GUAIBASAURIDAE, A NEW CLADE OF TRIASSIC BASAL SAUROPODOMORPHS

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

Bristol

Congreso; 69th Annual Meeting Society of Vertebrate Paleontology; 2009

The family Guaibasauridae was coined as a monospecific entity composed by Guaibasauruscandelariensis from the Norian Caturrita Formation of Brazil. Since then, the phylogeneticposition of Guaibasaurus remained controversial, being alternatively interpreted as a basalsaurischian or a basal theropod. Nevertheless, a new cladistic analysis depicts Guaibasaurusas closely related with Panphagia, Agnosphitys, Saturnalia, and two yet undescribed formsfrom Argentina and India. We employ the term Guaibasauridae for the monophyletic cladegathering these Late Triassic taxa, which lies at the base of Sauropodomorpha. In particular,Guaibasaurus is for the first time included within Sauropodomorpha, supported by thepresence of proximal caudal vertebrae with the base of the neural spine anteroposteriorlylonger than half the length of the neural arch, ilium with strongly laterally curved bladeand elongated pubic peduncle, and ischial shaft triangular-shaped in cross-section.Features supporting the monophyly of Guaibasauridae include: ilium with incipiently openacetabulum, elongated postacetabular process, femur with proximal anterior trochanter atlevel with femoral head, and tibial distal end with concave posterolateral corner. Contrarilyto recent interpretations, Panphagia is not depicted as the most basal sauropodomorph,and characters supporting its original assignment are more widely distributed among basalsaurischians (distally curved anterior dentary teeth and sub-circular distal ischium) or missinterpreted(symmetric proximal tibial condyles). Accordingly, Guaibasauridae is interpretedas a large monophyletic clade of early dinosaurs, which radiated during late Carnian-Noriantimes and attained a wide geographic distribution. Furthermore, no large monophyleticclades were previously recognized among basal dinosaurs, and Guaibasauridae constitutesthe first evidence of a large clade which radiated apart from the branch leading towardsJurassic and Cretaceous taxa, in this case the sauropod lineage.