Yuanjiawaornis viriosus, gen. et sp. nov., a large enantiornithine bird from the Lower Cretaceous of western Liaoning, China

Here we describe a new enantiornithine bird, Yuanjiawaornis viriosus gen. et sp. nov., based on a nearly complete, mostly articulated postcranial skeleton from the Lower Cretaceous Jiufotang Formation (120 Ma) of western Liaoning Province, northeastern China. It is similar in size to large bohaiornithids among the known Early Cretaceous enantiornithines; only Pengornis houi is larger. The semilunate carpal is incompletely fused to the metacarpals, suggesting that the holotype specimen was not fully mature at the time of death. The new specimen is distinguishable from other known enantiornithines by a unique combination of features including large body size, forelimb and hind limb subequal in length, a longitudinally grooved ventral surface of the synsacrum, robust and long transverse processes of the caudal-most sacral vertebra, a craniodorsally tapered acromion process of the scapula, a straight lateral margin of the coracoid, medially curved clavicular rami with tapered omal tips, a sternum that is oval in outline with slightly expanded distally caudolateral processes, a humeral head with a flat proximal surface, and a humerus with a deltopectoral crest that gradually decreases in height distally. A comparative analysis of some sacral features of basal birds including this taxon suggests that enantiornithines might have a uniquely shaped synsacrum, although more data is needed to confirm that hypothesis.