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Extant cyclostomes are jawless vertebrates and include hagfishes and lampreys. They are the closest extant relatives to jawed vertebrates (often called gnathostomes, which also include jawless extinct species) and share with them vertebrate-specific characters, like the presence of somites, neural crest cells, eyes (absent in hagfish), among others. There are also cyclostome-specific characteristics as the lingual apparatus, velum, and posterior hypophyseal process-derived cartilages. The comparison of development and anatomy between cyclostomes and gnathostomes provides insights into the evolution of the jaws and associated muscles. Furthermore, comparative anatomical and evolutionary developmental studies help to reconstruct the last common ancestor of vertebrates, cyclostomes, and gnathostomes. The evolutionary appearance of the jaw and associated muscles is of particular interest, as those are said to be one of the main reasons for the success of jawed vertebrates. Interestingly, the mechanisms to make an upper jaw are in place in cyclostomes and jawed vertebrates, but the lower jaw developmental program is only found in gnathostomes. The lower jaw is therefore a novelty that evolved in gnathostomes. Another example is the mandibular arch mesoderm and the associated neural crest cells which give rise to mandibular arch muscles and the jaws in gnathostomes, respectively. Cyclostomes have many muscles derived from the mandibular arch mesoderm, but only few can be homologized with those muscles from gnathostomes. However, those muscles that are homologous between those taxa must be present in their last common ancestor, i.e., vertebrates. Interestingly, some studies seem to indicate that mandibular arch (first arch) development was only secondarily included in the vertebrate branchial arch series, what would explain several differences in mandibular arch development between jawed and jawless vertebrates. However, this hypothesis is controversially discussed.

Recent findings that urochordates are the closest sister group of vertebrates have dramatically changed our understanding of chordate evolution and of the origin of the vertebrate head and its muscles. To better understand the evolution and diversity of chordates, in particular the morphological and taxonomical diversity of the vertebrates, it is crucial to investigate the origin, development, and comparative anatomy of not only hard tissues but also of soft tissues such as muscles. Building on the recent discovery of the cardiopharyngeal field in urochordates and on the comparative anatomy of chordate and vertebrate muscles, in this chapter we focus on the broader comparative and developmental anatomy of chordate muscles and the origin of vertebrate cephalic muscles.

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.