Proto-horses: Lengthening the legs
At the time that the sloths and other members of the Edentates (e.g. Glyptodon) were evolving in the south, on the other side of the Panamastrait in North America, another different group of grass-eaters were developing on the prairies. Their ancestors were forest-living creatures, not unlike tapirs but far smaller. Their molar teeth were rounded and suited to forest browsing. On the plains, in order to escape their predators, they began to run faster. The earliest forms (Hyracotherium) run on four toes on their frontlimbs and three toes on their hindlimbs. The longer the limbs, the better they serve as levers and, properly muscled, the faster they can propel their owners. As time passed these grazers lengthened their legs by rising off the ground onto their toes. The side toes started to dwindle and the animal, an early horse the size of a dog, was running on a single elongated middle toe (Mesohippus). The reduction of the side toes continued (Merychippus). The ankle bones thus became placed halfway up its legs, the side toes were reduced to internal vestiges called the splint bones, and the nail thickened to form the protective shock-absorbent hooves (Pliohippus).