Developmental variation in pterygoid segmentation clarifies patterns of avian bony palate evolution

The morphology of the palate has long constituted the primary basis for differentiating between the two deepest clades of crown group birds, Neognathae and Palaeognathae. However, published literature on the bird palate is dominated by classical anatomical descriptions pre-dating the advent of contemporary three-dimensional imaging techniques, hindering our understanding of bird palate disparity and development. Pterygoid segmentation, the process by which the rostral portion of the pterygoid separates and fuses with the palatine during ontogeny in neognathous birds, remains a poorly understood aspect of avian cranial development despite giving rise to an important component of the cranial kinetic system. Here, we use micro-computed tomography to explore ontogenetic change of the palate during the process of pterygoid segmentation across an unprecedentedly broad taxonomic sample of immature and mature birds. We found that direct evidence of post-hatching pterygoid segmentation was restricted to the major avian subclade Neoaves. Based on morphological and topological similarities, we hypothesise that the rostral projection of the pterygoid observed in Anatidae/Anseres and potentially Anhimidae and Megapodiidae, which we term the hemipterygoid process, is homologous with the hemipterygoid of neoavians, though it does not undergo segmentation. We posit that the origin of a discrete hemipterygoid (as observable in some crownward stem-birds) originated prior to the origin of the process of pterygoid segmentation; however, it remains ambiguous whether pterygoid segmentation is a synapomorphy of Neornithes, Neognathae, or Neoaves. Overall, our study clarifies the process of avian pterygoid segmentation and raises new questions regarding the major morphological modifications that have characterised the evolutionary history of the avian bony palate.