Morphological evolution of the pectoral girdle of pterosaurs: myology and function

Abstract

AbstractThe musculature of the pectoral region of representative rhamphorhynchoid (Campylognathoides) and large pterodactyloid (Anhanguera) pterosaurs was reconstructed in order to examine the function of various muscles and the functional consequences of the evolution of the advanced pectoral girdle of large pterodactyloids. The reconstructions suggest that m. supracoracoideus was not an elevator of the wing, but instead depressed and flexed the humerus. m. latissimus dorsi, m. teres major, m. deltoides scapularis, and m. scapulohumeralis anterior were wing elevators. Comparison of the origin, insertion and function of muscles in the rhamphorhynchoid and the large pterodactyloid suggests that the evolution of the advanced pectoral girdle: (1) straightened the pull of m. pectoralis, m. deltoides scapularis and m. teres major, improving their function in wing elevation; (2) allowed ligaments rather than muscles to resist the tendency of those muscles to move the scapula; and (3) braced the pectoral girdle against the vertebral column so that the tendency of m. latissimus dorsi and of aerodynamic lift on the wing to move the scapulocoracoid medially and dorsally, thereby compressing the thorax, could be resisted. The osteological and myological complexity of the advanced pectoral girdle, its uniqueness among tetrapods and its association with other complex osteological features argue that the advanced pectoral girdle is a synapomorphy complex of a single clade of large pterodactyloids, rather than a mere correlate of large size evolved convergently in various lineages.