomys castanops (Baird, 1852). Report Stanbury's Exp'd. to Great Salt Lake, p. 313, June. Type from "Prairie road to Bent's Fort," near present town of Las Animas, Colorado.
Pappogeomys merriami (Thomas, 1893). Ann. Mag. Nat. Hist., ser. 6, 12:271, October. Type from "southern Mexico," probably Valley of México (see Merriam, 1895:152).
gymnurus species-group
Pappogeomys fumosus (Merriam, 1892). Proc. Biol. Soc. Washington, 7:165, September 29. Type from 3 mi. W Colima, Colima.
Pappogeomys gymnurus (Merriam, 1892). Proc. Biol. Soc. Washington, 7:166, September 29. Type from Zapotlan (Ciudad Guzman), Jalisco.
Pappogeomys neglectus (Merriam, 1902). Proc. Biol. Soc. Washington, 15:68, March 22. Type from Cerro de la Calentura, about 8 mi. NW Pinal de Amoles, Querétaro.
Pappogeomys tylorhinus (Merriam, 1895). N. Amer. Fauna, 8:167, January 31. Type from Tula, Hidalgo.
Pappogeomys zinseri (Goldman, 1939). Jour. Mamm., 20:91, February 15. Type from Lagos, Jalisco.
The fossil record of the Geomyidae provides a sequence of morphotypes, each representing a stage in the phyletic development of the family. Most of the preserved specimens probably represent the stufenreihe rather than the ahnenreihe, as Simpson (1953:219-220) points out. Even so, the stufenreihe closely approximates the general trend of evolution, and the level of structural organization in the different stages of phyletic development may be ascertained. The actual ancestral series of most lineages probably will remain unknown, but hopefully some of the existing gaps will be filled by future discoveries. From the established record, several clearly defined lineages can be distinguished; in fact the sequence of origin, pattern of evolution, and specializations, of the principal lineages are reasonably well expressed.
In the earliest known geomyids from the Upper Oligocene and Lower Miocene, the premolars and molars are biprismatic and bilophodont. In rodents, this is itself a specialized pattern, and is thought to have evolved from a more primitive sextituberculate prototype by the union of individual cusps, and probably also cuspules, forming the two transverse enamel lophs. The primitive, common ancestor of the Geomyidae and Heteromyidae with sextituberculate teeth in the early Tertiary is unknown.
As soon as geomyids attained the early bilophodont stage of evolution, the basic morphological structure of the family was established. The family probably first became clearly distinguished from other Geomyoidea at this stage. In the early bilophodont stages of evolution, owing to the relatively deep valley between them, the two columns probably failed to unite in the normal cycle of wear, as they do in all later geomyids. Griphomys described by Wilson (1940:93) from the late Eocene of California, has a bilophate pattern in which the anterior and posterior lophs are separated by a persistent transverse valley. The occlusal pattern of Griphomys closely resembles a stage through which the ancestors of the early Miocene geomyids must have passed in their pre-Miocene evolution, as Wilson suggests (1949:115-116). Although he (1940:95; 1949:110-118) tentatively referred Griphomys to the superfamily Geomyoidea and Simpson (1945:80) went so far as to refer it to the family Geomyidae, with a notation of incertae sedis, its exact relationship to the pocket gophers is uncertain. However, the structure of the molariform dentition of Griphomys does not exclude it from the phyletic ancestry of the Geomyidae. In subsequent stages of evolution the anterior and posterior columns become united. Thereby part of the valley floor between the transverse prisms was progressively elevated, to the stage where attrition on the occlusal surface would unite the two columns. On the unworn enamel cap of living geomyids the two transverse enamel folds are separated by a shallow but well defined valley, briefly reflecting the ancient ancestral pattern.
Union of the lophs may have been either at the mid-points of the two columns or at the edge of their protomeres. [A protomere is the half of a tooth containing the protocone or protoconid—lingual side of upper tooth and labial side of lower tooth. The paramere is the opposite half of a given tooth—labial side of upper tooth and lingual side in lower tooth. See Miller and Gidley, 1918:434.] Union of the columns at the mid-points would have produced the figure-8 occlusal pattern (or H-pattern), which is characteristic of the early Miocene Geomyinae (Dikkomys). Union of the two columns at the protomeres would have produced the U-shaped pattern of the Entoptychinae, which also occurred in the early Miocene and were contemporary with the earliest Geomyinae. Since pre-Miocene geomyids are unknown, the actual phyletic development of the dentition is a matter of speculation. Probably the development of the two divergent lineages, one leading to the Entoptychinae and the other to the subfamily Geomyinae, occurred in the Oligocene (as depicted in Fig. 3). Of the two lineages, the subfamily Geomyinae, in my view, is the more primitive and less specialized. Support for this view is furnished by a reconstruction of the pattern of occlusal wear in Dikkomys and Pliosaccomys, especially on the first and second molars.
