Cranium, Cephalic Muscles, and Homologies in Cyclostomes

Abstract

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.