Calibrating the Early Cretaceous Urho Pterosaur Fauna in Junggar Basin and implications for the evolution of the Jehol Biota

Over the past decades, abundant and wellpreserved vertebrate fossils, known as the Urho Pterosaur Fauna, have been recovered from the Lower Cretaceous Tugulu Group in Junggar Basin, Xinjiang, NW China. Excavated materials belong to pterosaur, plesiosaur, dinosaur, crocodylomorph, and turtle taxa. As such, they provide key insights into the evolutionary history of several critical vertebrate groups in the Early Cretaceous. The Junggar assemblages have been interpreted as belonging to the Jehol Biota sensu lato, representing its northwesternmost known geographic extent. This research presents a new chemical abrasion-isotope dilution-thermal ionization mass spectrometry U-Pb age of 135.2 +/- 0.9 Ma (2 sigma internal error) from a tuffaceous bed stratigraphically below the fauna -bearing layers, indicating a Valanginian maximum age for the Urho fauna. Combined with available biostratigraphic data, the results bear several important paleobiologic implications for the Early Cretaceous vertebrates. First, the Dsungaripterus pterosaur and Psittacosaurus ornithischian fauna appear to have emerged earlier than previously believed. Second, the data suggest that the oldest carcharodontosaurids in Asia appeared during the Valanginian Stage and extend the age range of basal coelurosaurs and basal crocodyliforms. Our results do not support the notion of the Jehol Biota sensu lato migrating as far west as the Junggar Basin in their later stages. The new information calls into question the temporal and spatial bases for the conventional, threestage evolutionary theory of the Jehol Biota.