CONICET | Buscador de Institutos y Recursos Humanos

Previous authors have stated a replacement of the dinosaurian faunas after the Carnian-Norian (C-N) extinction event (Upper Triassic), in which the primitive Carnian saurischians, i.e., basal-most theropods (e.g., Eoraptor, herrerasaurids) and sauropodomorphs (e.g., Saturnalia), with still similar plesiomorphic pelvic and hindlimb morphology, became scarce, and the more advanced coelophysoids, prosauropods, and sauropods took part as the dominant Norian dinosaurian fauna. Recently, new discoveries and studies shed light on the early radiation of these lineages. Regarding to the coelophysoids, ?Camposaurus? constitutes their oldest record (late Carnian, USA), but its fragmentary nature prevents to reveal its phylogenetic placement. Thus, Coelophysis bauri (Norian, USA) constitutes one of the oldest coelophysoids with unequivocally resolved phylogenetic relationships, but interpreted as one of its more derived members. On the other hand, the stem-coelophysoid Dilophosaurus wetherilli is one of its youngest representatives. In this phylogenetic context, a ghost lineage of ?dilophosaurids? is depicted, far ago to Norian times, at least. In this regard, the presence of Norian ?dilophosaurids?, coelophysids, and intermediate forms (e.g., Liliensternus, Gojirasaurus, Zupaysaurus) suggest a fast morphological divergence and the establishment of the coelophysoid clades at least at this age. Furthermore, the recent re-interpretation of coelophysoids as the sister-group of all the remaining neotheropods (i.e., averostrans), suggest the origin of the latter during the middle-late Carnian, at least. Since oldest averostrans are recovered from late Early-Middle Jurassic outcrops, a large ghost lineage of basal averostrans is depicted. Regarding to the Norian sauropodomorphs, their exhibit a similar fast process of cladogenesis to that of coelophysoids. The Carnian fossil record depicts a scarce of sauropodomorphs, with only the small Saturnalia tupiniquim, coming from the Carnian (Ischigualastian) of Brazil, as an unequivocal member, and even the oldest. Only after the C-N boundary, an increase in size, geographic distribution, and diversity of sauropodomorphs occurred, reaching the position of important representatives in the Norian terrestrial faunas. The recent discoveries of Norian and Rhaetian sauropods and new insights on sauropodomorph phylogenetic relationships provide a different panorama to that previously thought. Yates and Kitching (2003), in light of new data provided by the Norian sauropod Antetonitrus ingenipes, stated that Prosauropoda is a lesser inclusive group that it was thought, with Anchisaurus and Melanorosaurus more closely related to Eusauropoda than to Plateosaurus (i.e., Anchisaurus and Melanorosaurus are sauropods). In this regard, both basal prosauropods (e.g., Plateosaurus, Coloradisaurus, Riojasaurus) and sauropods (e.g., Antetonitrus) are present in Norian outcrops (e.g., upper levels of the Los Colorados Formation, Lower Elliot Formation), thus depicting an evolutionary explosion during this age. In the C-N boundary a climatic change was documented around the world, indicating that the climate during part of the Carnian was significantly more humid than in the ensuing Norian and probably so than in much of the Ladinian (Simms et al., 1988). Bakker stated (1977) that Cretaceous sauropodomorphs were more common in seasonally dry climates and suggestively the Chinle Formation seems to be an exception for the world arid condition of the Norian (i.e., the C-N paleoclimatic change seems do not have occurred, showing evidences of a much longer period of humidity, from the late Carnian to even the late Norian; Simms et al., 1994), and in this outcrop, with well documented terrestrial tetrapod fauna, including dinosauriforms (Eucoelophysis, Chindesaurus, coelophysoids), only very scarce remains of basal sauropodomorphs were recovered (Long and Murry, 1995). Following this line of evidences, the arid conditions of the Norian may prompt a successful radiation of the early sauropodomorphs. In this context, both coelophysoids and sauropodomorphs seem to have a wide geographic distribution, in the case of the sauropodomorphs in arid environments, and fast diversification during the Norian, where the former may allow a high speciation rate.

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