Fossil wood: a key to understand paleoenvironmental and biogeographic evolutions in South and Southeast Asia

International audienceMonsoons are the defining climatic phenomenon of South and Southeast Asia. Monsoons are primarily controlled by topographic effects caused by Asian uplifts such as the Tibetan Plateau, the Himalayas or the Indo-Burman Ranges; their establishment during the Cenozoic and their evolution are yet not fully understood. Myanmar stands along the Bengal Bay, in the core engine of the South Asian monsoon, and is thus a relevant area of investigation. The country is separated today from India by the Indo-Burman Ranges. Fossil wood specimens from Myanmar offers the opportunity to follow the local evolution of the ecosystems and the biogeographical evolution of some taxa. A first comparison between the Pondaung (late Middle Eocene) and Natma (late Lower Miocene to early Middle Miocene) Formations allows us to identify long-term floristic changes. Eocene forest assemblages present taxa affiliated with modern analogues of dry dipterocarp forests (such as Sal, Shorea robusta C.F. Gaertn.). By contrast, Miocene forests are more analogous to modern tropical rainforests dominated by dipterocarps adapted to wetter conditions. These changes could first indicate a stronger monsoon regime during the Miocene than the Eocene, potentially due to regional uplift. From a biogeographical standpoint, these results support an emergence of the Dipterocarpaceae family in a dry seasonal climate, followed by a diversification and spreading toward everwet tropical zones. We also identify species similar to those of modern mangroves in both formations, confirming the continuous presence of this ecosystem in South Asia since at least since the middle Eocene. The expansion of the study to other geological units of Myanmar dating from the Paleocene, the Eocene, the Oligocene and the Miocene offers the possibility to draw up a more global overview of the climatic and biogeographic evolution of South Asia