Biotic interchange through lowlands of Tibetan Plateau suture zones during Paleogene

A substantial biotic interchange took place between Gondwanan India and Laurasia immediately following the India-Eurasia collision. Located just north of the collision zone, the role of the Tibetan Plateau in the geographic interchange of biota is still unknown because lack of fossil evidence. Here, well-preserved samara fossils of Ailanthus (the family Simaroubaceae) are described from the Lunpola and Nima basins, central Tibetan Plateau, ranging in age from the latest Paleocene to the Late Oligocene. These fossils are the largest samara fossils of this genus found so far. Characteristics such as the main ventral vein located in the intramarginal part of the samara, and the stylar scar located on the midline of the seed suggest that they represent a new species, which we herein name Ailanthus maximus J. Liu, T. Su et Z.K. Zhou, sp. nov. These findings greatly expand the known spatial distribution of Ailanthus in the Paleogene, and suggest that the Tibetan Plateau has played an important role in the migration of biota between Gondwanan India and the Northern Hemisphere. In addition, A. maximus is morphologically similar to A. triphysa, a living species which is naturally distributed in tropical and subtropical regions in Southeast Asia at elevation no more than 600 m, indicating a tropical or subtropical lowland environment in the central Tibetan Plateau during the latest Paleocene to Late Oligocene. Moreover, together with previous paleoaltimetric estimates, we propose that tropical or subtropical lowlands along the Bangong-Nujiang Suture Zone and the Yarlung-Zangpo Suture Zone existed in some parts of the Tibetan Plateau during the Paleogene only obtained their current elevation during the Neogene