Eohippus

Eohippus; Temporal range: Ypresian, 55.8–47.8 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	Reconstructed skeleton, National Museum of Natural History, Washington, D.C., United States
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
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]

Discovery

In 1876, Othniel C. Marsh described a skeleton as Eohippus validus, from Greek: ἠώς (eōs, 'dawn') and ἵππος (hippos, 'horse'), meaning 'dawn horse'.^{[citation needed]} 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.

Description

Eohippus stood at about 30 in (76 cm), or seven hands tall, at the shoulder.^[4] It has 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 its large canine teeth.^[4]^[5]

Stephen Jay Gould comments

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]