First Record of Cyclocarya from the Early Oligocene Qaidam Basin, North Tibet: Implications for the Paleogeography and Paleoecology

As the largest intermontane basin in the northeastern Tibetan Plateau (TP), the Qaidam Basin provides unique insight into paleoclimatic change and its relationship with global change and uplift on the TP. In this study, based on morphological comparison, fossil fruit of Cyclocarya from the Early Oligocene Shangganchaigou Formation in the Qaidam Basin is assigned as Cyclocarya cf. weylandii. The discovery of Cyclocarya cf. weylandii demonstrates the occurrence of Cyclocarya in the Oligocene sediment in Qaidam Basin. This is the first record of Cyclocarya fossil of Early Oligocene Age in China and indicates that Cyclocarya has existed on the northeastern TP since at least the Early Oligocene. The living analogues of the current fossil now lives in sub-tropical China, where the East Asian Monsoon is prevalent. Integrating the new fossils and previously reported fossil remains of plants and fishes, it can be inferred that the Early Oligocene Qaidam Basin was primarily influenced by westerly circulation and had a relatively warm and humid climate, which was in sharp contrast to the present-day climate in Qaidam Basin.