New World and Old World Horses

A graph of the interrelationships of New and Old World horses, showing the shift over time from browsing to grazing. In the Eocene, all horses were browsers. By the middle Miocene there was a mixture of browsing and grazing species in North America, but by the end of the Pleistocene only grazing species were present and they were confined to the Old World. From Bruce MacFadden, 1992. Fossil Horses.