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Temporal range: Ypresian
National Museum of Natural History, Washington, D.C.
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Perissodactyla
Family: Equidae
Genus: Eohippus
Marsh, 1876
Species: E. angustidens
Binomial name
Eohippus angustidens
(Cope, 1875)
  • Eohippus validus
  • Hyracotherium angustidens
  • H. a. angustidens
  • H. a. etsagicum
  • H. vasacciensis
  • H. v. vasacciensis
  • 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. Its remains have been identified in North America and date to the Early Eocene (Ypresian) stage.[2]


Restoration by Heinrich Harder
Restoration by Charles Knight

In 1876, Othniel C. Marsh described a skeleton as Eohippus validus, from the 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 Sir 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, Hyracotherium angustidens (Cope, 1875), and the resulting binomial is thus Eohippus angustidens.

Common misconception on size[edit]

In elementary-level textbooks,[which?] Eohippus is only described as being "the size of a small Fox Terrier", despite the Fox Terrier being about half the size[ambiguous] of Eohippus. This arcane analogy was so curious that Stephen Jay Gould wrote an essay about it ("The Case of the Creeping Fox Terrier Clone", essay #10 in his book, Bully for Brontosaurus), in which he concluded that Henry Fairfield Osborn had so described it in a widely distributed pamphlet. The reasons for this comparison are unclear, but Gould posits that Osborn, a keen fox hunter, could have made a natural association between horses and the dogs that accompany them.[3]

See also[edit]


  1. ^ MacFadden, B. J. (March 18, 2005). "Fossil Horses--Evidence for Evolution". Science. 307 (5716): 1728–1730. doi:10.1126/science.1105458. PMID 15774746. 
  2. ^ Froehlich, D. J. (2002). "Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla)". Zoological Journal of the Linnean Society. 134 (2): 141–256. doi:10.1046/j.1096-3642.2002.00005.x. 
  3. ^ Gould, S.J. (1991). Bully for Brontosaurus: Reflections in Natural History, W. W. Norton & Co., 540pp.