A new macronarian (DInosauria, Sauropoda) from the Late Jurassic of Argentina: Implications for our understanding of early macronarian evolution

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The Sauropoda are one of the main lineages of dinosaurs. They do not only include the largest terrestrial animals known, but also represent important components of Mesozoic terrestrial ecosystems, being the largest and most abundant herbivores in many known assemblages. The main radiation of sauropods happened shortly after their origin in the Jurassic, and by the end of this period, most major lineages were established. One of the most important events in Jurassic sauropod evolution was the origin and initial radiation of Neosauropoda, the major clade within sauropods to which the vast majority of taxa known from the Late Jurassic to the latest Cretaceous belong to. However, whereas one of the main subclades of Neosauropoda, the Diplodocoidea, are rather well characterized and there are few taxa of uncertain referral to this clade (e.g. Haplocanthosaurus), the inclusivity of the other major clade, Macronaria, and the interrelationships of its early branching members is much more contentious. One problem in this respect is that our knowledge of early neosauropod evolution is almost entirely based on the fossil record of the northern hemisphere, with the Late Jurassic sauropod fauna from the Tendaguru Formation of Tanzania being basically the only source of information from Gondwana.Recent fieldwork in the Late Jurassic Cañadón Calcáreo Formation of Argentina has resulted in the recovery of several new sauropod specimens. One of these specimens is a partial skeleton with similarities, but also some differences, to the only other known sauropod taxon from the Late Jurassic of South America, the early branching macronarian Tehuelchesaurus, from the same formation. The objective of the current project is thus a detailed study of this new specimen, including a large scale phylogenetic analysis to establish its phylogenetic position. In the phylogenetic analysis, we furthermore intend to use comparative cladistic methods both a priori and a posteriori, to gain a better understanding of the problems surrounding the early evolution of macronarians and to identify possible ways to improve resolution and stability in this part of sauropod phylogeny. Thus, we expect this project to not only result in additional information on 'basal' macronarians from Gondwana, but also in a better understanding of early neosauropod evolution and the problems and challenges in interpreting this evolutionary event.