ABSTRACT

Ischyromys is known from the early Duchesnean to the early Whitneyan of the Rocky Mountains and Great Plains of North America. Early species are morphologically diverse and are known from small and sometimes fragmentary samples, so the Duchesnean history of the group is hard to unravel. A diverse radiation of species with a derived but variable jaw musculature dominates the Chadronian of the Rocky Mountains. These species have been assigned to Ischyromys by some authors and Titanotheriomys by others. The name Titanotheriomys is recognized herein as a subgenus of Ischyromys. The Orellan of the Great Plains is dominated by two species of Ischyromys that lack the Titanotheriomys specializations. Both these species are also known from the Chadronian of the Great Plains based on a few fragmentary remains. Ischyromys became less abundant in the Rocky Mountains and more abundant in the Great Plains at the Chadronian/Orellan (Eocene/Oligocene) boundary, but no new species originated and no existing species underwent measurable change during the transition.