Eohippus

Eohippus, (genus Hyracotherium), extinct group of mammals that were the first known horses. They flourished in North America and Europe during the early part of the Eocene Epoch (56 million to 33.9 million years ago). Even though these animals are more commonly known as Eohippus, a name given by the American paleontologist Othniel Charles Marsh, they are properly placed in the genus Hyracotherium, the name given earlier by British paleontologist Richard Owen.

Hyracotherium was a form close to the common ancestry of all the odd-toed hoofed mammals, the perissodactyls. It stood 30–60 cm (1–2 feet) high at the shoulder, depending on the species. The skull varied in length; some species had a relatively short face, but in others the face was long and more horselike. Since the hind legs were longer than the forelegs, Hyracotherium was adapted to running and probably relied heavily on running to escape predators. The body was lightly constructed and raised well off the ground, its slender limbs supported by toes held in an almost vertical position. Although four toes were present on the front feet and three on the hind feet, all feet were functionally three-toed, and each toe ended in a small hoof. The incisors of Hyracotherium were small, and the cheek teeth had low crowns, which indicated that the animal was a browser that fed on leaves rather than grass. Hyracotherium was succeeded by Orohippus, which differed from Hyracotherium primarily in dentition.