Odd-toed ungulate

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The odd-toed ungulates are browsing and grazing mammals such as horses, tapirs and rhinoceroses whose hooves each feature an odd number of toes. The middle toe on each hoof is also usually larger than its neighbors. Together, odd-toed ungulates form the order Perissodactyla (perissos abundant/excessive + daktulos toes).

They are relatively large and, unlike the ruminant even-toed ungulates (artiodactyls), they have relatively simple stomachs. This is because they are hindgut fermenters, digesting plant cellulose in their intestines rather than in one or more stomachs.

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Evolution

Although no certain records are known prior to the early Eocene, the odd-toed ungulates probably[citation needed] 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;[2] 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 short tons (11 t), more than twice the weight of an elephant.

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