Hippopotamidae

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Hippopotamidae
Fossil range: Middle Miocene to Recent
Hippopotamus amphibius
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
(unranked) Whippomorpha
Family: Hippopotamidae
Gray, 1821
Genera

Hippopotamus
Choeropsis
†Hexaprotodon
†Archaeopotamus
†Saotherium

Hippopotamuses (colloquially also "Hippopotami") are the members of the family Hippopotamidae. They are the only extant artiodactyls which walk on four toes on each foot.

Contents

[edit] Characteristics

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 both smooth-skinned, and lack both sebaceous glands and sweat glands. The outer epidermis is relatively thin, so that hippos dehydrate rapidly in dry environments.[1]

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:

2-3.1.4.3
1-3.1.4.3

[edit] Evolution

Main article: Hippopotamus#Evolution

The hippopotamids are descended from the anthracotheres, a family of semi-aquatic artiodactyls that appeared sometime during 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 Africa.

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.

The enigmatic Kenyapotamus is insufficiently known to be assigned a place in the hippo phylogeny with any degree of certainty. In addition, the genus Hexaprotodon, now restricted to a group of animals living around the northern and northeastern Indian Ocean, formerly included most ancient hippos but this turned out to be paraphyletic.

[edit] Species

The systematics and taxonomy used here mostly follows the review of Boisserie (2005)[2].

Genus Hippopotamus - true hippopotami

Tentatively placed into Hippopotamus:

  • Hippopotamus karumensis (fossil)
  • Hippopotamus aethiopicus (fossil)
  • Hippopotamus protamphibius (fossil)
  • Hippopotamus coryndoni (fossil)
  • Hippopotamus afarensis (fossil) - formerly genus Trilobophorus

Genus Hexaprotodon - hexaprotodons or Asian hippopotami

  • Hexaprotodon palaeindicus - Indian Hippopotamus (fossil)
  • Hexaprotodon namadicus - Lesser Indian Hippopotamus (fossil) - possibly same as H. palaeindicus
  • Hexaprotodon bruneti - Afar Hexaprotodon (fossil)
  • Hexaprotodon sivalensis - Sivalik Hippopotamus (fossil)
  • Hexaprotodon sp. - Myanmar Hippopotamus (fossil)
  • and some Indonesian species from the Pleistocene

Genus Archaeopotamus - formerly included in Hexaprotodon

Genus Choeropsis - pygmy hippopotami

Genus Saotherium - formerly included in Hexaprotodon

[edit] References and Notes

  1. ^ Laws, Richard (1984). in Macdonald, D.: The Encyclopedia of Mammals. New York: Facts on File, 506-511. ISBN 0-87196-871-1. 
  2. ^ Boisserie identifies the species Hippopotamus minor as Phanourios minutus, but this genus is not widely recognized.