Phylogenetic history, allometry and disparate functional pressures influence the morphological diversification of the gekkotan quadrate, a keystone cranial element

Abstract

The functional components of the vertebrate skull-including the chondrocranial braincase, protective dermatocranium and lower jaw-are interconnected and become operational through the quadrate bone. This latter element is critical for cranial biomechanics and support of the auditory system in squamate reptiles, but the interspecific variation of quadrate anatomy has not been studied in detail. Our objectives were to determine the relative influence of phylogenetic history, allometry and functional selection pressures on the morphological diversification of this keystone cranial element across gecko genera using high-resolution X-ray micro-computed tomography, three-dimensional geometric morphometrics and phylogenetic comparative methods. Our results demonstrate substantial variation in gecko quadrate morphology. Two families possess highly derived quadrate morphologies, while the remaining gekkotans retain extensive overlap in quadrate shape. Allometric scaling has influenced shape across species; distantly related miniaturized taxa possess elongate, slender quadrate morphologies while large taxa have robust, laterally expanded quadrates. The relative height of the coronoid eminence and the loss of the peripheral auditory system co-vary with quadrate anatomy, indicating that multiple disparate functional pressures may act on this element. Our study has identified the disparity of quadrate morphology within geckos and has highlighted the importance of considering multiple factors that may influence the diversification of phenotypes.