Pectoral girdle morphology of Mesozoic birds and the evolution of the avian supracoracoideus muscle

The evolution of the avian flight musculature is poorly understood and most studies focused on Archaeopteryx. However, important evolutionary changes occurred within Aves, and here the morphology of the pectoral girdle of Mesozoic birds is interpreted in relation to the evolution of the supracoracoideus muscle, which elevates the wing in the upstroke. In the course of avian evolution, the main origin of this muscle shifted from the coracoid onto the sternum. It is hypothesized that resulting space constraints in the cranial portion of the sternum, which also served as the attachment site for the large pectoralis muscle, led to the evolution of a sternal keel. The cranial portion of the sternum of Early Cretaceous Enantiornithes—one of the major clades of Mesozoic birds—lacks a well-developed keel, and the development of the supracoracoideus muscle must have differed from extant birds. This assumption is in concordance with the fact that a triosseal canal, which forms a pulley for the tendon of this muscle in extant birds, appears not to have been developed in Enantiornithes.