Abstract

We review the enigmatic Chaoyangia beishanensis, one of the earliest birds described from the Jiufotang Formation, north-eastern China, and the first to be identified as an ornithurine (Aves: Ornithothoraces) and thus a member of the clade that includes living birds. A complete discussion of the validity of this taxon, which once included the holotype of Songlingornis, is provided, along with a revised diagnosis. The morphology of Chaoyangia is described, including extensive comparison with better known, recently discovered ornithurines as well as several other groups of Mesozoic birds (Confuciusornithiformes, Sapeornithiformes, Enantiornithes). Although preserved information is limited, the large number of fused sacral vertebrae and presence of a distal dorsal process on the ischium are among the features supporting early hypotheses that the only known specimen of Chaoyangia represents an ornithurine. Unique among ornithurines, Chaoyangia possesses two dorsal processes on the ischium, and thus remains a valid taxon. We include this taxon in a cladistic analysis to test morphological hypotheses regarding its systematic position. Although the results of the analysis are highly resolved and support the referral of Chaoyangia and Zhongjianornis to Ornithurae, support for the tree overall is very low. Recently discovered taxa have blurred the once clear morphological gap separating the two ornithothoracine clades (Ornithurae and Enantiornithes), and thus the increase in taxonomic diversity has caused a decrease in the stability of hypothetical&break; relationships.