Clade-wide morphological and functional variation of the sauropsid columella

The columella (=stapes) is the middle ear bone of reptiles that transmits vibrations from the environment to the inner ear. It has been shown to exhibit extensive interspecific morphological disparity in several clades; however, its morphological variation and associated functional consequences remain poorly described. Using published micro-computed tomography data of turtle, lepidosaur, crocodilian, and bird columellae, as well as images of non-avian dinosaur columellae, we provide qualitative and quantitative descriptions, and explore how phylogeny and function influence columellar morphology between and within sauropsid clades. Scaling relationships illustrate that birds have proportionally smaller columellae than other sauropsids, which may facilitate higher-frequency hearing. Columellar length scales proportionally with head width in both birds and turtles, whereas in lepidosaurs it increases disproportionately with head size. The sizes of other columellar features show varying degrees of negative allometry when compared to columellar length. Birds show high morphological lability in distal end and shaft shape but are conservative in footplate morphology, whereas lepidosaurs and turtles show more variability in footplate size and shape but more conservative shafts and distal structures. Snakes display multiple differences from non-ophidian squamates including longer columellae and similar relative footplate sizes to turtles and birds. Ecological signal appears evident in the footplate of the sea snake, Aipysurus laevis, which may have convergently evolved a mosasaur-like condition, as well as a derived columellar morphology in fossorial snakes that may be convergent in multiple lineages. Our findings show that columellar disparity reflects ecological, functional, and phylogenetic factors, and provide a framework for further investigations into the evolution of the sauropsid ear.