Species composition and distribution of middle pleistocene mammalian fauna in Khok Sung subdistrict, Nakhon Ratchasima province and its contribution to vertebrate community

The terrace deposit of Khok Sung, Nakhon Ratchasima province, has yielded the richest Pleistocene vertebrate fauna of Thailand, where abundant fossil mammals and reptiles (skulls, isolated teeth, and postcranial remains) were recovered. The mammalian fauna consists of at least 15 recognized species in 13 genera, including a primate, proboscideans, carnivores rhinoceroses, suids, bovids, and cervids, characterized by mostly extant elements associated to some extinct (Stegodon cf. orientalis) and extirpated (Crocuta crocuta ultima, Rhinoceros unicornis, Sus barbatus, and Axis axis) taxa. Three reptilian taxa: Crocodylus cf. siamensis, Python sp., and Varanus sp. are also identified. The Khok Sung mammalian taxa characterize the Pleistocene Ailuropoda–Stegodon faunal complex found throughout the subtropical to tropical forested regions of South China and mainland Southeast Asia. A chital, Axis axis, whose distribution is today restricted to the Indian Subcontinent, is reported here for the first time in Southeast Asia during the Pleistocene. The age of the Khok Sung fauna is tentatively attributed to the late Middle Pleistocene as either 188 or 213 ka, based on the paleomagnetic data and on the faunal comparisons. According to an analysis of the faunal similarity using the Simpson index, the Khok Sung mammalian fauna is most similar to that of Thum Wiman Nakin (northeastern Thailand), whose age has been dated to older than 169 ka. Compared to other Southeast Asian Pleistocene and extant faunas, the Khok Sung mammal assemblage yields most of mainland Southeast Asian taxa that migrated to Java during the latest Middle Pleistocene, supporting the hypothesis that Thailand was a part of the Sino-Malayan migration route from South China to Java. The Sunda shelf, forming when the sea-levels dropped during glacial stages, is supposed to provide the only possible route of mammalian dispersal between Southeast Asian mainland and islands. The Khok Sung fauna illustrates an open grassland landscape with abundant and diversified herbivores, close to the main river channel. A cenogram analysis of the mammalian fauna reflects a relatively humid condition for Khok Sung, similar to that of Thum Wiman Nakin, during the late Middle Pleistocene.