Developmental underpinnings of morphological disparity in the avian bony palate

The bony palate of palaeognaths was long thought to retain the plesiomorphic condition for crown birds, but recent fossil evidence suggests that aspects of palaeognath palate morphology are derived from a neognath-like ancestral state. Relatedly, heterochronic shifts have been proposed as the mechanism underpinning major evolutionary transitions in avian palate morphology, but this hypothesis has never been explicitly tested with a broad phylogenetic assessment of morphological variation through avian ontogeny. Here, we assess palatal changes through post-hatching ontogeny across the major extant avian subclades and find that although palaeognaths exhibit distinct ontogenetic changes relative to neognaths, no signatures of heterochrony underlie these developmental differences. However, we find that important patterns of morphological change appear to be dictated by variation in developmental mode. Our results clarify the ontogenetic mechanisms driving avian palate disparity and illustrate the influence of developmental mode on the evolvability of a key morphofunctional system in the avian skull.