A basal tetanuran theropod dinosaur from the Middle Jurassic of Argentina: implications for the early evolution of tetanuran theropods

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Theropod dinosaurs represent one of the most conspicuous components of Mesozoic vertebrate faunas and, in the form of birds, account for approximately one third of modern tetrapods. Recent research has demonstrated that theropods underwent an explosive radiation during the Jurassic, and most important lineages were established during this time. The main lineage radiating during this period are the tetanurans, which include well-known fossil forms, such as Allosaurus, Tyrannosaurus or Velociraptor, but also the birds. Tetanurans first appear in the early Middle Jurassic, and by the end of the period, all major lineages are established. However, especially the Middle Jurassic record of theropod dinosaurs is extremely poor, and most basal tetanuran taxa are represented by very incomplete material. Consequently, the early evolution and the phylogenetic interrelationships between basal tetanurans are still poorly understood. Recent fieldwork in the early Middle Jurassic (Aalenian-Bajocian) of Chubut Province, Argentina, resulted in the discovery of a partial theropod skeleton, including an almost complete skull, presacral vertebral column, forelimbs, and partial hindlimbs. This specimen represents the most complete basal tetanuran recovered from the Middle Jurassic and shows a unique combination of characters, including features that were hitherto considered apomorphic for different clades or even different species of basal tetanurans. The objective of the current project is thus a detailed description and analysis of this new basal tetanuran to evaluate its impact on our understanding of basal tetanuran evolution. We will describe and document the new taxon in detail and elaborate a large phylogenetic dataset, including several recently published phylogenetic matrices for basal theropods. The inclusion of the new taxon and the analysis of this matrix will result in new insights into the interrelationships of basal tetanurans as well as character distribution and homoplasy levels at the base of this important theropod lineage.