Shark Pharyngeal Muscles and Early Vertebrate Evolution

This study provides a new classification of shark pharyngeal muscles, leading to a re-interpretation of the evolutionary history of these muscles. It begins with Edgeworth's developmental classification but also incorporates phylogenetic and functional information. The two basic groups of pharyngeal muscles are (1) branchial (including deep interbranchials and superficial constrictors) and (2) spinal (including epaxial and hypobranchial muscles). Most of the branchial muscles are also present anteriorly in the mandibular and hyoid segments, indicating that these were once typical branchial segments. The extrabranchial cartilages and gill septa are used as landmarks for dividing the interbranchials from the superficial constrictors. These landmarks reveal that the most important expiratory muscles in the anterior pharynx (most parts of "constrictor hyoideus", interhyoideus, levator palatoquadrati, spiracularis, and intermandibularis) are actually enlarged interbranchial muscles that have taken on a surface location. "Shrinking" these enlarged interbranchials and returning them to a deep location produces a simple, metameric reconstruction of the pre-gnathostome pharynx that fits what is known about thelodonts and other fossil agnatha related to gnathostomes. Finally, the pharyngeal muscles of bony fishes are considered, and concluded to have evolved from a sharklike condition through an acanthodian-like intermediate. © 1997 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd.