Comparative Morphology and Mechanics of Aquatic and Terrestrial Feeding Salamanders

Salamanders are often used as models to understand the iconic vertebrate water-land transition as they exhibit similar morphology and, presumably, ecologies as early tetrapods. Recent studies have demonstrated previously unappreciated diversity in salamander feeding mechanisms, however, compared to mammals, and reptiles there is little data on the internal mechanical response of salamander skulls to feeding loads. In this study, we used data from microCT scans to compare skull osteology and jaw muscle architecture in an aquatic salamander (Necturus) and a terrestrial salamander (Salamandra), with resulting three-dimensional digital models and anatomical information used to carry out lever arm and finite element analyses to predict the mechanical behaviour of the skull under bite loads. Necturus and Salamandra exhibit markedly different skull osteology in terms of overall skull shape, bones composing the skull, and dentition. Contrary to our expectations, Necturus exhibited both absolutely and relatively larger jaw muscles than Salamandra and produced higher bite forces, even when scaled to the same size. At life-size, the skull of Necturus featured higher peak stresses than that of Salamandra; however, when scaled to the same surface area: muscle force ratio, the skull of Necturus featured lower peak stresses, suggesting its skull is stronger under biting loads than that of Salamandra. The skull of Salmandra did exhibit more evenly distributed stress than that of Necturus, suggesting skull shape in the terrestrial taxon may more closely reflect adaptation to resist biting loads. Our finite element models demonstrate that the inclusion of sutures in our models resulted in increased peak stresses; however, in terms of other performance metrics the impact of sutures on model behaviour was complex. Our results suggest that jaw muscle size, bite force and skull strength in salamanders are being driven by multiple factors, and are not solely related to differences in habitat.