Eocene United States

Mesonyx ("middle claw") is an extinct genus of mesonychid, one of the families that is part of the extinct order Mesonychia. Fossils of the various species are found in Early to Late Eocene-age strata in the United States and Early Eocene-aged strata in China, 51.8—51.7 Ma (AEO).

Rock formation

Eobasileus ("dawn-king") is a genus of dinoceratan mammal in the family Uintatheriidae. One species is currently considered valid, E. cornutus, though it has been suggested that the closely related Tetheopsis may also belong to this genus. The first specimen of Eobasileus, consisting of a skull, several vertebrae, and bones associated with the limbs, was collected from Haystack Mountain, from strata belonging to the Washakie Formation. The species became the subject of various letters written by Edward Drinker Cope (its describer) and Othniel Charles Marsh, and their disagreements over its tax

Sinopa ("swift fox") is an extinct genus of placental mammals from extinct clade Sinopinae within extinct order Hyaenodonta, that lived in North America and Asia from the early to middle Eocene.

Leptomeryx is an extinct genus of artiodactyl that lived in North America during the middle Eocene through early Miocene. One of the earliest ruminants, it belongs to the extinct family Leptomerycidae, which is related to today's chevrotains. Most fossils were found in the badlands of the Midwestern United States in sedimentary rocks of the White River Group, in the Brule and Chadron formations. Additional remains have been reported from southern Canada and Mexico. The genus was scientifically described in 1853 by Joseph Leidy and includes several species.

Chrysocetus is a genus of extinct archaeocete ceteceans belonging to the Basilosauridae that is known from the Late Eocene of the eastern United States and western Africa. Across its range two species are known, the American Chrysocetus healyorum and the African Chrysocetus fouadassii, named in 2001 and 2015 respectively. Chrysocetus was a comparably small basilosaurid, larger than Saghacetus and Tutcetus but smaller than most other members of this group. In addition to being noted for its well-preserved hip bones, Chrysocetus is unique amongst basilosaurids for possibly having only had a sing