New primates from the middle Eocene of the Sand Wash Basin, northwestern Colorado

Abstract

The transition between the Bridgerian and Uintan North American Land Mammal Ages of the middle Eocene is a pivotal time in the evolution of modern mammal ecosystems in North America, marking the beginning of a global cooling trend that led to the recession of tropical forests and gradual faunal turnover on the continent. However, few mammalian faunas are known from this time period, leading to difficulty characterizing and recognizing early Uintan faunal assemblages. The Sand Wash Basin in northwestern Colorado has been suggested to yield fossil faunas of early Uintan age, but fossils from the Sand Wash Basin have not been formally described since the 1970s despite active field work in the region. Here, I describe plesiadapiform and euprimate fossils from the Sand Wash Basin and compare them to other late Bridgerian and early Uintan North American primate assemblages. The Sand Wash Basin primate fauna comprises five species, all of which are known from the Washakie Basin in Wyoming. The presence of Ourayia uintensis suggests that at least some fossil localities within the Sand Wash Basin yield fossils that are Uintan in age; however, the rarity of primates and lack of a stratigraphic context in which to interpret localities make it difficult to determine whether some may be older.