The Buck Spring Quarries, located in the southern part of the type area of the Lost Cabin Member of the Wind River Formation, Wind River Basin, Wyoming, provide one of the richest assemblages of fossil vertebrates known from the latest Wasatchian Land-Mammal Age (ca. 50.5 Ma, Lostcabinian, early Eocene) of North America. More than 100 species of mammals, reptiles, birds, amphibians, and fishes are known. The quarries uniquely preserve associated skeletal remains, and complete skulls and dentitions of a large percentage of the vertebrates. The fossils come from a 2-m-thick sequence, which is composed primarily of mudstones, bioturbated limestone lenses,...