Anatomy of the Early Cretaceous Bird Rapaxavis Pani, a New Species from Liaoning Province, China

The lacustrine deposits of the Jiufotang Formation, Liaoning Province, China have yielded a diverse avifauna representing a wide spectrum of basal lineages. Here we add to this diversity by describing the longirostrine enantiornithean taxon Rapaxavis pani gen. et sp. nov. The specimen possesses the following autapormorphies: (1) caudoateral processes of the sternum with two prominent branches and a smaller third branch, (2) paired triangular thoracic elements of unknown homology, and (3) the combination of six sacral vertebrae and six free caudal vertebrae. Otherwise. this genus is closely allied with the Yixian Formation species Longirostravis hani, sharing elongate, curving rostrum with teeth restricted to the premaxilla and rostral-most dentary, and a derived manus lacking unguals. The holotype of Rapaxavis is exquisitely preserved, with such details as distal tarsals, keratinous sheaths of the pedal unguals and some three-dimensional features of the bones intact. The holotype appears to lack a fused tibiotarsus and it fused carpometacarpus. These features may indicate that the specimen is not yet fully grown, though the life stages of members of the Enantiornithes will not be clear until a complete ontogenetic series can be Constructed. A comparison of the proportions of the pedal phalanges of Rapaxavis to modern birds shows Rapaxavis to be highly adapted to a grasping, arboreal lifestyle. The combination of strongly arboreal pedal adaptations and the probing adaptations in the rostrum is here confirmed in a member of the Enantiornithes for the first time in the literature, adding to their already impressive known functional diversity.