Evolution, Homology, and Development of Tetrapod Limb Muscles

Since the early 1900s, researchers have attempted to unravel the origin and evolution of tetrapod limb muscles using a combination of comparative anatomy, phylogeny, and development. The methods for reconstructing soft tissues in extinct animals have been refined over time as our ability to determine muscle homology and phylogenetic relationships between tetrapods has improved. Since many muscles do not leave osteological correlates, muscle reconstruction in extinct animals is largely based on anatomy and development in extant animals. While muscle anatomy in extant tetrapods is quite conservative, the homologies of certain muscles between taxonomic groups are still uncertain. Comparative developmental studies can help to resolve these controversies, as well as revealing general patterns of muscle morphogenesis across tetrapod groups. We review the methods, results, and controversies in the muscle reconstructions of early members of the amniote, mammalian, and lissamphibian lineages, including recent attempts to reconstruct limb muscles in members of the tetrapod stem group. We also review the contribution of recent comparative developmental studies toward understanding the evolution of tetrapod limb muscles, including morpho-genic gradients, the origin of paired fins, and the evolution of morphological complexity. Finally, we discuss the role of broad, comparative myological studies as part of an integrative research program on vertebrate evolutionary biology.