Hexaprotodon sivalensis

Hexaprotodon sivalensis was an extinct hippopotamid from the Neogene and Quaternary of South Asia, known mainly from remains recovered from the Siwalik Group, one of the most important continental fossil-bearing sequences of the Indian subcontinent. Traditionally associated with deposits from the latest Miocene, Pliocene, and Pleistocene, this taxon represents one of the most characteristic forms of Asian fossil hippopotamids and helps illustrate the ancient expansion of Hippopotamidae outside Africa. Its generic name, Hexaprotodon, refers to the presence of six anterior incisors, a feature that distinguishes it from modern hippopotamuses of the genus Hippopotamus, while the specific epithet sivalensis refers to the Siwalik region, where many of its remains have been documented.

Anatomically, Hexaprotodon sivalensis was a large semi-aquatic mammal with a robust body, short and powerful limbs, a massive skull, and deep jaws, adapted to a lifestyle closely associated with rivers, lakes, and wetland environments. Like other hippopotamids, it had a specialized dentition, with well-developed incisors and canines at the front of the mouth, as well as broad-crowned molars suited to processing plant material. The presence of six incisors is one of the most important features for its identification, contrasting with the four-incisor condition seen in the living common hippopotamus, Hippopotamus amphibius. This dental difference reflects a greater evolutionary diversity among fossil hippopotamids than is observed in the two living species.

Remains attributed to Hexaprotodon sivalensis include mandibles, isolated teeth, cranial fragments, and postcranial elements recovered from several localities in India and Pakistan. In the Siwalik fossil record, the species appears relatively scarce but significant, associated with large-mammal faunas that include proboscideans, rhinoceroses, bovids, equids, suids, carnivorans, and primates. This faunal association indicates open and riverine ecosystems, with alternating floodplains, gallery forests, wetlands, and grassy areas, where Hexaprotodon sivalensis would have occupied an ecological niche similar to that of modern hippopotamuses: a large amphibious herbivore dependent on water for refuge, thermoregulation, and movement, while feeding mainly on land or along vegetated margins.

Although it is often compared with the modern common hippopotamus, Hexaprotodon sivalensis should not be understood simply as an ancient version of it, but rather as part of an Asian radiation of hippopotamids with its own distinctive features. Studies on fossil hippopotamids from South Asia have indicated that Hexaprotodon was especially representative in the Indian subcontinent and Southeast Asia, with a complex evolutionary history and a distribution that extended through different intervals of the late Neogene and Pleistocene. Its presence in the Siwalik fossil record makes it an important component of Asian megafaunal communities that predate modern faunas.

This replica represents Hexaprotodon sivalensis as a large fossil hippopotamid with a heavy build, robust head, low body, and powerful limbs, emphasizing its semi-aquatic adaptation and its distinctive six-incisor dental configuration. Its reconstruction integrates the available information on its geological provenance, mandibular anatomy, herbivorous ecology, and role within the riverine ecosystems of South Asia, offering a rigorous representation of one of the most representative extinct hippopotamuses from the Siwalik fossil record.

Approximate measurements of Hexaprotodon:

Scale 1:35 — Complete model
Length: 85 mm
Height: 39 mm
Width: 33 mm
Snout-to-tail length: 86 mm

Scale 1:20 — Complete model
Length: 149 mm
Height: 68 mm
Width: 58 mm
Snout-to-tail length: 150 mm