Hoofed Mammals

Ungulates (meaning roughly "hoofed" or "hoofed animal") make up several orders of mammals, of which six to eight survive. There is some dispute as to whether ungulate should be treated as an actual cladistic (evolution-based) group, or merely a phenetic group (similar, but not necessarily related), in light of the fact that all ungulates do not appear to be as closely related as once believed (see below). Ungulata was formerly considered an order which has been split into Perissodactyla and Artiodactyla. Members of these two orders are called the 'true ungulates' to distinguish them from 'subungulates' which include members from the Proboscidea, Sirenia, and Hyracoidea orders.

The even-toed ungulates form the mammal order Artiodactyla. They are ungulates whose weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in perissodactyls. There are about 220 artiodactyl species, including many that are of great economic importance to humans.

As with many mammal groups, even-toed ungulates first appeared during the Early Eocene (about 54 million years ago). In form they were rather like today's chevrotains: small, short-legged creatures that ate leaves and the soft parts of plants. By the Late Eocene (46 million years ago), the three modern suborders had already developed: Suina (the pig group); Tylopoda (the camel group); and Ruminantia (the goat and cattle group). Nevertheless, artiodactyls were far from dominant at that time: the odd-toed ungulates (ancestors of today's horses and rhinos) were much more successful and far more numerous. Even-toed ungulates survived in niche roles, usually occupying marginal habitats, and it is presumably at that time that they developed their complex digestive systems, which allowed them to survive on lower-grade feed.

The appearance of grasses during the Eocene and their subsequent spread during the Miocene (about 20 million years ago) saw a major change: grasses are very difficult to digest and the even-toed ungulates with their highly-developed stomachs were better able to adapt to this coarse, low-nutrition diet, and soon replaced the odd-toed ungulates as the dominant terrestrial herbivores.

The artiodactyls fall into two groups which, despite underlying similarities, are rather different. The suoids (pigs, hippos, and peccaries) retain four toes, have simpler molars, short legs, and their canine teeth are often enlarged to form tusks. In general, they are omnivores and have a simple stomach. (The two hippopotamus species are exceptions.)

The camelids and the Ruminantia, on the other hand, tend to be longer-legged, to have only two toes, to have more complex cheek teeth well-suited to grinding up tough grasses, and multi-chambered stomachs. Not only are their digestive systems highly developed, they have also evolved the habit of chewing cud: regurgitating partly-digested food to chew it again and extract the maximum possible benefit from it.

Lastly a group of artiodactyls, which molecular biology suggests were most closely related to Hippopotamidae, returned to the sea to become whales.

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: the suborder Hippomorpha, horses, which have only one toe and tend to be fast runners with long legs, and the suborder 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 were 12 families, of which only three survive. 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 tons, 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.

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