A moderate elevation and warm-humid climate of the Wulan Basin, NE Tibetan Plateau in the Middle Miocene indicated by Pueraria macrofossils

The phased uplift of the Tibetan Plateau since the Cenozoic strongly changed the Asian topography and greatly impacted not only the regional but also the global climate. Being sensitive to past climate changes, fossil plants are proven pivotal bio-indicators that can infer the paleoclimate and paleoelevation of different Tibetan Plateau parts. However, the timing and mechanisms of the uplift in the northeastern Tibetan Plateau remain debated due to limited evidence from plant fossils and inconsistent results. In this investigation, Pueraria leaflet and pod fossils collected from the Miocene Upper Youshashan Formation of the Wulan Basin, northeastern Tibetan Plateau, are reported. The leaflet fossils are assigned as a new species: Pueraria qinghaiensis Zhen-Dong Cao et San-Ping Xie sp. nov., while P. montana is recognized as its nearest living relative; the pod fossils are assigned as Pueraria sp. The leaflet margins of P. qinghaiensis and other fossils found in the same layer indicate that Pueraria from the Wulan Basin likely lived in open habitats and sprawled at the waterside. Moreover, the climate of the basin apparently was warm and humid in the Middle Miocene, and the elevation was about 723–2398 m based on the ecological niche of Pueraria. The occurrence of these new Pueraria fossils from the Wulan Basin supports the view that the elevation of the northeastern Tibetan Plateau did not attain its present elevation during the Middle Miocene. Subsequently, the plateau's uplift caused a series of environmental transformations, including increased elevation, reduced temperature and precipitation, and the intensification of Asian monsoons, which altogether led to the regional disappearance of the Pueraria population in the Wulan Basin.