The functional morphology of the pectoral fin girdle of the spiny dogfish (Squalus acanthias): Implications for the evolutionary history of the pectoral girdle of vertebrates

Fresh functional-morphological observations and theoretical considerations warrant a re-analysis of the traditional assumptions that the head skeleton of sharks consists of the chondrocranium and visceral arches and that the pectoral fin girdle is part of the appendicular skeleton. The scapulocoracoid cartilage of the Spiny Dogfish (Squalus acanthias) plays at least three major roles: (1) As a mechanical separator between the lateral undulations of the trunk and the vertical movements of the visceral arches: (2) as a structural basis for the anchoring of the pectoral fins: and (3) as the place of origin of the cucullaris and hypobranchial muscles which not only open the jaws and expand the branchial basket, but at the same time also affect the contractions of the heart and the timing of blood flow through the gills. Hence, the scapulocoracoid cartilage is an integral part of the head. It most likely evolved in connection with the evolution of the jaw apparatus and not simply as a part of the appendicular skeleton. Furthermore, the head is a mechanically coherent system, in which the central alimentary, circulatory, respiratory and nervous systems of the organism converge. These central feeding, respiratory and circulatory functions are not simply choreographed by the central nervous system, but are concatenated by the mechanical construction of the head itself. Such a conceptualization of the selachian head raises fresh questions concerning the constraints and potentialities of the evolutionary transformations of the various components of the head and pectoral appendages during the transition from aquatic pisciform to terrestrial tetrapod vertebrates.