Part 9

Discussion.--Features characterizing Thomomys and the tribe Thomomyini are more advanced than those characterizing the tribe Dikkomyini. Also, the Thomomyini retain more of the primitive features of the Geomyinae than do the more specialized tribe Geomyini.

Specializations are few, but include the third molar being a single column both above and below, enamel plates, and a masseteric ridge.

Key to the Subgenera of Thomomys

A Molars sub-crescent or ovate in cross-section, not becoming abruptly narrower at one end of tooth. Subgenus Pleisothomomys p. 519

A´ Molars pear-shaped, not sub-crescent or ovate, in cross-section, crown becoming abruptly narrow at one end of tooth. Subgenus Thomomys p. 520

Subgenus =Pleisothomomys= Gidley and Gazin

1933. Pleisothomomys Gidley and Gazin, Jour. Mamm., 14:354, November 13.

Type.--Pleisothomomys potomacensis Gidley and Gazin, 1933.

Chronologic range.--Late Pliocene (Hagerman local fauna, Idaho) to late Pleistocene. The latest records are from the fauna of Saber-tooth Cave, Florida, a late Pleistocene assemblage that probably was deposited in the Sangamon. The middle and late Pleistocene records are from the eastern United States, suggesting that the subgenus Pleisothomomys was restricted to that region while the subgenus Thomomys occupied the western United States and parts of Canada and México as it does today.

Description and Comparison.--Separated from subgenus Thomomys only on basis of sub-crescentic shaped molars (only jaw fragments and isolated teeth known), seemingly a primitive feature of the genus. This dental structure continued into the late Pleistocene; none of the Recent species expresses this feature of the molars, although the molars of Thomomys vetus of the late Pleistocene (Wisconsin deposits), referred to the subgenus Thomomys on the basis of its alleged relationship to Thomomys townsendii (see Davis, 1937:156-158), are less distinctly pear-shaped, and are more sub-crescentic, than in any other known species of the subgenus Thomomys. Pleisothomomys Gidley and Gazin (loc. cit.) was proposed as a genus but is here considered as of no more than subgeneric worth, and is recognized because of the apparent constancy of the sub-crescentic molars in the earlier members of the genus and in those populations of Thomomys occurring in Pleistocene times in the eastern United States.

Referred species.--Three (all extinct):

*Thomomys gidleyi Wilson, 1933. Carnegie Inst. Washington Publ. 440:122, December. Type from Hagerman beds, late Pliocene, Idaho.

*Thomomys potomacensis Gidley and Gazin, 1933. Jour. Mamm., 14:354, November 13. Type from Cumberland Cave, middle and late Pleistocene, Maryland.

*Thomomys orientalis Simpson, 1928. Amer. Mus. Novit., 328:6, October 26. Type from Saber-tooth Cave, late Pleistocene, Florida.

Subgenus =Thomomys= Wied-Neuwied

1839. Thomomys Wied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar. Leop. Carol., 19(1):377.

1903. Megascapheus Elliot, Field Columb. Mus., Publ. 76, Zool. Ser., 3 (11):190, July 25. Type: Diplostoma bulbivorum Richardson, from Columbia River, probably near Portland, Oregon.

Type.--Thomomys rufescens Wied-Neuwied, 1839.

Chronologic range.--Early Pleistocene (Broadwater-Lisco local fauna, Nebraska) to Recent. Numerous records, mostly isolated teeth, from nearly all stratigraphic levels of the Pleistocene (for details, see account of fossil record).

Description.--Molars pear-shaped in cross-section, becoming abruptly narrow at one end of the tooth. The teeth of the late Pleistocene species Thomomys vetus are less distinctly pear-shaped than other referred species (see remarks in the description of the subgenus Pleisothomomys).

Essentially on the basis of its significantly larger size and details of the skull, Elliott (1903:190) proposed subgeneric recognition of Thomomys bulbivorus and described the subgenus Megascapheus to include it. Also the molars of Thomomys bulbivorus usually have a small enamel plate, both above and below, bordering the persistent inflection on the protomere end of the tooth; each lateral plate is isolated from the transverse plates on the anterior and posterior walls of the tooth. In my opinion these features do not warrant subgeneric recognition; however, these characters do distinctly separate Thomomys bulbivorus from other groups of species, and the character of the molars suggests retention of a primitive trait. Therefore, I propose that the unique structure of this species be recognized by setting it apart in the bulbivorus species-group.

Referred species.--Ten species, three extinct, placed in three species-groups (the numerous subspecies of this genus are listed in Miller and Kellogg, 1955:276-332, and Hall and Kelson, 1959:412-447).

bulbivorus species-group

Thomomys bulbivorus (Richardson, 1829). Fauna Boreali-Americana, 1:206. Type from Columbia River, probably near Portland, Oregon.

umbrinus species-group

*Thomomys scudderi Hay, 1921. Proc. U. S. Nat. Mus., 49:614. Type from Fossil Lake beds, late Pleistocene, Oregon.

Thomomys umbrinus (Richardson, 1829). Fauna Boreali-Americana, 1:202. Type from southern México, probably near Boca de Monte, Veracruz.

Thomomys bottae (Eydoux and Gervais, 1836). Mag. de Zool., Paris, 6:23. Type from coast of California, probably near Monterey.