DESCRIPTION OF THE DORSAL VERTEBRAE OF DREADNOUGHTUS SCHRANI, A LARGE TITANOSAURIAN SAUROPOD FROM THE CRETACEOUS OF SOUTHERN PATAGONIA, ARGENTINA

Dreadnoughtus schrani is a new, well-preserved, super-massive titanosaur from the Campanian-Maastrichtian Cerro Fortaleza Formation in Patagonia Argentina. Of the five largest titanosaurs known, this taxon includes only the second nearly complete dorsal vertebral series, enhancing our fragmentary knowledge of these extreme giants. It includes portions of eight dorsal vertebrae representing the anterior, middle, and posterior sections of the vertebral series. All vertebrae of Dreadnoughtus are strongly opisthocoelous, camellate, and have deep lateral pneumatic fossae. Although not all the vertebrae can be definitively discerned between the holotype or paratype, there is minimal, if any, overlap of preserved elements between the two individuals and variation within the series is identifiable. This includes a hook-shaped postzygodiapophyseal lamina (podl) arising in the middle dorsal vertebrae. This is similar in form to the “disconnection” or lamina capture of the podl in Bonitasaura. Also, in the posterior dorsal vertebrae the spinodiapophyseal lamina divides and posterior centroparapophyseal lamina arises and becomes more developed. Such serial variation allows the vertebrae to be arranged in sequential order despite being disarticulated in the quarry. As in Opisthocoelicaudia, several vertebrae of Dreadnoughtus preserve scarring on the distal dorsal margin of the transverse process, previously suggested to denote the attachment site of a muscle or ligament. When compared to other super-massive titanosaurs both Dreadnoughtus and Futalognkosaurus have dorsoventrally narrow transverse processes, unlike Puertasaurus, but Dreadnoughtus and Argentinosaurus have ventromedially inclined prezygapophyses while Futalognkosaurus has almost horizontal prezygapophyses. Even though many titanosaur taxa are fragmentary, the completeness of Dreadnoughtus schrani permits comparisons among them. By parsing out the relationships of these giant dinosaurs, we can enhance our understanding of the evolution of gigantism in Titanosauria. Because the only overlapping elements between many of these giant titanosaurs are vertebrae, the new taxon will be integral to this endeavour until more complete skeletons are discovered.