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. See also Evolution of the horse References Evolution of the Horse Horse Evolution, by Kathleen Hunt Evolution of Horses Fossils of Canada Paleontology in Alberta Eocene horses Oligocene horses Eocene mammals of North America Oligocene mammals of North America Prehistoric placental genera Prehistoric mammals of North America Eocene genus first appearances Oligocene extinctions Fossil taxa described in 1874 Taxa named by Othniel Charles Marsh