Ontogenic Development of the Acrocoracoid Process Responsible for the Evolution of Avian Flapping Flight

When modern birds engage in powered flight by flapping their wings, the upstroke movement is produced mainly by a ventrally located muscle called the musculus (m.) supracoracoideus. To achieve this unique mechanism for elevating the humerus, birds have evolved the triosseal canal, a tunnel-like structure which deflects the tendon of the m. supracoracoideus. This canal comprises bony projections of three different skeletal elements of the pectoral girdle (i.e., scapula, coracoid, and clavicle). Importantly, the acrocoracoid process of the coracoid structurally constitutes an essential component for the realization of the triosseal canal's function. Consequently, it has been identified as a key morphological feature for flapping in modern birds. Although limb and girdle development has been extensively studied using avian embryos, the morphogenesis of the avian coracoid itself remains poorly described. To address this knowledge gap, we performed morphological and histological analyses of the shoulder musculoskeletal system of the avian embryo, focusing on the development of the acrocoracoid process. We found that the acrocoracoid process emerges during embryonic development as a lateral protrusion of the dorsal coracoid. Meanwhile, the tendon of the m. supracoracoideus elongates laterally. Using histological analysis, we demonstrate that several shoulder and limb muscles, including the mm. biceps brachii, coracobrachialis, and deltoideus minor, attach to the acrocoracoid process during its development. Our results provide the first detailed description of the ontogenic development of the acrocoracoid process.