New Patagonian Cretaceous theropod sheds light about the early radiation of Coelurosauria

摘要: Here we describe a new theropod, Bicentenaria argentina nov. gen. et sp., from the early Late Cretaceous of Patagonia. It is represented by more than hundred bones belonging to different sized individuals, which were buried together in disarticulation after little transportation. The available association skeletal elements suggests gregarious behaviour for , an ethological trait also recorded among other theropod clades. Increasing documentation monospecific assemblages groups theropods that ehaviour may have constituted ancestral condition Theropoda, at least. characterizes surangular bone with high dorsal margin and prominent lateral shelf, retroarticular process low, wide spoon-shaped, quadrate its condyle larger medial one. Phylogenetic analysis found Chinese Tugulusaurus atagonian as successive sister taxa all coelurosaurs, thus revealing importance taxon understanding diversification Coelurosauria. In particular, amplifies array basal coelurosaurs inhabited Gondwana during Cretaceous, including compsognathids, Aniksosaurus Santanaraptor. Although still restricted handful forms, information indicates was cradle evolution lineages those documented Upper beds northern landmasses. Analysis body size distribution averostran results identification two main episodes drastic reduction evolutionary history Coelurosauria: one occurred initial radiation group (as Bicentenaria, Zuolong, Tugulusaurus, Aniksosaurus), second episode diverification Paraves or avialans. Reduction allowed adult coelurosaurians exploit ecological niches not occupied before averostrans.