Abstract
Ischyromyids are a group of large rodents with the earliest fossil record known from the late Paleocene (Clarkforkian) of North America; they are considered the earliest fossil representatives of Rodentia of modern aspect. Ischyromyids dominated early Paleogene small-mammal assemblages of North America and in the latest Paleocene migrated to western Europe and to Asia; in the latter they survived only to the beginning of the late Eocene, but were never abundant. Here we describe for the first time the calcanei of ischyromyids from the early middle Eocene of the Erlian Basin in Nei Mongol, northern China. These calcanei document the existence of three species. The morphology of the studied tarsal bones overall suggests ambulatory locomotion for these animals ('slow cursors'), similar to that of the coypu and porcupines, but one form shows more marked cursorial capabilities. These differences show that Chinese ischyromyids, although rare, had attained greater taxonomic diversity by the middle Eocene in Nei Mongol than estimated from dental remains. We also address the question of the morphological and ecological divergence of these ischyromyids in relation to their North American counterparts, as well as the issue of a direct dispersal route from North America to Asia in the early Eocene.