ABSTRACT
This chapter provides an updated, comprehensive overview of the cephalic muscles of nonosteichthyan vertebrates. It describes dissections of cyclostomes (sea lamprey; Atlantic hagfish) and chondrichthyans (spiny dogfish, common smooth-hound, spotted ratfish, and little skate) and a synthesis of primary, including more recent, literature about the evolution and development of the cephalic muscles in vertebrates. The chapter compares the cephalic muscles of the nonosteichthyan groups with those of osteichthyans, based on previous dissections and review of the literature. The comparative analysis thus uncovers evolutionary patterns of the cephalic muscles across early vertebrate lineages in order to address long-standing challenges about vertebrate comparative anatomy, such as the evolution of cephalic muscles and the connection between the head and pectoral girdle. Cyclostomes are the main taxa people use for comparison with vertebrate cephalic muscles. Despite having many highly peculiar morphological features, hagfishes and lampreys are crucial to study the evolutionary transitions from nonvertebrate animals to gnathostomes.
