New Information on Lessemsaurus sauropoides (Dinosauria, Sauropodomorpha) from the Late Triassic of Argentina

POL, D.; POWELL, J.E.

Mesa, Arizona, EEUU

Congreso; 64th Meeting of the Society of Vertebrate Paleontology; 2005

Society of Vertebrate Paleontology

New information on Lessemsaurus sauropoides from the Upper Triassic of Argentina is presented here. These materials provide unrecorded transitional morphologies between the generalized anatomy of basal forms and the derived condition of Sauropoda. The phylogenetic relationships of this form are approached here through a cladistic analysis of basal sauropodomorphs recorded worldwide in Upper Triassic and Lower Jurassic beds of Pangea. The most parsimonious trees recovered in the cladistic analysis support a paraphyletic arrangement most taxa traditionally referred as Prosauropoda, being depicted as series of clades that form the successive sister-taxa of Sauropoda. The inclusion of the usually overlooked ‘melanorosaurids’ (e.g., Lessemsaurus sauropoides, Melanorosaurus readi, Antetonitrus ingenipes) is critical due to their unique combination of plesiomorphic and apomorphic characters. The analysis shows that several of the currently recognized sauropod synapomorphies had actually appeared earlier in the evolutionary history of Sauropodomorpha.The stratigraphic fit of alternative phylogenetic hypotheses of basal Sauropodomorpha is contrasted here. This comparison shows that all previously proposed phylogenies show similar degrees of agreement with the temporal data from the fossil record and results in the discovery of a large Norian radiation of sauropodomorph dinosaurs that includes the origins most known ‘prosauropod’ lineages as well as the sauropod lineage.