New data on the cranial morphology of the tapejarid Sinopterus from the Early Cretaceous Jehol Biota

Here, we describe a new Jehol tapejarid skeleton tentatively referred to Sinopterus dongi, revealing some valuable cranial information about the palate and mandibular symphysis of this species. In the palate, the absence of the lateral process of the pterygoid indicates that the subtemporal fenestra is undivided, in contrast to the well-developed lateral process that subdivides the subtemporal and secondary subtemporal fenestrae in other azhdarchoids. The mandibular symphysis is elongated to about 58.7% of the mandible length, significantly longer than the other two Jehol tapejarids (52.9-53.7%) and much longer than other tapejarids (38.0-51.0%), except for the basal tapejarid Caupedactylus ybaka (similar to 65%). The symphyseal shelf is extensive, flattened, and sculptured by sinuous ridges and pits. The ratio of its length to the symphysis is 34.4%, between that of the possible tapejarid Bakonydraco galaczi (43.9%) and the coexistent chaoyangopterid Eoazhdarcho liaoxiensis (26.7%), and much distinct from the well-reduced condition in other tapejarids. This discovery highlights the variation of the cranial morphology in the Tapejaridae and even the Azhdarchoidea.