A NEW GENUS OF BEAVER (MAMMALIA, CASTORIDAE, RODENTIA) FROM THE ARIKAREEAN (OLIGOCENE) OF MONTANA AND ITS BEARING ON CASTORID PHYLOGENY

The holotypes of two Arikareean castorids from Montana, Steneofiber co mplexus Douglass and S. hesperus Douglass were reexamined in light of recent synonymies of these species with other European and Asian speci es. The two Montana species are synonyms but are not synonymous with a ny previously described species of castorid from either Europe or Asia . A new genus, Neatocastor, is named for the Montana beaver, N. hesper us. The new genus is most closely related to the otherwise Chadronian to Whitneyan-aged castorid Agnotocastor Stirton and the Hemingfordian to Barstovian Amblycastor Matthew. A combination of retained primitive characters and derived characters of the skull and dentitions of thes e genera unite them systematically. This group of genera represents an early offshoot of the Castoridae that is not ancestral to any other g roup of castorids.