The odd-toed ungulates or Perissodactyla are large to very large browsing and grazing mammals with relatively simple stomachs and a large middle toe. The members of the order fall into two groups: horses, which have only one toe and tend toward being fast runners with long-legs, and the sub-order Ceratomorpha, which contains two families of slower-moving, thick-set animals with several functional toes: the tapirs and the rhinoceroses.
The odd-toed ungulates arose in what is now North America in the late Paleocene, less than 10 million years after the dinosaurs died out. 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 reinvaded the Americas during the middle Eocene (about 45 million years ago). There was a great diversity of form among them, with 12 families in total (of which only three survive), and enormous diversity of size as well, with the largest species, an Asian rhinoceros called Paraceratherium reaching 12 tonnes—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.
Of the original 12 families of odd-toed ungulate, only 3 remain today.