Key to the species of Thomomys

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A modified (based on recent taxonomic changes by Álvarez-Castañeda and Segura-Trujillo (2025)) key to species of Thomomys from Verts & Carraway (2005) (modified from Hall 1981; Jameson and Peeters 1988; and Verts and Carraway 1998, with characters from Hoffmeister 1986; Thaeler 1972, 1980; and Thaeler and Hinesley 1979).

1 Sphenoidal fissure present ---- genus Megascapheus
  Sphenoidal fissure absent ---- genus Thomomys, go to 2
2 Length of ear from notch 8-9 mm; nasals extend posteriorly as far as premaxilla; 'pelage dark brown dorsally; feet white--------- Mountain Pocket Gopher (Thomomys monticolus)
  Length of ear from notch usually <8 mm; premaxilla extend posteriorly beyond nasals; feet gray ----- 3
3 Mandible with wide (~1.5 mm) flange on posteriolateral edge; length of baculum 22-31 mm in adult males; pelage black, blackish with reddish dorsal stripe, or reddish brown--------- Western Pocket Gopher (Thomomys mazama)
  Mandible with narrow (~1 mm) flange on posteriolateral edge; length of baculum 10.5-23.4 mm in adult males; pelage yellowish gray to yellowish brown ---- 4
4 Karyotype diploid number (2n) = 40, 42, 44, 46, 48, 52, 54, 56, or 60 and broadly distributed from Cascade and Sierra Nevada ranges of British Columbia, Oregon, Washington, and California; east to Alberta and western Nebraska; and south to central New Mexico ------------------------------Northern Pocket Gopher (Thomomys talpoides)
  Karyotype 2n = '46, 56, or 58 and restricted to small areas in Montana, Idaho, Utah, and Wyoming ---- 5
5 Karyotype 2n = 46 and restricted to extreme southern Wyoming Wyoming Pocket Gopher (Thomomys clusius)
  Karyotype 2n = 56 and restricted to small areas in southern Montana and northeastern Idaho, or karyotype 2n = 58 and restricted to extreme southeastern Idaho, northern Utah, and western Wyoming------------- Idaho Pocket Gopher (Thomomys idahoensis)

References

Álvarez-Castañeda ST, Segura-Trujillo CA. 2025. Genus-level review of pocket gophers in the family Geomyidae. Therya 16(1): 41-75.

Hall ER. 1981. The mammals of North America. 2 vols. John Wiley and Sons, New York.

Hoffmeister DF. 1986. Mammals of Arizona. Tucson (AZ, USA): University of Arizona Press.

Jameson Jr EW, Peeters HJ. 1988. California mammals. Berkeley (CA, USA): University of California Press.

Thaeler Jr CS. 1972. Taxonomic status of the pocket gophers, Thomomys idahoensis and Thomomys pygmaeus (Rodentia, Geomyidae). Journal of Mammalogy 53:417-428.

Thaeler Jr CS. 1980. Chromosome numbers and systematic relations in the genus Thomomys (Rodentia: Geomyidae). Journal of Mammalogy 61:414-422.

Thaeler Jr CS, Hinesley LL. 1979. Thomomys clusius, a rediscovered species of pocket gopher. Journal of Mammalogy 60:480-488.

Verts BJ, Carraway LN. 1998. Land mammals of Oregon. Berkeley (CA, USA): University of California Press.

Verts BJ, Carraway LN. 2003. Thomomys townsendii. Mammalian Species 2003(719):1-6.