Modularity without convergence: integrated evolution of the salamander autopodium

Abstract

Morphological modularity is known to favour evolutionary diversification by enabling different body regions to vary in response to functional and ecological demands. Despite their remarkable diversity in limb morphology and ecology, amphibians—and salamanders in particular—remain significantly under-represented in empirical tests of modularity. Using geometric morphometrics and phylogenetic comparative methods, we investigate patterns of shape variation, integration and modularity in the salamander autopodium. We first validated a landmark-based method to outline foot shape, demonstrating that image orientation does not affect shape analyses and that species can be reliably distinguished. We then applied this approach to a larger comparative dataset encompassing 149 salamander taxa across major lineages and ecological strategies. Our findings demonstrate that the salamander autopodium is functionally modular: the distal region (digits and interdigital region) shows more evolutionary lability than the proximal pes, while remaining integrated within a coordinated evolutionary system. Furthermore, the combinations of allometric, ecological and life-history factors account for a small but significant percentage of shape variation, with a low evolutionary signal in foot shape. Our results suggest that modular organization of the salamander foot may allow for morphological variation while still being limited by a common evolutionary and developmental history. By focusing on an understudied group and morphological system, this study provides new insight on how modularity influences limb evolution in tetrapods.