AGES AND PALEOENVIRONMENT OF MIOCENE MAMMALIAN FAUNAS FROM THAILAND

The geological survey of continental basins in northern Thailand has l ed to the discovery and the study of several mammalian assemblages in that area. All localities from northern Thailand occur in a time span ranging from about 16 to 14 Ma. In addition, a diachronism is apparent among the different localities and is indicated by the different stag es of evolution in the tooth morphology of rodents, ruminants, rhinos and mastodonts. Mae Long (Li Basin) would be the oldest locality (abou t 16 Ma), followed by Had Pu Dai (Lampang Basin), and then Huai Siew a nd Ban San Klang (both in Pong Basin). Ban Na Sai (Li Basin) and Mae T eep would be contemporaneous with Ban San Klang, and Mae Moh is though t to be the youngest locality. In order to study the paleoenvironment of the Thai Neogene faunas, we have used the cenogram method in which structure reflects the general environment of a faunal community. We a lso compared the faunal communities from Thailand with those from the Pakistan Siwaliks and from China. In the latter areas, the microfauna is well represented and the large mammals are numerous, thus suggestin g a wet environment with a likely monsoonal climate. The Middle Miocen e paleoenvironment in South Asia seems to have been quite open, and sm all areas of forests were probably intermixed with grassland. The ceno gram structure completely excludes an environment dominated by primary forest or by steppe.