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

Abstract

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. With this large dataset, we present direct evidence for the full process of post-hatching pterygoid segmentation being exclusively restricted to the major avian subclade Neoaves. Further, our investigation helps refine several hypotheses of homology related to the pterygoid-palatine contact in crown birds and their close stem-group relatives: we hypothesise that the rostral projection of the pterygoid observed in Anseres (and potentially Anhimidae and Megapodiidae) is homologous with the hemipterygoid of neoavians, and that the discrete and unfused hemipterygoids of some crownward stem-birds suggest that a discrete hemipterygoid originated prior to the origin of the process of pterygoid segmentation. However, whether pterygoid segmentation represents a synapomorphy of Neornithes, Neognathae, or Neoaves remains ambiguous in light of elusive data from the fossil record and early stages of cranial development. Overall, our study clarifies the process of pterygoid segmentation in crown birds and raises new questions regarding the major morphological modifications that have characterized the evolutionary history of the avian bony palate.