Two extant ground squirrel species are included in the Spermophilus richardsonii group (Nadler et al., 1982). The Wyoming ground squirrel (Spermophilus elegans) occupies open habitats in the central Rocky Mountains and northern Basin and Range regions (Zegers, 1984); Richardson's ground squirrel (Spermophilus richardsonii) occurs on the northern Great Plains (Michener and Koeppl, 1985). The two species hybridize along a narrow contact zone in southwestern Montana (Nadler et al., 1971; Koeppl et al., 1978). A member of the S. richardsonii group was present by the middle Pleistocene on the Great Plains (Cudahy Local Fauna; Paulson, 1961), but is poorly known before the late Pleistocene when fossils are common on the central and unglaciated northern Great Plains (Neuner, 1975). Based on a multivariate discriminant analysis (Neuner, 1975) and study of bacular morphology of a well-preserved fossil (Neuner and Schultz, 1979), this middle to late Pleistocene squirrel was judged intermediate between and likely ancestral to S. elegans and S. richardsonii, indicating a latest Pleistocene or Holocene origin of extant species.