Equine Evolution

Horses as we know them today look nothing like their earliest ancestors did when they first appeared. The first horse-like creature lived in the Nearctic and Palearcitc zones during the Eocene period, about 54 million years ago. Unlike modern horses, Hyracotherium boreale (also called Eohippus) was adapted for life in the woodlands and forests that dominated the Eocene. Hyracotherium was much smaller than the modern horse, with an arched back, a short snout, and a small cranium. Its legs were short, and ended in padded four-toed forefeet and three-toed hind feet with a functional hoof on each toe. Hyracotherium was a browsing animal that fed on shrubs, leaves, and branches, as evidenced by its low-crowned teeth and distinctive molars built for grinding. As environmental changes began to occur, Mesohippus bairdi emerged in the Oligocene approximately 33 million years ago. While also a forest browser, Mesohippus had a longer face and snout than Hyracotherium did, and developed more complex premolars with defined cusps. Mesohippus had three toes on both its fore and hind feet, as the fourth toe that Hyracotherium had was reduced to a vestigial nub, and unlike Hyracotherium, Mesohippus had longer legs and a relatively straight and stiff spine that allowed it to run over hard ground.

Rapid environmental change in the Miocene saw the coevolution of abrasive siliceous grasses and the herds of long-legged ungulates that were adapted to eat them. One such ungulate was Merychippus sejunctus, which emerged about 15 million years ago. Merychippus was taller than Mesohippus and its head morphology was much different, as Merychippus was adapted to a diet of tough grasses instead of leaves: it had an elongated muzzle with deeper jaws, and eyes that were set further back in its head to accommodate the large roots of its ever-growing teeth. In addition, to enable it to survive on its diet of abrasive grasses, Merychippus had high-crowned teeth with distinctive cusps and cement between the cusps. It was also adapted for rapid running across grasslands: the two bones in its forearm were fused to eliminate arm rotation, and although it was three-toed the outer toes were reduced while the center toe developed a large, convex hoof.

One of the last equids native to North America was Equus scotti, which lived during the Pleistocene and most resembled today’s horses. Equus scotti had a single hoof on each foot, with side ligaments to prevent twisting, and the remnants of the side toes found in earlier equines were retained as splint bones. Like Merychippus, it had high-crowned, ever-growing with complex cusps, and was well-suited for life in open grasslands.