Macroevolutionary drivers of morphological disparity in the avian quadrate

Abstract

In birds, the quadrate connects the mandible and skull, and plays an important role in cranial kinesis. Avian quadrate morphology may therefore be assumed to have been influenced by selective pressures related to feeding ecology, yet large-scale variation in quadrate morphology and its potential relationship with ecology have never been quantitatively investigated. Here, we used geometric morphometrics and phylogenetic comparative methods to quantify morphological variation of the quadrate and its relationship with key ecological features across a wide phylogenetic sample. We found non-significant associations between quadrate shape and feeding ecology across different scales of phylogenetic comparison; indeed, allometry and phylogeny exhibit stronger relationships with quadrate shape than ecological features. We show that similar quadrate shapes are associated with widely varying dietary ecologies (one-to-many mapping), while divergent quadrate shapes are associated with similar dietary ecologies (many-to-one mapping). Moreover, we show that the avian quadrate evolves as an integrated unit and exhibits strong associations with the morphologies of neighbouring bones. Our results collectively illustrate that quadrate shape has evolved jointly with other elements of the avian kinetic system, with the major crown bird lineages exploring alternative quadrate morphologies, highlighting the potential diagnostic value of quadrate morphology in investigations of bird systematics.

Remarkable insights into modern bird origins from the Maastrichtian type area (north-east Belgium, south-east Netherlands)

“…Those results demonstrate the necessity of incorporating fossil taxa into ancestral state reconstructions of skeletal geometry whenever possible – a conclusion applicable well beyond Galloanserae. Moreover, subsequent work by Kuo et al (2024) has illustrated widespread decoupling between quadrate morphology and feeding specialisation across the avian tree of life, emphasising the potential value of quadrate morphology in assessments of fossil phylogenetic affinities. As such, the morphological similarity of the quadrate of Asteriornis to those of extant Galliformes is likely to reflect genuine phylogenetic proximity.…”