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 were smaller than those of other proto-horses. In later genera, these were lost altogether as a result of the development of side ligaments that helped stabilize the middle toe during running. References External links Photo of a Merychippus tooth. Teeth of Merychippus californicus A classification scheme from The Paleobiology Database Florida Museum of Natural History The Evolution of the Horse Biodiversity.org Miocene horses Miocene odd-toed ungulates Transitional fossils Miocene mammals of North America Prehistoric placental genera Miocene genus first appearances Fossil taxa described in 1856