Evolutionary Variation
The pectoral girdle demonstrates an enormous variation in amniotes (animals adapted to a 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 were, however, present in early bony fishes before there were even limbs. In digitless tetrapods 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 more derived tetrapods the dermal part of the girdle was gradually reduced and the scapulocoracoid split into a dorsal scapula and a ventral coracoid.
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