| Hexaprotodon | |
|---|---|
 | |
 | |
| Hexaprotodon sivalensis skull elements | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Family: | Hippopotamidae |
| Subfamily: | Hippopotaminae |
| Genus: | †Hexaprotodon Falconer & Cautley, 1836 |
| Type species | |
| †Hexaprotodon sivalensis | |
| Species | |
See text | |
| Synonyms | |
Hexoprotodon [sic][1] | |
Hexaprotodon is an extinctgenus ofhippopotamid known from Asia and possibly Africa and Europe. The nameHexaprotodon means "six front teeth" as some of the fossil forms have three pairs ofincisors.[2][3] Thepygmy hippopotamus was historically placed in the genus, but today is generally placed in its own genus. The core Asian members of the genus ranged from the Indian subcontinent to Southeast Asia, and are thought to have had an aquatic ecology similar to that of the livingcommon hippopotamus. The last members of the genus became extinct during theLate Pleistocene, perhaps as recently as 15,000 years ago.
The nameHexaprotodon was often applied to thepygmy hippopotamus before its reclassification into the genusChoeropsis. The genus has been historically applied to numerous fossil hippopotamus species spanning Asia, Africa and Europe. The genussensu lato, has been suggested to beparaphyletic with respect to both species of living hippopotamus.[2] The uncontroversial, core Asian members of the genus most closely related to thetype speciesH. sivalensis first appeared around 6 million years ago, during the latestMiocene and were widespread throughoutSouth andSoutheast Asia, with the oldest records coming from theSiwalik Hills of the northern Indian subcontinent.[4][5] The African speciesHexaprotodon bruneti from the Early Pleistocene of Ethiopia may be closely related to the AsianHexaprotodon species, and thus belong in the genus in its more narrow sense. If so, it likely originates from a migration from Asia.[2]
The Asian species ofHexaprotodon, like the livinghippopotamus (Hippopotamus amphibius), but unlike the pygmy hippopotamus are thought to have had a semiaquatic ecology, with their skull shape greatly resembling that ofH. amphibius, with elevatedorbits that allowed them to see above water while submerged. This lifestyle likely evolved independently in bothHexaprotodon and the genusHippopotamus. In comparison toHippopotamus, themandibular symphysis is much more robust, the canine processes do not extendlaterally outwards, and the molar teeth are lower crowned. The more slender and less massive postcranial skeleton compared toH. amphibius also suggests thatHexaprotodon was less adapted to walking in mud.[6]Dental microwear suggests a grazing diet for AsianHexaprotodon species, similar toH. amphibius.[7]
Hexaprotodon was largely extinct by the LateMiddle Pleistocene in Southeast Asia[4] but survived inSumatra into the early Late Pleistocene, with one tooth dated to around 70,000 years ago.[5] The last known populations survived on theIndian Subcontinent to the very end of the Pleistocene,[4] with among the latest dates being around 16,467–15,660 cal yearsBefore Present from bones found in theNarmada River valley in central India. Fossil evidence from a late-surviving IndianHexaprotodon indicates that it lived during a catastrophicdrought caused by the latestHeinrich event, leading to an extremely weakIndian monsoon. It is thought that these drought conditions led to a heavyhabitat fragmentation due toHexaprotodon depending on aquatic habitats, prompting anextinction vortex. Humans may have also facilitated the extinction by hunting the hippopotamuses during this vulnerable state, although no evidence of hippopotamus butchery is known from the Indian subcontinent.[4]
The genusHexaprotodon contains the following species, all from Asia and Africa:
Genussensu lato:
Genussensu stricto:
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