The pectoral girdle and forelimb of the basal theropodHerrerasaurus ischigualastensis

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Abstract
New specimens of Herrerasaurus ischigualastensis shed light on the structure and function of the pectoral girdle and forelimb in early theropod dinosaurs. As in tetanurian theropods, the scapulocoracoid has a broadly expanded acromion and strap-shaped scapular blade. The forelimb is less than one-half the length of the hind-limb and is specialized for prey capture and manipulation. The short proximal segments of the forelimb provide mechanical advantage and are marked by prominent tuberosities. The well-ossified carpus is divided into two functional units, one composed of the radiale and ulnare and the other composed of a centrale and distal carpals, and is designed functionally for dorsoventral extension and flexion of the manus against the forearm. The manus is longer than the humerus, radius, or ulna and is specialized for grasping and raking, as shown by the marked metacarpal extensor depressions, long penultimate phalanges, and trenchant unguals in digits I–III. Upon flexion or extension, the unguals of digits I–III converge. Manual digits IV and V are very reduced, and the phalangeal formula is 2–3–4–1–0. Increase in the length of the deltopectoral crest and decrease in the length of manual digit IV and loss of its terminal ungual constitute the only dinosaurian synapomorphies in the pectoral girdle and forelimb. Many previously cited dinosaurian synapomorphies in the pectoral girdle and forelimb cannot be substantiated. Several synapomorphies in the scapula and manus unite Herrerasaurus ischigualastensis with other theropods.