Intercalary elements, treefrogs, and the early differentiation of a complex system in the Neobatrachia

Intercalary elements are additional skeletal structures of digits of many anuran amphibians. Twelve terminal clades in the neobatrachian lineage of frogs have intercalary elements revealing it is a homoplastic character with five to seven gains and two to four losses along a consensus phylogeny of the Neobatrachia. We analyzed anatomical variation of intercalary elements, related structures (distal phalanges, tendons, and muscles), and articulations of digits of 45 anuran species, representing eight suprageneric terminal taxa. The intercalary elements are integrated in a complex system that is probably related to different types of movements, which are produced by a similar set of muscles and tendons with limited variation among the studied taxa. Species in the clades Hyloides and Ranoides show distinctive patterns of morphostructural features in their intercalary elements that are usually wedge-shaped and composed of hyaline cartilage in Ranoides, and biconcave and composed of embryonic cartilage in Hyloides. Features derived from the typical hyloid condition may only be interpreted in some Hylidae (Pseudis and Lysapsus) and Centrolenidae. In Ranoides, the described features of the intercalary elements are found in all taxa examined with the exception of Leptopelis, which have an intercalary element similar to the other Ranoides but formed by connective tissue. Several features are shared by all taxa having intercalary elements: (1) the intercalary elements differ from the phalanges by lacking terminal epiphyses, (2) they are present in hands and feet, and (3) they appear in all digits. This finding suggests that the genetic basis for presence of intercalary elements may be homologous in all these taxa and may have evolved only once early in neobatrachian history.