Ontogeny and Homology of the Shoulder Joint and Forelimb Muscles in Frogs and Toads (Lissamphibia: Anura)

Recent studies revealed inaccuracies in the naming and homologization of anuran shoulder joint muscles, and highlighted the value of muscle ontogeny and innervation to derive homology hypotheses. To expand the taxonomic range of previous work and add forearm muscles, the ontogenetic development, innervation, and morphology of shoulder joint and forelimb muscles were described based on tadpoles (Gosner Stages 31-41; Nieuwkoop-Faber Stages 53-54 to 59) and adult specimens of 11 anuran species. Digital dissections of volume data (histology, episcopic microtomy, µCT) were performed and 3D reconstructions generated. The developmental and anatomical observations were used to support existing or suggest new hypotheses on muscle homology across species. Results indicate that, like in all tetrapods, the shoulder joint and forelimb muscles are ontogenetically derived from the dorsal and ventral pre-muscle masses in the developing limb, which split and differentiate into individual muscles. The splitting pattern appears mostly conserved across sampled species. Interspecific variations are observed in the shoulder joint muscles, which comprise homologous muscles being present as one muscle unit in some species and subdivided into two heads or portions in others. The m. supracoracoideus, for example, is undivided in some species but splits into portiones anterior and posterior during ontogeny in others, resulting in a muscle with a common insertion but two distinct origins. Interspecific variation among forearm muscle units concerns morphological differences rather than divergence in developmental splitting pattern, the most notable examples being the m. extensor digitorum communis longus and the m. abductor indicis longus. For the former, we observed three distinct heads in some species and suggest naming them capita digiti medii, annularis, and minimi, respectively, for digits III-V. To reflect homology across Anura, we recommend using the name m. abductor indicis longus instead of m. supinator manus as previously used in Ascaphus truei.