Macroevolutionary drivers of morphological disparity in the avian quadrate

In birds, the quadrate bone acts as a hinge between the lower jaw and the skull, playing an important role in cranial kinesis. As such, the evolution of avian quadrate morphology may plausibly be assumed to have been influenced by selective pressures related to feeding ecology. However, variation in quadrate morphology across living birds and its potential relationship with ecology have never been quantitatively characterised. Here, we used three-dimensional geometric morphometrics and phylogenetic comparative methods to quantify morphological variation of the quadrate and its relationship with an array of key ecological features across 200 bird species covering all major extant lineages. We found non-significant associations between quadrate shape and several aspects of feeding and foraging ecology across different scales of phylogenetic comparison. By contrast, allometry and phylogeny exhibit stronger relationships with quadrate shape than other ecological features. We show that the avian quadrate evolves as an integrated unit and exhibits strong associations with the morphologies of neighbouring bones. Our results collectively suggest a macroevolutionary scenario in which the shape of the quadrate evolved jointly with other elements of the avian kinetic system, with the major lineages of birds exploring alternative quadrate morphologies--highlighting the potential diagnostic value of quadrate morphology in investigations of fossil bird systematics.