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Merychippus is a proto-horse that lived between 17 and 11 million years ago. It had three toes on each foot and was the first horse known to graze. Though its name means "ruminant horse," it is not thought that Merychippus ruminated.
Merychippus lived in herds. It was about twelve hands (48 inches) tall, making it the tallest equine yet. The muzzle was longer, the jaw deeper, and the eyes wider apart than any other horse-like animal to date. The brain was also much larger, making it smarter and more agile. Merychippus was the first equine to have the distinguishable head of today's horses. The foot was fully supported by ligaments, and the middle toe developed into a hoof, which did not have a pad on the bottom. In some Merychippus species, the side toes were larger, whereas in others, they had become smaller and only touched the ground when running. Its teeth were like those of Parahippus (the extra crest that was variable in Miohippus became permanent and the other teeth were beginning to form a series of tall crests with higher crowns).
By the end of the Miocene era, Merychippus was one of the first speedy grazers. It gave rise to at least nineteen different species of grazers, which can be categorized into three major groups. This burst of evolution is often known as the "Merychippine radiation."
The first was a series of three-toed grazers known as hipparions. These were very successful and split into four genera and at least sixteen species, including small and large grazers and browsers with large and elaborate facial fossae. The second was a group of smaller horses, known as protohippines, which included Protohippus and Calippus. The last was a line of "true equines" in which the side toes became smaller. These were eventually lost altogether as a result of the development of side ligaments that helped stabilize the middle toe during running.
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