Lower jaw modularity in the African Clawed Frog (Xenopus laevis) and Fire Salamander (Salamandra salamandra gigliolii)

Modularity describes the degree to which the components of complex phenotypes vary semi-autonomously due to developmental, genetic, and functional correlations. This is a key feature underlying the potential for evolvability, as it can allow individual components to respond to different selective pressures semi-independently. The vertebrate lower jaw has become a model anatomical system for understanding modularity, but to date, most of this work has focused on the mandible of mammals and other amniotes. In contrast, modularity in the mandible of lissamphibians has been less well-studied. Here, we used geometric morphometrics to quantify the static (intraspecific) modularity patterns in Xenopus laevis and Salamandra salamandra gigliolii. We tested developmental and functional hypotheses of modularity and demonstrate that both species exhibit significant modularity. Functional modularity was supported in Xenopus, yet the lack of definitive support for both the developmental and functional hypotheses in Salamandra suggests influences on modularity are much more complex. Allometry has a small yet significant impact on lower jaw shape in both taxa and sex has a significant effect on shape in Xenopus. The high modularity seen in both species mimics the results of other studies on the amphibian cranium, suggesting that modularity is a ubiquitous feature of the tetrapod jaw.