Abstract
Several well-preserved jaws of the rare North American omomyid primate Steinius vespertinus, including the first known antemolar dentitions, have been discovered in 1989 and 1990 in the early Eocene Willwood Formation of the Bighorn Basin, Wyoming. They indicate that its dental formula is as primitive as those in early Eocene Donrussellia (Adapidae) and Teilhardina (Omomyidae)--widely considered to be the most primitive known euprimates--and that in various dental characters Steinius is as primitive or more so than Teilhardina. Therefore, despite its occurrence at least 2 million years later than Teilhardina, S. vespertinus is the most primitive known omomyid and one of the most primitive known euprimates. Its primitive morphology further diminishes the dental distinctions between Omomyidae and Adapidae at the beginning of the euprimate radiation