Meeting Abstract

P3.40  Jan. 6  Morphological evolution of the vertebrate pectoral girdle SHEARMAN, Rebecca, M; Wesleyan University rmshearm@uchicago.edu

Accounts of pectoral girdle evolution often focus on sarcopterygians and the morphological transition from fins to limbs. Actinopterygii, however, constitutes half of the extant vertebrates on the planet. Excluding them from a study on the evolution of the vertebrate skeleton lessens our appreciation of the morphological diversity among vertebrates and obstructs our ability to identify trends in the morphological evolution of the shoulder girdle. I conducted a broad survey of the embryology and morphology of the pectoral girdle of several vertebrate taxa, and identified trends in morphological evolution of the shoulder within several gnathostome lineages. The shoulder skeleton consists of multiple large elements with a large dermal component in basal lineages of both the Sarcopterygii and the Actinopterygii. In more derived lineages, the shoulder consists of relatively fewer elements and has a reduced dermal skeleton. Additionally, the Lissamphibia uniquely exhibit a range of characteristics of the shoulder skeletons of many vertebrates and provide a morphological intermediate between the disparate pectoral girdle morphologies of extant fish and amniotes. Homologies within the vertebrate pectoral girdle have been debated for over 200 years. My analysis suggests that the procoracoid (as recently defined by Vickaryous and Hall, 2006) appeared independently in amniotes and amphibians. As in many extant lizards, the amphibian procoracoid lies anterior to the coracoid, does not contribute to the glenoid, and in frogs, contacts the dermal clavicle. Additionally, the three-part endochondral shoulder skeleton, consisting of the scapula, coracoid, and procoracoid, is unique among tetrapods and is absent in both actinopterygian and sarcopterygian fish.