At a crossroads The late Eocene flora of central Myanmar owes its composition to plate collision and tropical climate

Myanmar was shaped by the India-Asia collision, fusion of the Burma Terrane (BT) with Asia, and mountain building. Throughout this process new elevational gradients and habitats were formed, which affected the regional climate, but also forged new dispersal routes into Asia and India. In spite of its importance, the vegetation history of Myanmar is poorly known, and this hinders our understanding on the origins and evolution of SE Asian biodiversity. In this study we reconstruct the late Eocene flora in central Myanmar, based on samples from a sedimentary succession in Kalewa, and extend on the Sapotaceae fossil record with additional early Eocene–early Oligocene samples. We then study the morphology, botanical affinity, source ecology and biogeography of selected sporomorphs, and assess the Gondwanan and Laurasian components. Our results show that this late Eocene palynoflora is representative for evergreen forests, typical in a seasonal wet climate, with dryer vegetation away from the area of sedimentation. The abundance of Sapotaceae further suggests that this family became an important component of the SE Asian flora shortly after the India-Asia collision. We conclude that the late Eocene geographic position and Gondwanan origin of Myanmar facilitated floristic exchange between the Indian Plate, BT, mainland and SE Asia, making the BT a crossroads for plant dispersals between Gondwana and Laurasia. The shift from late Eocene seasonal evergreen to present-day moist deciduous forests was likely due to the northward drift of Myanmar, the subsequent two-stage uplift of the Indo-Burman Ranges and posterior Neogene global cooling and drying.