A Subadult Specimen of the Early Cretaceous Bird Sapeornis chaoyangensis and A Taxonomic Reassessment Of Sapeornithids

We describe the anatomy and bone microstructure of a new subadult specimen of Sapeornis chaoyangensis from the Early Cretaceous of Liaoning Province, China. To date, this is the only known sapeornithid that preserves substantial portions of its plumage. Found in the Yixian Formation, it also represents the oldest known sapeornithid, and as such, extends the stratigraphic range of this lineage 3–5 million years. This specimen also increases our knowledge of sapeornithid skeletal and integumentary anatomy, including previously unrecognized details of the primary and secondary wing feathers. Examination of the characters used to diagnose other named sapeornithid species reveals that such diagnoses have incorporated morphologies that are influenced by either taphonomy or ontogeny. Based on qualitative and quantitative comparisons between the new specimen and other sapeornithid species, we argue that all other named sapeornithids are junior synonyms of S. chaoyangensis.