A new small enantiornithine bird from the Jehol Biota, with implications for early evolution of avian skull morphology

Enantiornithes is the most diverse Mesozoic avian clade. Approximately half of the known global diversity of Enantiornithes is from the Early Cretaceous Jehol Biota of China. The Jehol enantiornithines are usually articulated and complete, but the bones are overlain by each other and preserved in two dimensions, severely limiting the number of cranial characters that can be recognized. Here we describe a new enantiornithine bird, Pterygornis dapingfangensis gen. et sp. nov., from the Jehol Biota. The new taxon has a unique sternal morphology with an external rostral spine and a pair of craniolateral processes. Phylogenetic analysis resolves the new taxon in a derived position within Enantiornithes. The specimen is disarticulated with several exceptionally well-preserved cranial bones, including the jugal and quadratojugal, morphologies of which remain poorly understood for enantiornithines. Our results indicate that the quadratojugal is an inverted L-shaped element, morphologically similar to that of more basal birds Archaeopteryx bavarica, Jeholornis prima, Confuciusornis sanctus and Sapeornis chaoyangensis. Our findings also illustrate that the quadratojugal underwent large modifications with the reduction of the caudoventral and squamosal processes sequentially during early avian evolution, contributing to the refinement of the cranial kinesis in early birds. http://zoobank.org/urn:lsid:zoobank.org:pub:384D11F5-1CD3-4447-B01E-58D320B42D49