A Unique Rodent Fauna from the Whitneyan (Middle Oligocene) of Southwestern Montana

ABSTRACT Nine rodents are described from the Whitneyan (middle Oligocene) White Hills fauna of Montana. Of these, three new genera are described: the aplodontid Altasciurus, the cylindrodontid Lophicylindrodon, and the castorid Montanacastor. Prosciurus relictus (Cope, 1873) is named as the type species of Altasciurus. The other genera are unique to this fauna and are based on new species; Lophicylindrodon expiratus and Montanacastor simplicidens. In addition, two more new species are described: the eomyid Orelladjidaumo exiguus and the cricetid Willeumys argosorus. The rodent fauna from White Hills is believed to be Whitneyan in age because of the occurrence of Eumys brachyodus Wood, 1937, and Leptodontomys douglassi (Burke, 1934); both elsewhere known from the Whitneyan. The rodent fauna is unique for several reasons: 1) a much lower diversity of rodents than in comparably aged faunas; 2) the number of unique species (four of nine); 3) the relict occurrence of a cylindrodontid (otherwise latest occurrence in the Chadronian); and 4) the extremely high proportion of a single species (A. relictus makes up over 70% of the number of specimens in the fauna). The uniqueness of this fauna may be explained by the fact that the other well-known Whitneyan faunas are from the Great Plains, whereas the White Hills fauna was likely from an isolated intermountain basin. However, taphonomic factors may have controlled the relative numbers of specimens of each species.