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Fragmentary skeletons of the earliest and most primitive dinosaurs were discovered more than 30 years ago in Upper Triassic rocks in South America and include Staurikosaurus pricei from the Santa Maria Formation of southern Brazil (1) and Pisanosaurus mertii and Herrerasaurus ischigualastensis from the Ischigualasto Formation of northwestern Argentina (2-5). These formations, deposited approximately 225 million years ago, record the initial phase of dinosaur evolution before their domination of terrestrial vertebrate communities (6). The remains of these early dinosaurs, however, are too fragmentary to provide a coherent view of the origin and early radiation of dinosaurs. Recent work in the Ischigualasto Formation resulted in the discovery of several skeletons of Herrerasaurus (7) that provide the first complete picture of an early dinosaur and clarify the sequence of anatomical changes that occurred during early dinosaur evolution.
The skull, previously undiscovered, lacks nearly all of the specializations of later dinosaurs. Narrow from side to side and low in profile (Fig. 1), the skull has small external nares and a narrow antorbital fossa without diverticulae (8). (Figure 1 omitted) The most significant cranial specialization is a well-developed sliding joint within each lower jaw. As in theropods, this intramandibular joint allowed the lower jaws to flex, with the anterior toothed half capable of rotating approximately 15 deg against the posterior half in Herrerasaurus. Extant anguimorphine lizards have evolved an analogous intramandibular joint and a flexible, grasping bite, which may be adaptations to subdue live prey (9).
The neck, pectoral girdle, and forelimb are also represented among the new fossils and show that Herrerasaurus was an obligatory biped with a slender, flexible neck (Fig. 2) (8, 10). (Figure 2 omitted) The forelimb, which is less than half the length of the hind limb, has short proximal segments (humerus and radius-ulna) and an elongate manus (Figs. 3 and 4A). (Figures 3 and 4A omitted) Manual digits IV and V are reduced and would not have extended as free digits beyond the palm of the hand (phalangeal formula 2-3-4-1-0). The elongate penultimate phalanges and large recurved unguals on digits I through III indicate that the manus is specialized for grasping (8).
This material of Herrerasaurus clarifies anatomical features that arose in the common dinosaurian ancestor and revises our understanding of the structure...