A new articulated forelimb of Hamipterus tianshanensis from the Lower Cretaceous of Xinjiang, China

Hamipterus tianshanensis, a major component of the Early Cretaceous Hami Pterosaur fauna in northwestern China, is represented by numerous three-dimensionally preserved bones, eggs, and embryos. Although exquisitely preserved, most of these elements are disarticulated. Here, we describe an almost complete articulated forelimb of Hamipterus collected from the same stratigraphic horizon as the prior specimens. It shows several novel anatomical features previously unknown in this taxon, including a well-developed flexor tendon process of the proximal syncarpal; ventral surface of the distal syncarpal with a crescent outline; straight proximal portion of the pteroid; metacarpal I articulating with the carpus, coupled with a reduction in metacarpals II and III; unguals short and deep; a large longitudinal pneumatic foramen between the cotyles of the first wing phalanx; an oval pneumatic foramen on the posteroventral surface of the second wing phalanx; and a shallow groove extending along the posteroventral shaft of both the second and third wing phalanges. During ontogeny, all forelimb bones of Hamipterus grew isometrically, or nearly so, except for the antebrachium and the second wing phalanx. In addition, a comparison of the limb bone proportions with other ornithocheiroids reveals that the limb proportions of Hamipterus are more consistent with those of boreopterids and anhanguerids.