A comparative approach to the microstructure in the carpus and tarsus in anurans

Despite the significant literature about morphological features of limb skeletons involved in tetrapod limb evolution, some questions about carpal and tarsal elements remain. In anurans, the ecomorphological and biomechanical approaches studied long hind limbs (to jump) and forelimbs (to land) and emphasized the role of the long bones in locomotion but disregarded what happens with the nodular elements of the carpus and tarsus. Here, we present a comparative study of nodular elements of the carpus and tarsus in anurans based on whole-mount specimens stained with Alcian Blue (cartilage) and Alizarin Red S (bone and calcified cartilage). The sample comprises 113 species belonging to 33 anuran families and postmetamorphic series in selected species. Further, we analyze the histology of the carpus and tarsus in individuals of nine species. In most anurans, the carpal and tarsal elements are cartilaginous in adult stages. The cartilaginous matrix may present different degrees of calcification. Few taxa present truly ossified carpals and tarsals with marrow cavity, blood cells, and hematopoietic cells. Interpretation of the interspecific variation in the carpus and tarsus skeletons on the most recent anuran phylogeny suggests that the delayed ossification of carpals and tarsals has evolved in derived lineages (e.g. Pelobatoidea and Neobatrachia).