Anuran limbs reflect microhabitat and distal, later-developing bones are more evolutionarily labile

Tetrapod limbs have been used as a model system to investigate how selective pressures and constraints shape morphological evolution. Anurans have had many independent transitions to various microhabitats, allowing us to dissect how these factors influence limb morphology. Furthermore, anurans provide a unique system to test the generality of developmental constraints proposed in mammals, namely that later-developing limb bones are under less constraint and show more variation. We used micro-computed tomography scans of 236 species from 52 of 55 families, geometric morphometrics, and modern phylogenetic comparative methods to examine how limb bones are related to microhabitat, phylogeny, allometry, and developmental timing. Although there was significant phylogenetic signal, anuran limb shape showed a relationship with microhabitat and to a lesser extent, body size. We found that distal bones had higher evolutionary rates than proximal bones, providing evidence that developmental constraints are reduced in later-developing bones. Distal bones also showed increased selection related to allometry and microhabitat, providing an additional explanation for higher evolutionary rates. By looking at the evolution of limb shape across a diverse clade, we demonstrated that multiple factors have shaped anuran limbs and that greater evolutionary lability in later-developing limb bones is likely a general trend among tetrapods.