- This is the article on the family Hippopotamidae; for the main article on hippos, see Hippopotamus
Temporal range: Middle Miocene–Recent
†Trilobophorous Geze, 1985
Hippopotaminae Gray, 1821
†Kenyapotaminae Pickford, 1983
Hippopotamuses are stout, naked-skinned, and amphibious artiodactyl members of the family Hippopotamidae possessing three-chambered stomachs and walking on four toes on each foot. While they resemble pigs physiologically, their closest living relatives are actually the whales.
There are two living species of hippopotamus among two genera; the pygmy hippo, Choeropsis liberiensis of the forests of west Africa, and the common Hippo, Hippopotamus amphibius, which kills more humans yearly than any other mammal.
Hippopotamuses 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.
Both the incisors and canines are large and tusk-like, although the canine tusks are by far the larger. The tusks grow throughout life. The postcanine teeth are large and complex, suited for chewing the plant matter that comprises their diet. The number of incisors varies even within the same species, but the general dental formula is as given below.
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 relationship to each other - apart from being fairly distant relatives - is not well resolved.
The enigmatic Miocene Kenyapotamus is insufficiently known to be assigned a place in the hippo phylogeny with any degree of certainty. In addition, the genus Hexaprotodon, in a sense now restricted to an extinct group of animals once living around the northern and northeastern Indian Ocean, which formerly included most ancient hippos, has turned out to be paraphyletic.
Analogous structures 
The lower canine teeth of hippopotamids are similar in function and structure to the tusks of elephants. While hippopotamids and elephants are only very distantly related within the Mammalia, the lower canine teeth of both groups are long and have a slight curve, and species of both families use this structure when fighting.
- Genus Hippopotamus - true hippopotami
- Hippopotamus amphibius - Hippopotamus Africa
- †Hippopotamus antiquus - mainland Europe & British Isles; Pleistocene
- †Hippopotamus creutzburgi - Crete; Pleistocene
- †Hippopotamus minor - Cyprus; Pleistocene to Holocene
- †Hippopotamus melitensis - Malta; Pleistocene
- †Hippopotamus pentlandi - Sicily; Pleistocene
- †Hippopotamus lemerlei - Madagascar; Holocene
- †Hippopotamus laloumena - Madagascar; Holocene
- †Hippopotamus gorgops - Africa & mainland Europe; Late Miocene–Middle Pleistocene
- Tentatively placed into Hippopotamus:
- Genus Hexaprotodon - hexaprotodons or Asian hippopotami
- †Hexaprotodon bruneti - Ethiopia; Pliocene
- †Hexaprotodon coryndoni - Ethiopia; Pliocene
- †Hexaprotodon crusafonti - Spain; Late Miocene (syn. Hexaprotodon primaevus)
- †Hexaprotodon hipponensis - Algeria
- †Hexaprotodon imagunculus - Uganda & Congo; Pliocene
- †Hexaprotodon iravaticus - Myanmar; Pliocene - Pleistocene
- †Hexaprotodon karumensis - Kenya & Eritrea; Pleistocene
- †Hexaprotodon madagascariensis - Madagascar; Holocene
- †Hexaprotodon namadicus - India; (possibly same as Hex. palaeindicus)
- †Hexaprotodon palaeindicus - India;
- †Hexaprotodon pantanellii - Italy; Pliocene
- †Hexaprotodon protamphibius - Kenya & Chad; Pliocene
- †Hexaprotodon siculus -
- †Hexaprotodon sivalensis - India
- †Hexaprotodon sp. (undescribed) - Myanmar
- †Undescribed Indonesian species; Pleistocene
- Genus Archaeopotamus - formerly included in Hexaprotodon
- 1-2 undescribed species
- Genus Choeropsis - pygmy hippopotamus
- Choeropsis liberiensis - Pygmy Hippopotamus
- Genus Saotherium - formerly included in Hexaprotodon
- Saotherium mingoz Chad; Pliocene
References and notes 
- Boisserie, Jean-Renaud (2005): The phylogeny and taxonomy of Hippopotamidae (Mammalia: Artiodactyla): a review based on morphology and cladistic analysis. Zool. J. Linn. Soc. 143(1): 1-26. doi:10.1111/j.1096-3642.2004.00138.x (HTML abstract)
- "Hippopotamidae". Integrated Taxonomic Information System. Retrieved 11 August 2004.
- Petronio, C. (1995): Note on the taxonomy of Pleistocene hippopotamuses. Ibex 3: 53-55. PDF fulltext