Eohippus

*Eohippus angustidens*

Julius T. Csotonyi Artwork

Eohippus is an extinct genus of hoofed mammals in the family Equidae. The only recognized species is E. angustidens , which was traditionally considered a species of the genus Hyracotherium . His remains have been found in North America.

## Discovery

In 1876, Othniel C. Marsh described a skeleton as Eohippus validus , from the Greek ηώς (eōs, "dawn") and ιππος (hippos, "horse"), which would mean "dawn horse". Its similarities to the fossils described by Richard Owen were formally noted in a 1932 article by Sir Clive Forster Cooper. E. validus was reclassified in the genus Hyracotherium , which had priority as a genus name, so Eohippus became the most modern synonym of this genus. More recently Hyracotherium was found to be a paraphyletic group of species, and this genus now only includes the species H. leporinum . For its part, E. validus was determined to be identical to a previously named species, Hyracotherium angustidens (Cope, 1875), and the resulting binomial name is then Eohippus angustidens.

In school textbooks, it is common for Eohippus to be described as an animal "about the size of a small Fox Terrier", despite the fact that Fox Terriers are half the size of Eohippus . This arcane analogy was so curious that even Stephen Jay Gould wrote an essay about it ("The Case of the Creeping Fox Terrier Clone," collected in his book Brontosaurus and the Minister's Buttock), in which he concluded that Henry Fairfield Osborn , who had described him in a widely distributed pamphlet, being an enthusiastic fox hunter made a natural association between horses and the dogs that accompany them.

## Description

The only species is E. angustidens, which was long considered a species of Hyracotherium. Living during the Eocene era  approximately 55 to 58 million years ago, Eohippus, the “dawn horse” or more correctly called *Hyracotherium*, is the most ancient ancestor of today’s horse.  From this species descended many “horse-like” animals that make up the “family tree” or in this case, the family bush of the horse family and its cousins.  Its head was short and the eyes were in the middle of the skull.  Its back was rounded and it may have had a striped coat for camouflage.  It lived in the Old World and in North America.

Eohippus had 4 toes on each front foot and 3 toes and a splint bone on the hind feet.  It stood about 12 inches tall at the shoulders. It is hard to estimate the weight but some researchers have suggested that the weight of Hyracotherium may have ranged from the size of a domestic cat (3-5 kg) to a much larger animal of 25-35 kg (MacFadden).

Its teeth were low crowned and included a large canine tooth, theoretically used for fighting for females for the harem. It presumably fed on herbaceous plants and may have followed seasonal migrations. The lower limb bones, called the radius and ulna are not fused together as in the modern horse but are distinct as two separate bones – this permits some rotational movement and may have been helpful in moving through the uneven terrain of the forests.  Hyracotherium may also be the ancestor of the other perrisodactyls (hoofed odd-toed animals) like the tapirs and rhinos, and the extinct strange chalicotheres and enormous titanotheres.

## Appearance in other media

### *Jurassic Park*

* It appears in illustrations in the Smithsonian Jurassic World Dominion Prehistoric Projector Kit alongside Gastornis.

## References

* https://www.jstor.org/stable/2400511
* https://www.britannica.com/animal/dawn-horse
* https://www.equineguelph.ca/equimania/EvolutionTimeline/eohippus.html
* https://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.1988.tb00457.x
* https://www.researchgate.net/publication/229963781\_Quo\_vadis\_Eohippus\_The\_systematics\_and\_taxonomy\_of\_the\_early\_Eocene\_equids\_Perissodactyla
* https://www.researchgate.net/publication/276906615\_From\_%27dawn\_horse%27\_to\_racehorse
* https://www.researchgate.net/publication/229944669\_Fossil\_horses\_from\_Eohippus\_Hyracotherium\_to\_Equus\_2\_rates\_of\_dental\_evolution\_revisited