Rodents (Mammalia) from Diamond O Ranch Local Fauna, Southwestern Montana

ABSTRACT Eleven species of rodents are described from the Diamond O Ranch local fauna from the Beaverhead Basin, southwestern Montana. Of the species recognized, seven have been previously reported from the Chadronian of Montana or adjacent Wyoming and Saskatchewan or Texas: Ardynomys occidentalis Burke, 1936; Pseudocylindrodon sp., cf. Pseudocylindrodon neglectus Burke, 1935; Eutypomys parvus Lamb, 1908; Adjidaumo minimus (Matthew, 1903); Paradjidaumo disjunctus Korth, 2013; Paradjidaumo nanus Emry and Korth, 2013; and Aulolithomys vexilliames Korth and Emry, 1997. Isolated molars of two different indeterminate species of Ischyromys Leidy, 1856, are also recognized. Two new species are described: the ischyromyine Spurimus hoffmani and the eomyid Metanoiamys norejkoi. Both these species represent the latest occurrence of these genera, which are elsewhere limited to the earlier Eocene. The rodents from Diamond O Ranch suggest an early Chadronian age for the fauna; however, other components of the fauna make this age determination uncertain (Tabrum et al. 1996:295; Fostowicz-Frelik and Tabrum 2009).