DAWN HORSE

extinct genus (Hyracotherium) of ancestral horses that flourished in North America and Europe during the Early Eocene Epoch (57.8 to 52 million years ago). The North American species were formerly grouped separately in the genus Eohippus, but all known species of dawn horses are now recognized within the single genus Hyracotherium. The dawn horse was a form close to the common ancestry of all the odd-toed hoofed mammals, the perissodactyls, that stood 30 to 60 cm (1 to 2 feet) high at the shoulder, depending on the species. It was adapted to running, with hind legs longer than the forelegs. The body was lightly constructed, with slender limbs and elongated feet. The skull varied in length in different individuals: some specimens have a relatively short face or region in front of the eyes, a primitive condition; in other specimens, the face is relatively long and more horselike in appearance. The body was raised well off the ground and supported by the toes, which were held in an almost vertical position. Even though four toes were present on the front feet and three on the hind feet, all the feet were functionally three-toed. Each toe ended in a small hoof. In the dawn horse the incisor teeth were small, and the cheek teeth had exceptionally low crowns. In general the teeth were still primitive, but progressive trends are evident. It was a browsing form rather than a grazer and had probably already come to rely heavily upon running in order to escape predators. The dawn horse was succeeded by Orohippus, which differed from it primarily in dentition.