The extinct genus Hexaprotodon Falconer & Cautley, 1836 (Mammalia, Artiodactyla, Hippopotamidae) in Asia: Paleoecology and Taxonomy

A remarkable aspect, in which the Asian Hexaprotodon (hippopotamus) differs from other fossil mammal taxa, is that in some localities they are abundant, while in other localities, which are fairly similar in faunal composition, their fossils are lacking completely or are very scarce. During geological-paleontological fieldwork in 1989 and 1990 in Pakistan near New Mirpur (Azad Kashmir), it turned out that in the upper part of the Plio-Pleistocene Samwal Formation fossils of hippopotamus are scarce, while in the lower part, also of Plio-Pleistocene age, in the same zoogeographical area and also in fluviatile sediments, they occur frequently To identify whether the presence or absence of hippopotamus in Asian localities can be explained on the basis of paleoecological factors a comparison is made between the sediments, the taphonomy and the fossil assemblages of the Upper and Lower Samwal Formation. It is concluded that the paleoecological characteristics of the faunas are different because of deposition in different sedimentary environments. Several (sub)species of hippopotamus in Africa and Asia have been described. To determine if really so many subspecies need to be distinguished in Asia, a taxonomical study was done, which starts with an overview of the studied material (skulls, mandibles and dentition) and the fossil localities. The basis of taxonomy is the description of the fossils. Therefore, the material is measured and morphological and biometrical descriptions of the material are given. The variation within a species is important for the taxonomy. Yet, certain differences might be related to sexual dimorphism. Also the variation within a species is important for the taxonomy. Therefore, the sexual dimorphism and variation in the extant Hippopotamus amphibius is discussed. It is concluded that the variation in size and morphology in Hippopotamus amphibius is large. A detailed comparison is made of the morphology and biometric data of the studied fossils. Special attention was paid to the localities Kali Glagah, Kedung Brubus and Ngandong on Java (Indonesia), because the bio- and chronostratigraphical positions of these localities are well known. Differences in the hippopotamus material from these three localities are not significant enough to warrant a subdivision in (sub)species. The general trend is that the Kali Glagah hippopotamus is smaller than the one from Kedung Brubus, which again is smaller than that from Ngandong. Biometry shows that the material from Java is clearly smaller than that from continental Asia. A revision of the genus Hexaprotodon from Asia is made. After reviewing the literature and based on the results in this study, it is concluded that during the Plio-Pleistocene, Hexaprotodon sivalensis (Falconer & Cautley, 1836) was the only species in continental Asia. Also on Java there was only one species, Hexaprotodon sivajavanicus Dubois, 1908. Apparently, not as many fossil (sub)species occur in Asia as was suggested by authors like Falconer & Cautley (1836), Dubois (1908), Deraniyagala (1936) and Hooijer (1950). They neglected the large intraspecific variation in the fossil and extant Hippopotamidae.