Eohippus

Eohippus Temporal range: Ypresian,
Reconstructed skeleton, National Museum of Natural History, Washington, D.C., United States
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Placentalia
Order:	Perissodactyla
Family:	Equidae
Genus:	†Eohippus Marsh, 1876
Species:	†E. angustidens
Binomial name
†Eohippus angustidens (Cope, 1875)
Synonyms
Eohippus validus Hyracotherium angustidens H. a. angustidens H. a. etsagicum H. vasacciense H. v. vasacciense H. cusptidatum H. seekinsi H. loevii Orohippus angustidens Orohippus cuspidatus Orohippus vasacciensis Lophiotherium vasacciense

Eohippus is an extinct genus of small equid ungulates.¹ The only species is E. angustidens, which was long considered a species of Hyracotherium (now strictly defined as a member of the Palaeotheriidae rather than the Equidae). Its remains have been identified in North America and date to the Early Eocene (Ypresian stage).²

In 1876, Othniel C. Marsh described a skeleton as Eohippus validus, from Greek: ἠώς (eōs, 'dawn') and ἵππος (hippos, 'horse'), meaning 'dawn horse'. Its similarities with fossils described by Richard Owen were formally pointed out in a 1932 paper by Clive Forster Cooper. E. validus was moved to the genus Hyracotherium, which had priority as the name for the genus, with Eohippus becoming a junior synonym of that genus. Hyracotherium was recently found to be a paraphyletic group of species, and the genus now includes only H. leporinum. E. validus was found to be identical to an earlier-named species, Orohippus angustidens Cope, 1875,³ and the resulting binomial is thus Eohippus angustidens.

Eohippus stood at about 12 in (30 cm), or three hands tall, at the shoulder.⁴ It had four toes on its front feet and three toes on the hind feet, each toe ending in a hoof. Its incisors, molars and premolars resemble modern Equus. However, a differentiating trait of Eohippus is the large canine teeth.⁴⁵