Dinosaurs from the Jurassic of Chubut: Evolutionary implications

RAUHUT, O.W.; POL, D.

Simposio; V Simposio Argentino del Jurásico; 2013

Dinosaurs experienced their most important radiation during the Jurassic, but the dinosaur fossil record for this time is rather poor, and our current understanding of this radiation is mainly based on the fossil record of the Northern Hemisphere. In South America, Jurassic dinosaurs are mainly known from the Cañadón Asfalto and Cañadón Calcáreo Formations in Chubut Province, Argentina. The Toarcian?Bajocian Cañadón Asfalto For- mation has yielded at least ten different taxa of dinosaurs, including the sauropods Patagosaurus, Volkheimeria and two undescribed sauropodomorphs, the theropods Eoabelisaurus, Piatnitzkysaurus, Condorraptor and an undescribed basal tetanuran, and the heterodontosaurid Manidens as well as an undescribed basal ornithischian. The fauna thus represents a transitional fauna between Early Jurassic faunas known from southern Africa and other localities, and Late Jurassic and Cretaceous faunas. As a heterodontosaurid, Manidens represents a typical component of Early Jurassic southern Gond- wanan faunas, whereas the theropods are among the earliest known representatives of important clades (ceratosaurs and tetanurans, respectively) that replaced Early Jurassic coelophysoids. All sauropodomorphs are sauropods at different evolutionary stages on the stem-lineage to neosauropods, but non-sauropodan sauropodomorphs are missing. This faunal composition supports the idea of a drastic faunal turnover at the end of the Early Jurassic and indicates a rapid radiation especially of theropod dinosaurs at about this time. From the Oxfordian?Tithonian Cañadón Calcáreo Formation, only neosauropodan sauropods have been described so far (the macronarians Tehuelchesaurus and an unnamed brachiosaurid as well as the diplodocoid Brachytrachelopan), indicating a complete replacement of basal sauropod lineages by the Late Jurassic.