The Bighorn Basin has produced the richest and most diverse early Eocene mammalian faunas in the world and is the principal source of our knowledge of skeletal anatomy in these mammals. Until recently, most of our information on postcranial anatomy in early Eocene mammals came from the works of Matthew and his contemporaries. Considerable new evidence has been unearthed in the last 25 years, but very little of it has yet been described or even reported in the literature. Since 1979, a USGS–Johns Hopkins project working in the Wasatchian part of the Willwood Formation has collected more than 150 skeletal...