PHYLOGENETIC RELATIONSHIPS OF BASAL SAURISCHIANS: TESTING THE EVIDENCE FOR THE HERRERASAURIAN AFFINITIES OF TAWA

NOVAS, F. E.; EZCURRA, M. D.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontologia de Verterbados; 2011

Universidad Nacional de San Juan

In the last few years several new basal saurischians and theropods have been described from Triassic beds of South America (Martinez and Alcober, 2009; Alcober and Martinez, 2010; Ezcurra, 2010; Martinez et al., 2011), North America (Nesbitt et al., 2009; Sues et al., 2011), and India (Novas et al., 2011), increasing our knowledge about the early evolutionary radiation of these groups. Such discoveries were accompanied by their respective phylogenetic analyses, with contrasting results. Among these taxa, the North American Tawa Nesbitt, Smith, Irmis, Turner, Downs and Norell 2009 exhibits a mosaic of plesiomorphic and derived features that led to its being interpreted as a transitional form between herrerasaurids and neotheropods (Nesbitt et al., 2009). However, our phylogenetic analysis recovers Tawa as a basal member of Herrerasauria, based on the following synapomorphies: jugal with anterodorsally inclined main axis, expanded anterior process with well-developed dorsal prong, and minimum height equal or more than 30% of the orbit height; mandible with blocky and dorsally raised retroarticular process; scapular blade poorly expanded distally and with a straight anterior margin; metacarpal IV:metacarpal III ratio less than 0.6; ilium with a pubic peduncle distinctly more ventrally extended than the ischial one; and pubis with distal pubic boot. In the context of the present study, Herrerasauria is composed of ISI R282, Tawa, Staurikosaurus Colbert 1970, Herrerasaurus Reig 1963, and Sanjuansaurus Alcober and Martinez 2010, ordered in a pectinate topology. Moreover, Herrerasauria is recovered as the sister-group of Eusaurischia (= Theropoda + Sauropodomorpha), in agreement with Langer (2004), but contrary to Nesbitt et al. (2009) and Martinez et al. (2011), who depicted Herrerasauria as basal theropods. We agree with Martinez et al. (2011) in considering Eodromaeus Martinez, Sereno, Alcober, Colombi, Renne, Montañez and Currie 2011 as a basal theropod and Eoraptor Sereno, Forster, Rogers and Monetta 1993 as the most basal sauropodomorph. The results of the present analysis indicate that the early evolution of Saurischia was more complex than thought, and included branches that did not belong to the main lineages into which Dinosauria is usually divided. In our view, Herrerasauria and the basal sauropodomorphs Guaibasauridae Bonaparte et al., 1999 represent evolutionary radiations that started before the flowering of neotheropods and more derived sauropodomorphs, and that become extinct during the latest Triassic.