Today there is only one horse genus: Equus. However, in the past 56 million years there have been many different kinds of horses. The ancestor of the horse is the dog-sized Hyracotherium or Eohippus. These small, forest-dwelling animals had four-toed front feet, three-toed hind feet, and were designed for running. About 50 million years ago, these small ancestors of the horse became extinct in Europe, but continued to evolve in North America.
The San Bernardino County Museum in Redlands, California, has a display on the evolution of the horse. According to the Museum display:
“Horses originated in North America. They became extinct here at the end of the Ices Ages, but survived in other parts of the world.”
The evolution of the horse involved changes in the feet. The small forest-dwelling horses with multiple toes gradually evolved into animals with three toes and then just one toe.
According to the Museum display:
“Forest-dwelling horses munched on leaves. Horses on open plains grazed on grass. Grass contains silica, which is very abrasive.”
The evolution from leaf-eating to grass-eating required changes in the teeth which led to tall, long faces to provide room for large, long teeth. The evolution from small forest browsers to large grassland grazers occurred by 1.9 million years ago. According to Ian Lange, in his book Ice Age Mammals of North America: A Guide to the Big, the Hairy, and the Bizarre:
“Leg and foot length and body size of the Ice Age horses generally increased, with their brains increasing in size relative to their body mass. With these developments came a reduction in number of toes as the keratin-covered central to became a hoof.”
Shown above is a comparison of the modern horse skull (top) with that of Archaeohippus, a 15-million-year-old horse skull uncovered in San Bernardino County.
The display shown above shows the evolution of the horse’s toe. The horse in the background in a life-sized model of Eohippus. From left to right: (1) Archaeohippus from 15 million years ago with three toes touching the ground; (2) Scaphohippus intermontanus from 14 million years ago with three toes but only one touching the ground; and (3) Equus africanus asinus (i.e. burro) with one toe touching the ground.
Shown above is the skull of Equus scotti. Arabian horses may be descendants of Scott’s horse.
Scott’s horse was the size of a large modern horse and remains of this horse have been found in sites in Texas, Oklahoma, Florida, and Alberta.
Ancient horses migrated from North America into Asia and eventually Europe. By about 8,000 years ago, all of the North American horses had become extinct.
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