CALIBRATING THE EVOLUTIONARY TREE OF SAUROPODOMORPH DINOSAURS IN THE JURASSIC AND CRETACEOUS OF SOUTH AMERICA (Invited Presentation)

Sauropodomorph dinosaurs dominated the herbivorous niches throughout most of the Mesozoic Era. Although their origin dates back to the Late Triassic, the most successful lineages (eusauropods) radiated during the Jurassic and Early Cretaceous. Despite recent progress in unraveling the phylogenetic relationships among major groups of Sauropoda, our understanding of the timing of sauropod diversification events remains rather poor. The Cañadón Asfalto Basin (Patagonia, South America) provides a unique opportunity to place radioisotopic age constraints on sauropod-bearing successions that record major evolutionary event of this group. Within this basin, two Jurassic geological units have yielded important eusauropod assemblages. The Cañadón Asfalto Formation has yielded the basal Patagosaurus, Volkheimeria, and two undescribed taxa, which have been largely constrained to late Early Jurassic. The Cañadón Calcáreo Formation (Oxfordian-Kimmeridgian) includes two lineages of neosauropods, the macronarian Tehuelchesaurus and a brachiosaurid, as well as the diplodocoids Brachytrachelopan and an unnamed diplodocid. These units are unconformably overlain by the Cretaceous Los Adobes and Cerro Barcino formations, the latter of which has yielded some of the most complete titanosauriform remains (the titanosauriform Chubutisaurus and an undescribed gigantic basal titanosaur) from the Early Cretaceous of South America. Here we present new taxonomic, phylogenetic and U-Pb age data from the Jurassic and Cretaceous successions that help bracket three major evolutionary events of Sauropodomorpha. An initial faunal replacement event involving the extinction of non-sauropod species and subsequent radiation of large-bodied eusauropods that has been bracketed between Pliensbachian and Toarcian; a second major radiation event represented by the appearance of a neosauropod fauna including macronarian and diplodocoids in the Cañadón Calcáreo Formation that is inferred to have occurred at least by the Oxfordian-Kimmeridgian; and finally, the newly investigated radiation of an endemic clade of gigantic titanosaur sauropods from the late Albian of Patagonia, coincident with the establishment of the mid-Cretaceous Gondwanan fauna and the emergence of angiosperms in the continental ecosystems.