Direct ESR dating of the Pleistocene vertebrate assemblage from Khok Sung locality, Nakhon Ratchasima Province, Northeast Thailand

We report here the first direct dating study of the faunal assemblage from Khok Sung locality, Thailand. This palaeontological site is of great biochronological, palaeo-environmental and biogeographical significance. Firstly, it has yielded a rich and diversified Pleistocene vertebrate fauna with up to 15 mammalian species from 13 genera, 10 reptile species, as well as fish and bird remains. Interestingly, while most of the mainland Southeast Asian Pleistocene mammal fossils originate from cave deposits, the Khok Sung fossil layer is located within an 8 m thick fluvial terrace. Secondly, it is geographically located in an area of major importance for reconstructing the migration pathway of large mammals between South China and Java. Combined US-ESR dating of five teeth provides two possible ages of 130 +/- 29 ka and 217 +/- 36 ka for the fossils. The reason for the occurrence of these two age groups lies in the fact that it was not possible to obtain sediments that were directly associated with the measured samples, nor was it possible to carry out in situ gamma dose rate measurements due to the high water level. Sediment samples recovered from museum specimens show significantly variable concentrations of radioelements. Both options are equally plausible, as independent methods did not provide firm evidence favouring one or the other age range. The results illustrate the intrinsic limitations of the ESR dating method when fossil teeth are not collected in situ during the excavation. Regardless whether the age of the Khok Sung fauna corresponds to MIS 7 or early MIS5, it is nevertheless one of the oldest assemblages directly dated in the region. Further efforts are required for more accurate fossil identification and attribution and for additional numerical dating in this region to improve the biochronological framework of the Pleistocene mammalian faunas in Southeast Asia, which presently remains quite imprecise.