Moropus merriami in the Early Barstovian Lower Snake Creek Fauna of Nebraska, with comments on biogeography of North American Chalicotheres

Moropus merriami Holland and Peterson (1914), an early Barstovian schizotheriine chali-cothere from North America, is rediagnosed and redescribed on the basis of additional material. While originally recognized from the Virgin Valley and High Rock Canyon local faunas of northwest Nevada, M. merriami can also be identified from the Lower Snake Creek fauna preserved in the Olcott Formation of northwest Nebraska. It represents a derived species of Moropus in which the astragalus and metatarsals have become relatively broad and short. The large claw associated with digit II of the manus is laterally compressed, and proximal and middle phalanges of digit II of the pes are uniformly fused. In northwest Nebraska, M. mer-riami is the early Barstovian faunal replacement for the dome-skulled chalicothere Tyloce-phalonyx skinneri, which is found in the Sheep Creek Formation. The early Barstovian ap-pearance on the Great Plains of a species best known from the Great Basin and Columbia Plateau is a significant event at a time when specific-level faunal differences between these regions were pronounced