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

NOVAS, F. E.; EZCURRA, M. D.; AGNOLIN, F. L.; POL, D.; ORTIZ, R.

Revista del Museo Argentino de Ciencias Naturales

Museo Argentino de Ciencias Naturales

Lugar: Buenos Aires; Año: 2012 vol. 14

1514-5158

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