Diversity of Heads, Jaws, and Cephalic Muscles in Amphibians

Living amphibians include caecilians, salamanders, and frogs. They share many features like a biphasic lifestyle with aquatic larvae and terrestrial adults, cranial skeletal elements that are derived from neural crest cells and mesoderm, and muscles derived from branchiomeric mesoderm and somites. Meckel’s cartilage, which forms the larval lower jaw, derives, for example, from mandibular arch neural crest cells. Mandibular arch mesoderm gives rise to the intermandibularis muscle, which forms the floor of the mouth, and the adductor mandibulae muscles, which are laterally located muscles that close the jaw. For the following arches (hyoid and branchial arches), similarities in muscles and cartilages can also be found. However, taking a closer look, it turns out that amphibians differ also in many things. Direct development evolved in all three taxa as did neoteny, i.e., the specimens reach sexual maturity while still having larval characters. The crania range from robust in caecilians to very light in anurans. Neural crest cells contribute in different amounts to the same cranial structures as the skull roof, and in all muscle groups, some muscles changed their attachments, others are only present in one or two taxa, and again others developed several heads. The knowledge of the diversity and in particular the similarities of amphibian crania and cranial muscles can help us to shed light onto the anatomy of the last common ancestor of tetrapods and onto variations in head anatomy and development in amniotes, which include reptiles and mammals.