Hindlimb adaptations in Ourayia and Chipetaia, relatively large-bodied omomyine primates from the Middle Eocene of Utah.

Formation ABSTRACT North American omomyids represent a tremendous Eocene radiation of primates exhibiting a wide range of body sizes and dietary patterns. Despite this adaptive diversity, relatively little is known of the postcranial specializations of the group. Here we de- scribe hindlimb and foot bones of Ourayia uintensis and Chipetaia lamporea that were recovered from the Uinta B member (early Uintan Land Mammal Age), Uinta For- mation, Utah. These specimens provide insights into the evolution of postcranial adaptations across different body sizes and dietary guilds within the Eocene primate radi- ation. Body mass estimates based on talar measure- ments indicate that Ourayia uintensis and Chipetaia lamporea weighed about 1,500-2,000 g and 500-700 g, respectively. Skeletal elements recovered for Ourayia include the talus, navicular, entocuneiform, first meta- tarsal, and proximal tibia; bones of Chipetaia include the talus, navicular, entocuneiform, and proximal femur. Both genera had opposable grasping big toes, as indi- cated by the saddle-shaped joint between the entocunei- form and first metatarsal. Both taxa were arboreal leap- ers, as indicated by a consistent assemblage of charac- ters in all represented bones, most notably the somewhat elongated naviculars, the high and distinct trochlear crests of the talus, the posteriorly oriented tib- ial plateau (Ourayia), and the cylindrical head of the fe- mur (Chipetaia). The closest resemblances to Ourayia and Chipetaia are found among the Bridger omomyines, Omomys and Hemiacodon. The results of our compari- sons suggest that the later, larger, more herbivorous omomyines from Utah retained a skeletal structure char- acteristic of earlier, smaller North American omomyids. Am J Phys Anthropol 131:303-310, 2006. V C 2006 Wiley-Liss, Inc.