Meeting Abstract
Postcranial skeletal pneumaticity is the invasion and resorption of bone by epithelial extensions of the respiratory anatomy into vertebrae and other postcranial bones, and is a hallmark innovation of birds and their ornithodiran ancestors. Various lines of evidence suggest that pneumatic spaces arise in part by environmentally-mediated opportunism of pneumatizing epithelia, with the biomechanics of a bone delimiting the extent of pneumatization that can be safely tolerated. Variably pneumatized birds have quantifiable differences in the cortical thickness and trabecular architecture of their vertebrae, but it is not known whether gross vertebral shape also varies with extent of pneumaticity. Because differences in bone shape necessarily entail biomechanical differences, and given the evidence that pneumatization of a bone is mediated by feedback from its biomechanical milieu, I hypothesize that extent of pneumaticity is correlated with vertebral shape. I test this hypothesis using geometric morphometric methods on three-dimensional models of vertebrae from the neognath bird family Ciconiidae (storks), focusing on whether serial variation in vertebral shape corresponds to variation in pneumaticity. Pneumatization (the fraction of total vertebral volume occupied by air space) shows substantial variation along the axial column, and is strongly correlated with serial variation in vertebral shape (R2=0.498, p<0.001). More elongate vertebrae are significantly less pneumatic than craniocaudally shorter, more upright vertebrae, a pattern that cannot be explained by variation in vertebral size. This finding indicates that not all vertebral shapes are equally amenable to pneumatization, and is consistent with the hypothesis that the functional and biomechanical idiosyncrasies of a vertebra are important determinants of the degree to which it is pneumatized.