In Dikkomys, the anterior and posterior column first unite near their mid-points in the first stages of wear thus producing a figure-8 shaped (H-shaped) occlusal pattern in the premolar and all three molars. Evidently in the first two upper molars, the columns unite closer to their lingual margins than their mid-points, but at any rate both outer and inner re-entrant folds are evident at this stage of wear. With continued attrition on m1 and m2 of Dikkomys, the anterior and posterior columns secondarily unite at the edge of their labial margins thus enclosing a fossette of enamel in the labial half of the tooth. The lateral coalescence at the ends of the protomeres occurs because of the shallow vertical depth of the labial re-entrant fold, and the fossette itself does not reach the base of the crown and with continued wear it too would disappear, but not until the last stages of wear, at least in Dikkomys matthewi. The lingual re-entrant fold is deep, and therefore, persistent through all stages of wear. Although the amount of wear required for its effacement would be great, the occlusal configuration of the first and second lower molars in Dikkomys could be eventually ground down to a U-pattern as in the entoptychids. Only one upper molar of Dikkomys, the first, has been recovered (see Wood, 1936:23, fig. 32B). Although the tooth is in an early stage of wear, the lingual valley is minute. Less attrition than required in m1 and m2 would progressively reduce the lingual fold until it too would essentially form a U-pattern, perhaps retaining a slight lingual inflection. Hence, the first upper molar becomes a mirror image of the first lower molar, and the second upper molar probably had the same pattern as the first (at least it does so in Pliosaccomys). Both of the lateral re-entrant folds of the premolar are deep vertically, and consequently would not disappear with occlusal wear. Therefore, the H-pattern of the premolars is retained throughout life.
The m3 (M3 unknown for Dikkomys or Pliosaccomys) also has deep lateral folds; hence, it too retains the H-pattern in all stages of attrition, although the isthmus between the two prisms may become wider in the final phases of wear (as it does in Pliosaccomys).
In Pliosaccomys, the stages of wear are essentially the same as those described for Dikkomys, except that the anterior and posterior loph of the first and second molars tend to unite closer to one side of the tooth, lingual side in upper molars and labial in lower. Only a slight inflection of the re-entrant fold is evident on the side of union, and the inflection disappears in the first phases of wear as the columns unite. Concomitant with the lateral shift in the initial point of coalescence of the transverse lophs, the occlusal penetration of the re-entrant fold from the opposite side increases in horizontal depth, and the fold extends medially more than half way across the occlusal surface, thus forming a pattern essentially like that of the entoptychids. The U-pattern in Pliosaccomys appears in the initial stages of wear without going through an earlier H-pattern as is the case in its Miocene ancestors of the genus Dikkomys, unless the minute inflection is considered as indicative of that stage. The two columns of the premolar and m3 are joined near their mid-points as in Dikkomys; therefore, they retain their primitive H-pattern, a feature unique to the Geomyinae.
The evolutionary trend toward an ontogenetically earlier U-pattern in the first two molars in the primitive lineage of the Geomyinae suggests that the U-pattern characteristic of the Entoptychinae was simply an earlier tendency toward the same specialization that occurred later in the subfamily Geomyinae. If so, early entoptychines would have been characterized by an H-pattern in the first stages of attrition, like Dikkomys, and later developed union at the edge of the protomeres. However, in the entoptychines, all the molariform dentition, and not merely the first and second molar, became specialized; consequently the U-pattern was produced on the occlusal surfaces of each of the cheek teeth. As in Pliosaccomys, the transitional phase, in which the two columns were united at their mid-points, was eventually eliminated from the pattern of wear and only the U-pattern, that now appeared in the initial stages of wear, was retained. In the entoptychines of the early Miocene there is no suggestion of the H-pattern that characterizes the Geomyinae, except in the position of the cusps before wear in the lower molars of Pleurolicus sulcifrons, which, according to Wood (1936:6), suggests the H-pattern. In earlier unknown Oligocene stages of evolution, the prisms possibly united first at their mid-points, and the columns may have joined at the side of the tooth only in the terminal stages of wear. The U-pattern of pre-Miocene entoptychines, therefore, may have become the dominant occlusal pattern only in the later stages of phyletic development.
