Extinct hippopotamus genus
Hexaprotodon is an extinct genus of hippopotamid known from Asia and possibly Africa and Europe. The name "Hexaprotodon" means "six front teeth" as some of the fossil forms have three pairs of incisors. The pygmy 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 living common hippopotamus. The last members of the genus became extinct during the Late Pleistocene, perhaps as recently as 15,000 years ago.
Taxonomy.
The name "Hexaprotodon" was often applied to the pygmy hippopotamus before its reclassification into the genus "Choeropsis". The genus has been historically applied to numerous fossil hippopotamus species spanning Asia, Africa and Europe. The genus "sensu lato", has been suggested to be paraphyletic with respect to both species of living hippopotamus. The uncontroversial, core Asian members of the genus most closely related to the type species "H. sivalensis" first appeared around 6 million years ago, during the latest Miocene and were widespread throughout South and Southeast Asia, with the oldest records coming from the Siwalik Hills of the northern Indian subcontinent. The African species "Hexaprotodon bruneti" from the Early Pleistocene of Ethiopia may be closely related to the Asian "Hexaprotodon" species, and thus belong in the genus in its more narrow sense. If so, it likely originates from a migration from Asia.
Description.
The Asian species of "Hexaprotodon", like the living hippopotamus ("Hippopotamus amphibius"), but unlike the pygmy hippopotamus are thought to have had a semiaquatic ecology, with their skull shape greatly resembling that of "H. amphibius", with elevated orbits that allowed them to see above water while submerged. This lifestyle likely evolved independently in both "Hexaprotodon" and the genus "Hippopotamus". In comparison to "Hippopotamus", the mandibular symphysis is much more robust, the canine processes do not extend laterally outwards, and the molar teeth are lower crowned. The more slender and less massive postcranial skeleton compared to "H. amphibius" also suggests that "Hexaprotodon" was less adapted to walking in mud. Dental microwear suggests a grazing diet for Asian "Hexaprotodon" species, similar to "H. amphibius". 
Extinction.
"Hexaprotodon" was largely extinct by the Late Middle Pleistocene in Southeast Asia but survived in Sumatra into the early Late Pleistocene, with one tooth dated to around 70,000 years ago. The last known populations survived on the Indian Subcontinent to the very end of the Pleistocene, with among the latest dates being around 16,467–15,660 cal years Before Present from bones found in the Narmada River valley in central India. Fossil evidence from a late-surviving Indian "Hexaprotodon" indicates that it lived during a catastrophic drought caused by the latest Heinrich event, leading to an extremely weak Indian monsoon. It is thought that these drought conditions led to a heavy habitat fragmentation due to "Hexaprotodon" depending on aquatic habitats, prompting an extinction 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.
Species.
The genus "Hexaprotodon" contains the following species, all from Asia and Africa:
Genus "sensu lato":
Genus "sensu stricto":
References.
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