A lower jaw of an eohippus ( Hyracotherium cf. H. angustidens ) from late Paleocene strata in Wyoming has extended the geological record of fossil horses into pre-Eocene time and suggests that the order Perissodactyla had an origin earlier than that heretofore conjectured. This specimen, together with equid teeth also possibly of late Paleocene age from Baja California, indicates that early perissodactyls were widespread on the North American continent before the Eocene epoch. Late Paleocene and early Eocene deposits of northwestern Wyoming have yielded many vertebrate rarities and "first or earliest occurrences" which require reappraisals of inter-and intracontinental dispersal patterns of the vertebrate groups involved.