Evolutionary and ecological correlates of avian uncinate process morphology: insight from a bizarre shape found in Early Cretaceous Enantiornithes

The avian uncinate process, a bony projection of the thoracic ribs, has been linked to flight performance and physiology. Its morphology exhibits considerable shape variation, which is potentially driven by ecological factors. Here, we describe a new enantiornithine bird (specimen STM11-128) from the Lower Cretaceous Jehol Group with unusual uncinate processes and comprehensively compare shape variance of the uncinate process between Mesozoic stem avian clades and extant Neornithes. Using landmark-based geometric morphometric and phylogenetic comparative methods, we quantify morphological variation of the uncinate process and explore its relationship with key ecological features across a broad phylogenetic sampling. We found that shape variation of the uncinate process shows a weak association with the examined ecological categories. In contrast, uncinate process length shows a stronger association with several ecological attributes, suggesting that avian uncinate process length strongly influences its mechanical benefits, with little impact from its shape. Finally, when comparing morphological variation across different clades of non-neornithine Mesozoic birds and modern birds, Mesozoic avian uncinate processes are characterised by a slender profile, whereas the shape of neornithine uncinate processes shows significantly higher diversity. This at least partially reflects a sampling bias as these small processes are rarely preserved in fossils.