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Miohippus (meaning "small horse") is an extinct genus of horse existing longer than most Equidae. It lived in what is now North America from 32 to 25 million years ago, during the late Eocene to late Oligocene. According to the Florida Museum of Natural History, Othniel Charles Marsh first believed Miohippus lived during the Miocene and thus named the genus using this incorrect conclusion. More recent research provides evidence that Miohippus actually lived during the Paleogene period.
Miohippus species are commonly referred to as the three-toed horses. Their range was from Alberta, Canada to Florida to California.
Miohippus was larger than Mesohippus and had a slightly longer skull. Its facial fossa was deeper and more expanded, and the ankle joint was subtly different. Miohippus also had a variable extra crest on its upper molars, which gave it a larger surface area for chewing tougher forage. This would become a typical characteristic of the teeth of later equine species.
Miohippus had two forms, one of which adjusted to the life in , while the other remained suited to life on . The forest form led to the birth of Kalobatippus (or Miohippus intermedius), whose second and fourth finger again elongated for travel on the softer primeval forest grounds. The Kalobatippus managed to relocate to Asia via the Bering Strait land bridge, and from there moved into Europe, where its were formerly described under the name Anchitherium. Kalobatippus is then believed to have evolved into a form known as Hypohippus, which became extinct near the beginning of the Pliocene.
As many as eight species of Miohippus were described from the John Day Formation of Oregon, but recent work on the dental variation has determined that only one species of Miohippus was present within a given member.
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