Cartilage on the furculae of living birds and the extinct bird Confuciusornis: a preliminary analysis and implications for flight style inferences in Mesozoic birds？
WU Qian，Jingmai O’CONNOR，LI Zhi-Heng，Alida M. BAILLEUL
Abstract The early evolution of flight is one of the most studied topics in vertebrate paleontology. Living birds have evolved to utilize a variety of flight styles, but studies focused on inferring flight strategies in Mesozoic birds are often contradictory and without a clear consensus, making it necessary to find additional informative characteristics that can be useful for inferences in fossils. Virtually nothing is known about the histology of the avian pectoral girdle, even though skeletal and joint tissues are key candidates to solve form-function relationships. Avian secondary cartilage found on the dermal bones of the avian skeleton is influenced by epigenetics and only forms when joints are stimulated by muscle contractions. As the only dermal bone in the avian postcranium, the furcula is a potential site for the formation of furcular secondary cartilage and merits further attention. It is still unknown whether living and fossil birds have furcular secondary cartilage. Here we present histological analyses conducted on the furcula-coracoid articulation in three living birds (Spilopelia chinensis, the Spotted dove; Passer montanus, the Eurasian tree sparrow; and Apus apus, the Common swift), taxa that utilize different flight styles, and one of the most common fossil birds of the Jehol Biota, Confuciusornis. Secondary cartilage was identified on the furculae of the living birds and of Confuciusornis, representing the first report of furcular secondary cartilage in the fossil record. Clear differences in secondary cartilage morphologies were observed in the living species, but additional data is required to establish a strong form-function relationship that could be useful for making inferences in Mesozoic birds.
Key words Evolution of flight, birds, Confuciusornis, furcula, histology, secondary cartilage