Habitat Reconstruction of the Pleistocene Hippopotamidae from the Siwaliks of Pakistan Using Stable Isotopes

Stable isotopes of carbon and oxygen (δ13C and δ18O) archived in fossil tooth enamel are essential tools to reconstruct the palaeoecology and palaeohabitat of mega-herbivores. Among the mega-herbivores, hippopotamus (Hippos) are the third-largest existing artiodactyls. The family Hippopotamidae consists of semi-aquatic, mostly herbivorous non-ruminant animals. Hippos first appeared in the late Miocene (~ 6 Ma) which was the time of substantial faunal turnover and expansion of C4 vegetation in low latitude regions globally, due mainly to lowered atmospheric CO2 as well as other environmental factors. However, studies regarding the habitat of mega-herbivores are inadequate. In this paper, we reconstruct the habitat of Hexaprotodon sivalensis recovered from the early Pleistocene (ca. 2.5–2.2 Ma), Pabbi Hills locality of the Siwalik sub-Group of Pakistan. We used bulk stable isotope analyses of oxygen and carbon which provide temporal and environmental information on the animal’s ecology and diet. The average δ13Cenamel and δ18Oenamel are − 0.6 ± 1.4‰ and − 2.1 ± 2.1‰, respectively, which indicate that during the early Pleistocene, the diet of H. sivalensis was dominantly composed of C4 vegetation which was present mainly in the form of open grasslands. The high δ18Oenamel values indicate that H. sivalensis inhabited freshwater ponds under drier climatic conditions where they fed upon the C4 vegetation exclusively. However, enamel thickening/complexity and high crowns also provide direct evidence for pure C4 grass-dominated vegetation along with higher δ13Cenamel in the Siwaliks, like modern hippos from Africa.