Mesohippus

Mesohippus is an extinct genus of early horses that lived during the Oligocene, roughly 37 to 32 million years ago. Its forefeet and hind feet each had three toes, with the enlarged middle toe bearing most of its weight.

Equidae: Equidae is the mammal family containing horses, asses, and zebras, along with their extinct relatives. Mesohippus belongs to this family of living and extinct equids.
Graviportal locomotion: Graviportal locomotion is weight-bearing movement supported by robust limbs, typical of very large terrestrial mammals. Mesohippus was a lightweight runner rather than a graviportal animal.
White River Formation: The White River Formation is a sequence of Oligocene sedimentary rocks in the Great Plains of North America. Its fossil beds preserve abundant remains of Mesohippus and other Oligocene mammals.
Mesohippus bairdi: Mesohippus bairdi is a named species of the extinct equid genus Mesohippus. It is among the species used to describe the genus's Oligocene diversity.
Perissodactyla: Perissodactyla is the mammal order of odd-toed ungulates, including horses, rhinoceroses, and tapirs. Its odd-toed ancestry places Mesohippus within this broader mammal order.
Mesaxonic foot: A mesaxonic foot has its central third digit as its main axis of support. Mesohippus carried most of its weight on its enlarged middle toe.
John Day Fossil Beds: John Day Fossil Beds are fossil-bearing rock formations in Oregon spanning much of the Cenozoic. They preserve Oligocene equid fossils associated with Mesohippus.
Mesohippus westoni: Mesohippus westoni is a named species of the extinct equid genus Mesohippus. Its fossils provide another species-level example within the genus.
Oligocene: The Oligocene is the geological epoch from about 33.9 to 23 million years ago. Most Mesohippus fossils come from this epoch.
Digit reduction in horses: Digit reduction in horses is the evolutionary loss of side toes as the central digit becomes dominant. Mesohippus retained three toes, an intermediate condition in this long-term trend.

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