We summarize and add to the biostratigraphy and magnetic polarity stratigraphy of the Washakie Formation, Washakie Basin, Wyoming. Previously the Washakie Formation (divided into the lower Kinney Rim Member and the upper Adobe Town Member) was thought to contain rocks of early Bridgerian through late Uintan age. Continuing collection efforts in the Washakie Basin by the Field Museum of Natural History (FMNH) allow us to revise the biochronologically determined age of the Washakie Formation to late Bridgerian through early Uintan age.
A late Bridgerian age for the poorly fossiliferous Kinney Rim Member of the Washakie Formation is indicated by the presence of Hyrachyus eximius (a taxon with a late Bridgerian first occurrence elsewhere). In addition, the following taxa are also known from the Kinney Rim Member: Peratherium cf. P. knighti, cf. Apatemys bellus, Hyopsodus sp., Orohippus sp., Mesatirhinus sp., Helaletes nanus, and Hyrachyus modestus (all known from the Bridgerian elsewhere, but none restricted to the late Bridgerian, except possibly Mesatirhinus). In addition, taxa restricted to the early Bridgerian of the Bridger Basin (e.g., Smilodectes) have not been recovered from the Washakie Formation, except for a possible new species of tillodont from the Kinney Rim Member.
An early Uintan age for the upper unit of the Adobe Town Member, the uppermost unit in the Washakie Formation, is indicated by the occurrence of Pareumys grangeri (restricted to the early Uintan elsewhere). In addition, Paramys compressidens and Epihippus gracilis (both known from the Uintan elsewhere, but neither restricted to the early Uintan) are also known from the upper unit.