Miohippus

Miohippus (meaning "small horse") (whose species are commonly referred to as the three-toed horses) was a genus of prehistoric horse that lived in what is now North America during the late Eocene to early Oligocene Period some 36 million years ago. It is believed to have branched off from Mesohippus, and the two coexisted for about four-eight million years.

Mesohippus had died out by the mid-Oligocene, and Miohippus became much larger than it. They weighed around 40 to 55 kilograms. They were somewhat larger than most earlier Eocene horse ancestors, but still much smaller than modern horses, which typically weigh about 500 kilograms.

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 forests, while the other remained suited to life on prairies. 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 fossils were formerly described under the name Anchitherium. Kalobatippus is then believed to have evolved into a form known as Hyohippus, which became extinct near the beginning of the Pliocene.