Perissodactyla

The order Perissodactyla are browsing and grazing mammals which includes horses, tapirs and rhinoceroses. The odd-toed ungulates (animals having an odd number of toes on each hoof) are usually large, have relatively simple stomachs and a large middle toe. In contrast to the ruminant Artiodactyl ungulates, perissodactyls are hindgut fermenters; that is, they digest plant cellulose in their intestines rather than stomach.

Evolution
Although no certain records are known prior to the early Eocene, the odd-toed ungulates probably arose in what is now Asia during the late Paleocene, less than 10 million years after the Cretaceous-Tertiary extinction event. By the start of the Eocene (55 million years ago) they had diversified and spread out to occupy several continents. The horses and tapirs both evolved in North America; the rhinoceroses appear to have developed in Asia from tapir-like animals and then recolonised the Americas during the middle Eocene (about 45 million years ago). There were approximately 15 families, of which only three survive (McKenna and Bell, 1997; Hooker, 2005). These families were very diverse in form and size; they included the enormous brontotheres and the bizarre chalicotheres. The largest perissodactyl, an Asian rhinoceros called Paraceratherium, reached 12 ST, more than twice the weight of an elephant.

Perissodactyls were the dominant group of large terrestrial browsers right through the Oligocene. However, the rise of grasses in the Miocene (about 20 million years ago) saw a major change: the even-toed ungulates with their more complex stomachs were better able to adapt to a coarse, low-nutrition diet, and soon rose to prominence. Nevertheless, many odd-toed species survived and prospered until the late Pleistocene (about 10,000 years ago) when they faced the pressure of human hunting and habitat change.