Complexities and novelties in the early evolution of avian flight, as seen in the Mesozoic Yanliao and Jehol Biotas of Northeast China

Recently reported specimens from the Mid-Late Jurassic Yanliao (or Daohugou) Biota and Early Cretaceous Jehol Biota of Northeast China suggest that the early evolution of avian flight involved a surprising amount of homoplasy and evolutionary experimentation. Pennaceous feathers of variable size, structure, and extent occur on the hindlimbs of numerous Jehol and Yanliao paravian theropods, including some basal birds, and clearly had an aerodynamic function at least in the dromaeosaurid Microraptor. However, their function in many cases may have been primarily ornamental, and it is unclear whether aerodynamically useful hindwings represent a widespread paravian feature or an evolutionary novelty limited to Microraptor and possibly a few other taxa. Clearer examples of novelties related to aerial locomotion are the tail plumage of the basal bird Jeholornis, in which a proximal fan of feathers is present and the ancestral distal frond is somewhat reduced, and the membranous wings of the Yanliao scansoriopterygid Yi. Early paravian evolution evidently involved a rapid diversification of aerodynamic structures, and ancestral paravians may have been volant. It is also possible that the avian lineage passed through a four-winged “tetrapterygian” stage, but current phylogenies suggest that aerodynamic hindwings were more likely acquired independently by different paravian groups.