Comparative Morphology and Macroevolution of the Avian Quadrate

In birds, the quadrate acts as a hinge between the lower jaw and the skull, playing an important role in cranial kinesis. Therefore, the evolution of avian quadrate morphology presumably has been influenced by selective pressures related to feeding. Owing to its complex and variable morphology, the avian quadrate has been relatively overlooked with respect to other aspects of the crown bird skeleton, and previous researchers have frequently used different names to describe the same anatomical characters. Meanwhile, none have attempted to quantify evolutionary changes in quadrate shape to examine its geometric variation in light of ecological information. Here, we investigated the morphological variation of the quadrate across over 249 bird species covering all major lineages of extant birds to produce a clearly labelled anatomical atlas of the avian quadrate and describe the shape variance among different clades. Following this anatomical investigation, we used three-dimensional geometric morphometrics to quantify morphological variation of the quadrate to examine its relationship with an array of key ecological categories and perform ancestral shape reconstructions in the context of an up‐to‐date galloanseran phylogeny as a case study. We found non-ecological factors, such as allometry and phylogeny, exhibit stronger relationships with quadrate shape than ecological factors do. The avian quadrate evolved as an integrated unit and exhibits strong associations with the morphologies of neighbouring bones with which it articulates. Our results also illustrate the importance of incorporating fossil taxa into ancestral shape reconstructions and helps elucidate important aspects of the morphology and function of the avian feeding apparatus early in crown Galloanserae evolutionary history. Collectively, our results suggest a complex macroevolutionary scenario in which quadrate morphology evolved jointly with other elements of the avian kinetic system. With the knowledge of quadrate morphologies in crown birds, we plan to apply this explicitly quantitative framework to avian quadrates of stem group birds in future investigations.

I would like to thank Cambridge Trust and the Ministry of Education, Taiwan for the support during my PhD.