According to the recently expressed views of several paleontologists, the Entoptychinae constitute the primitive lineage of the family and the early Geomyinae constitute a specialized offshoot of the entoptychine ancestral assemblage. The structure of the Entoptychinae, especially of the less advanced genera, closely approximates that of the hypothetical primitive morphotype. But, according to my view, the subfamily Geomyinae constitutes the ancestral assemblage and its structure is essentially that of the primitive morphotype of the family. At any rate the structure of the early geomyines more closely approximates the structure of the ancestral stock than the more divergent entoptychines. Therefore, the genus Dikkomys of the early Miocene, the first known geomyine, is considered to be a generalized geomyid, and, although it is a contemporary of the more specialized entoptychid assemblage, is considered to be more closely allied to the ancestral stock.
The entoptychines were the dominant and most highly differentiated geomyids of the early and middle Miocene. Nevertheless, they became extinct in the middle Miocene, and the geomyines of that time survived and later gave rise to the modern pocket gophers. Therefore, the early history of the family Geomyidae is characterized by an early radiation and trend toward specialization, followed by survival of the less specialized Geomyinae and extinction of the more specialized Entoptychinae.
The most abundant geomyids of the early and middle Miocene, the Entoptychinae, consisted of at least 24 species (see Wood, 1936:4-25) classified in four genera: Pleurolicus, Gregorymys, Grangerimus, and Entoptychus. The genera were essentially contemporaneous (see Figure 3). Even so, the subfamily was morphologically varied, pointing to an earlier origin in the Oligocene (actually a part of the John Day Fauna, including Pleurolicus may be correlated with late Oligocene Whitneyian age) followed by a relatively rapid radiation including all four genera in the early Miocene. Two genera, Pleurolicus and Gregorymys, continued into the Middle Miocene (Hemingfordian). This divergence, specialization, and subsequent radiation suggest that the entoptychines evolved into a new major adaptive zone, in the sense described by Simpson (1945:199-206).
The radiation is correlated geographically and temporally with the southward retreat of the Neotropical flora of the Tertiary from the western United States and southward movement of the Arctic flora of the Tertiary (see Axlerod, 1950; Berry, 1937:31-46; Chaney, 1947:139-148; and Kendeigh, 1961:280-283). In the early Tertiary the Neotropical-tertiary geoflora occurred northward to at least 49° latitude in western North America, and the boreal Arctic-tertiary flora was restricted to a circumpolar zone. The southward and eastward shift of the Neotropical-tertiary flora, associated with the drying and chilling of the continent, began in the middle or late Oligocene and was concurrent with the divergence and radiation of the Entoptychinae. Beginning in late Oligocene and continuing at least into middle Miocene, most of the region in which the entoptychines occurred was occupied by the Arcto-tertiary geoflora of which the temperate forest division contributed the dominate plant associations. The maples, chestnuts, dogwoods, beeches, walnuts, oaks, elms, birches, and sycamores of that flora were the forerunners of today's eastern deciduous forest. It is my view that the entoptychines became adapted to the conditions of this paleoecological environment and radiated rapidly in the Arikareean when the major change occurred in climax vegetation. The ancestral stock of the Geomyinae was not so successful in the Arcto-tertiary climax, and most of it probably was displaced southward along with the tropical flora.
The skeleton in the entoptychines is not so strongly fossorial as in the modern geomyids (Wilson, 1949:117), and these early geomyids probably were semi-fossorial with somewhat the same burrowing habits as those of the living mountain beaver (Aplodontia). Inasmuch as the morphology and taxonomy of the entoptychines were discussed in detail by Cope (1884) and reviewed later by Wood (loc. cit.), there is no need to recount the details here. According to Wood (op. cit., 27-28), Pleurolicus occupied a central position in the entoptychid radiation and perhaps appeared slightly earlier than the other genera. Wilson (1949) suggested that the lower part of the John Day may actually be Upper Oligocene rather than Lower Miocene, and this arrangement is followed here. Also, Pleurolicus is less specialized than the other genera and occurs in deposits of both the Great Plains and the Pacific Coast. Gregorymys, also little specialized, occurred only on the Great Plains. The more specialized genera, Grangerimus and Entoptycus, evidently appeared somewhat later than Pleurolicus and evolved from it. Except for a record from southern Texas reported recently by Hibbard and Wilson (1950:621-623) and the new species described by MacDonald (1963:182) from the Sharps Formation of South Dakota (early Arikareean), Grangerimus is known only from the Pacific coast. Entoptycus was restricted to the Pacific Coast (John Day fauna).
