Developmental underpinnings of morphological disparity in the avian bony palate

Abstract

The deepest phylogenetic divergence in crown birds gave rise to Palaeognathae and Neognathae, clades exhibiting divergent bony palate morphologies. This observation led to the longstanding hypothesis that the distinctive palate arrangement of palaeognaths retains the ancestral crown bird condition, but recent fossil evidence instead suggests that aspects of palaeognath palate morphology are derived from a neognath-like ancestral condition. While neoteny has been hypothesised as a mechanism underpinning the distinctive palatal morphologies of palaeognaths, this hypothesis has never been tested with a broad phylogenetic assessment of morphological variation through avian palate ontogeny. Here, we quantitatively assess morphological variation of the palate through the post-hatching ontogenies of 70 bird species representing all major extant phylogenetic subclades, clarifying the ontogenetic mechanisms giving rise to avian palate disparity. Though palaeognaths exhibit distinct ontogenetic changes in the PPC relative to neognaths, we find no signatures of heterochrony--including neoteny--underlying these developmental differences. However, important patterns of morphological change in the avian palate appear to be dictated by variation in developmental mode. Our results document the effects of post-hatching development on a key morphofunctional system in the avian skull, and, more broadly, highlight the influence of developmental mode on morphological evolvability across crown group birds.