The small equids in the extinct ancient genus Eohippus are ungulates. The sole species is E. angustidens, which was once thought to be a Hyracotherium species. Its remnants, which belong to the Early Eocene (Ypresian stage), have been found in North America.
Discovery
Othniel C. Marsh named a skeleton Eohippus validus in 1876. The Greek terms for "dawn horse" are ἠώς (eōs, "dawn") and ἵππoς (hippos, "horse"). Its resemblance to the fossils that Richard Owen described was officially noted by Clive Forster Cooper in a 1932 publication. Eohippus was made a junior synonym of the genus Hyracotherium, which was given precedence over E. validus as the name of the genus. As a result of the recent discovery that the species in Hyracotherium are paraphyletic, only H. leporinum remains in the genus. Eohippus angustidens is the resultant binomial, as E. validus was discovered to be similar to an earlier-named species, Orohippus angustidens Cope, 1875.
Description
At the shoulder, Eohippus measured around 30 cm (12 in), or three hands' height. Each toe on its front foot ends in a hoof, while its rear feet have three toes. Modern Equus is similar to its incisors, molars, and premolars. However, Eohippus's massive canine teeth set it apart from other species.
Stephen Jay Gould's Comments
Even though most readers would be fairly unfamiliar with that breed of dog, Stephen Jay Gould bemoaned the ubiquity of a much-used term to denote Eohippus size ("the size of a small Fox Terrier") in his 1991 article, "The Case of the Creeping Fox Terrier Clone". He came to the conclusion that the term first appeared in a widely circulated book written by Henry Fairfield Osborn. Osborn was an avid fox hunter, so it seems to reason that he would have drawn a connection between his horses and the dogs that went with them.
Sources
A restoration of Eohippus by Charles Knight.
E. angustidens skeleton in the National Museum of Natural History in Washington, D.C., United States.