Entoptycus is the most specialized of the known genera; it has pronounced fossorial adaptations, especially in the skull. Its molariform teeth are rootless and ever-growing as in the modern geomyines. Moreover, the continuous enamel bands on only moderately worn teeth become separated in the final stages of wear into anterior and posterior enamel plates by tracts of dentine that extend toward the crown on the sides of each tooth. This extension was made possible by the union of the two columns at both the lingual and labial margins of the tooth forming an O-pattern, and the crown is essentially monoprismatic save for the isolated enamel fossette in the center of the tooth. The fossette is all that remains of the lateral re-entrant fold that characterized the preceding U-pattern of the earlier stages of wear. Late in the sequence of wear, the anterior enamel plate is lost in the lower molars and the posterior plate in the upper molars. The U-pattern characterizes the final stages of attrition in the other genera of the Entoptychinae; none developed the dental specializations seen in Entoptycus. Rootless, ever-growing cheek teeth, discontinuous enamel patterns, and monoprismatic molars were not evolved in the subfamily Geomyinae until the late Pliocene.
Phyletic Trends in Subfamily Geomyinae
The subfamily Geomyinae is made up of three groups, recognized taxonomically for the first time in this account as tribes—Dikkomyini, Thomomyini, and Geomyini (for full discussion of classification, see previous account). The phylogeny proposed by me is illustrated in Figure 3. The tribe Dikkomyini is characterized by generalized and primitive features that together form the basic structural foundation of the subfamily. Evolution within the Dikkomyini resulted in the acquisition and perfection of fossorial adaptations. The Thomomyini and Geomyini are considerably more specialized than the ancestral Dikkomyini from which they evolved. The Geomyini are clearly more specialized than the Thomomyini, suggesting closer affinity between the Thomomyini and the Dikkomyini than between the Geomyini and the Dikkomyini. The specializations in the dentition and the associated changes in the skull of the Thomomyini and Geomyini permit more efficient mastication of fibrous vegetation. Along with these specializations, fossorial adaptations inherited from the Dikkomyini are retained without noteworthy modification.
Dikkomys, the earliest known genus of the tribe Dikkomyini, can be taken as a starting point of evolution for the subfamily Geomyinae. The Pliocene genus Pliosaccomys is the only other known geomyine having primitive features closely resembling those of Dikkomys. The relatively close but previously unrecognized relationship between Dikkomys and Pliosaccomys can be understood when patterns of wear on the occlusal surfaces of the cheek teeth are taken into account. It appears that Pliosaccomys descended from Dikkomys-like stock, if not Dikkomys itself. Although Dikkomys is towards the beginning of this phyletic sequence and Pliosaccomys towards the end of the sequence, the primitive features shared by the two provide a generalized morphotype for the subfamily Geomyinae.
In the molariform dentition, an almost complete series of stages of wear in Pliosaccomys has been preserved, and those of Dikkomys can be reconstructed with reasonable accuracy from those that are known (see Fig. 4):
(1) In the initial stage of wear in Dikkomys the anterior and posterior columns are separated by an intervening valley (Fig. 4A), and the occlusal surface of each column bears a loph of dentine surrounded by a ring of enamel: protoloph on the anterior column and metaloph on the posterior column of the upper teeth (protolophid and hypolophid in corresponding positions in the lower teeth). Actually this stage is not preserved in the known material of Dikkomys, but does occur in both geomyines and entoptychines in all stages of evolution, and it must have also occurred in Dikkomys in order for the next two stages, which are preserved, to have developed.
(2) The occlusal surfaces are ground down to a level where the enamel loops of the two columns join at their mid-points, thus forming an H-shaped pattern (Fig. 4B), or more exactly a pattern resembling a figure 8. Probably this was the primitive pattern in the final stage of wear in the geomyid ancestor of the Oligocene.