†Trilobophorous Geze, 1985
Hippopotaminae Gray, 1821
†Kenyapotaminae Pickford, 1983
Hippopotamuses are the members of the family Hippopotamidae. They are the only extant artiodactyls which walk on four toes on each foot.
Hippopotamids are large mammals, with short, stumpy legs, and barrel-shaped bodies. They have large heads, with broad mouths, and nostrils placed at the top of the snout. Like pigs, they have four toes, but unlike pigs, all of the toes are used in walking. Hippopotamids are unguligrade, although, unlike most other such animals, they have no hooves, instead using a pad of tough connective tissue. Their stomach has three chambers, but they are not true ruminants.
The living species are smooth-skinned and lack both sebaceous glands and sweat glands. The outer epidermis is relatively thin, so that hippos dehydrate rapidly in dry environments.
Both the incisors and canines are large and tusk-like, although the canine tusks are by far the largest. The tusks grow throughout life. The postcanine teeth are large and complex, suited for chewing the plant matter that composes their diet. The number of incisors varies even within the same species, but the general dental formula is:
The hippopotamids are descended from the anthracotheres, a family of semi-aquatic artiodactyls that appeared in the late Eocene, and are popularly thought to have resembled small- or narrow-headed hippos. More specifically, the hippos split off from the anthracotheres some time during the Miocene. After the appearance of the true hippopotamids, the anthracotheres went into a decline brought about by a combination of climatic change and competition with their descendants, until the last genus, Merycopotamus, died out in the early Pliocene of India.
There were once many species of hippopotamid, but only two survive today: Hippopotamus amphibius, and Choeropsis liberiensis. They are the last survivors of two major evolutionary lineages, the hippos proper and the pygmy hippos, respectively; these lineages could arguably be considered subfamilies but their relationships to each other - apart from being fairly distant relatives - are not well resolved.
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