Pectoral girdle

Other animals

The pectoral girdle demonstrates an enormous variation in amniotes (vertebrates adapted to a full terrestrial life), both among extant species and along evolutionary lines, and determining homologies for individual pectoral elements is difficult. Except for the sternum, these elements (along with the pelvic girdle) were, however, present in early bony fishes before there were even limbs, arising from their ancestral external armor plates. In digitless choanates, the cleithrum, clavicle, and interclavicle are dermal and linked to the caudal part of the head while the humerus articulates with a small scapulocoracoid bone. As the first digits appeared, the pectoral structure lost its direct connection to the head skeleton while the scapulocoracoid grew more prominent and started to face laterally. In true tetrapods, the dermal part of the girdle was gradually reduced and the scapulocoracoid split into a dorsal scapula and a ventral coracoid. As for the sternum, it also came from the fusion of the inner ends of the pectoral girdles in tetrapods, subsequently growing between the ribs.