The Cook Ranch local fauna is from the Sage Creek Basin in southwestern Montana. Fossil mammals have been described from the Cook Ranch local fauna as early as the 1930s. The age has been identified as early Oligocene (late Orellan) based on the mammalian fauna. Twelve species of rodents are recognized herein from Cook Ranch. Two new species are described: an eomyid Metanoiamys woodi and a cricetid Wilsoneumys focarius. The aplodontiid Campestrallomys annectensKorth, 1989a, is referred as a new combination, Ninamys annectens (Vianey-Laud et al., 2013), and distinguished from North American species of CampestrallomysKorth, 1989a, based on dental characters (e.g., partial ectoloph on upper cheek teeth, morphology of hypolophid on lower cheek teeth). This is the first recognition of this genus in North America of this otherwise Eurasian genus. A neotype for the heliscomyid Heliscomys gregoryi is assigned, and it is demonstrated that the species is distinct from other species of the genus based on size and style and stylid development of the molars.

The bulk of the species present are typical of other North American Orellan faunas but one species, Adjidaumo minimusMatthew, 1903, is elsewhere limited to the Chadronian (latest Eocene) and another species, Agnotocastor praetereadensStirton, 1935, is elsewhere limited to the Whitneyan (early middle Oligocene). These identifications extend the known ranges of both species.