The Evolution of the Avian Shoulder Joint

Abstract

The concavoconvex glenoid and bulbous humeral head of modern birds represent both structurally and functionally a hemisellar (half saddle) joint modified from a pattern common among post-Paleozoic tetrapods. Birds share with crocodilians, their nearest living relatives, similarities in joint architecture, including scapulohumeral and coracohumeral ligaments. The glenoid underwent a major reorientation during the evolution of the avian shoulder from the primitive condition of being posteroventrally directed as in Deinonychus antirrhopus and coelurosaurs generally to being dorsolaterally directed as in modern forms. The laterally facing glenoid of Archaeopteryx lithographica was intermediate in orientation and provided for a substantial degree of wing elevation but not the fully abducted, sagittal positioning that modern birds employ at the upstroke-downstroke transition.