Eohippus is an extinct genus of small equid ungulates.[1] The only species is E. angustidens, which was long considered a species of Hyracotherium. 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, Modern (1453-);: ἠώς (Greek, Modern (1453-);: eōs, 'dawn') and Greek, Modern (1453-);: ἵππος (Greek, Modern (1453-);: 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, or three hands tall, at the shoulder.[4] It has four toes on its front feet and three toes on the hinds, each toe ending in a hoof. Its incisors, molars and premolars resemble modern Equus. However, a differentiating trait of Eohippus is its large canine teeth.[4] [5]
In his 1991 essay, "The Case of the Creeping Fox Terrier Clone",[6] Stephen Jay Gould lamented the prevalence of a much-repeated phrase to indicate Eohippus size ("the size of a small Fox Terrier"), even though most readers would be quite unfamiliar with that breed of dog. He concluded that the phrase had its origin in a widely distributed pamphlet by Henry Fairfield Osborn, and proposed that Osborn, a keen fox hunter, could have made a natural association between his horses and the dogs that accompanied them.[6]