Osteology of the Wide-Hipped Titanosaurian Sauropod Dinosaur Savannasaurus Elliottorum from the Upper Cretaceous Winton Formation of Queensland, Australia

ABSTRACT

The titanosaurian sauropod dinosaur Savannasaurus elliottorum is represented by a partial postcranial skeleton from the lower Upper Cretaceous (Cenomanian–lowermost Turonian) Winton Formation of Queensland, northeast Australia. Here, we present a detailed description of this specimen, as well as an emended diagnosis for this titanosaur. Savannasaurus elliottorum displays numerous character states that are generally regarded as plesiomorphic for Titanosauria, as well as several traits that are often regarded as apomorphic of that clade or a less inclusive subset thereof. Several features of Savannasaurus support a close relationship with the coeval Diamantinasaurus matildae, and this clade appears to occupy an early-branching position within Titanosauria. Relative to body size, the thoracic and abdominal breadth of Savannasaurus is greater than that seen in giant titanosaurs such as the contemporaneous South American lognkosaurians; however, this relative breadth is not quite as extreme as that of the small-bodied latest Cretaceous saltasaurines, or Opisthocoelicaudia skarzynskii. The possible advantages engendered by the barrel-shaped thorax, robust limbs, wide-gauge gait, and lack of hyposphene-hypantrum articulations are explored, and it is hypothesized that these traits were positively selected by the wet, temperate floodplain environment in which Savannasaurus lived. Greater stability and flexibility might have reduced the risk of bogging, and/or facilitated more expedient self-extraction from muddy waterholes. Similar environmental pressures acting upon other titanosaurian taxa or clades elsewhere might have led to the repeated independent development, or accentuation, of the bauplan regarded as ‘typical’ for the clade Titanosauria. This would explain the many observed convergences between Savannasaurus and Diamantinasaurus, and Saltasauridae.

ACKNOWLEDGMENTS

The authors would like to thank: additional members of the Elliott family for facilitating the excavation of ‘Wade’ on Belmont Station in 2005; the volunteers who participated in the ‘Wade’ digs in 2005 and who prepared the specimen over the course of the next decade; the staff at the Australian Age of Dinosaurs Museum of Natural History; A. Kramarz and M. Ezcurra (Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’), A. Otero (Museo de La Plata), J. Powell (Universidad Nacional de Tucuman), B. González Riga and L. Ortíz David (Universidad Nacional de Cuyo), J. Calvo and J. Mansilla (Museo de Geología y Paleontología de la Universidad Nacional del Comahue), V. Zurriaguz and I. Cerda (Universidad Nacional de Río Negro; Museo Provincial de Cipolletti ‘Carlos Ameghino’), D. Pol and J. Carballido (Museo Paleontológico Egidio Feruglio), R. Martínez and M. Luna (Universidad Nacional de la Patagonia San Juan Bosco), L. Ibiricu (Instituto Patagónico de Geología y Paleontología), and others for allowing S.F.P., P.D.M., and P.U. to make firsthand observations of Argentinean sauropod specimens in their care; and reviewers J. Whitlock (Mount Aloysius College) and J. Carballido (Museo Paleontológico Egidio Feruglio), and editor M. D’Emic (Adelphi University) for their constructive comments that greatly improved this manuscript. S.F.P.’s early work on this project was funded by an Australian Research Council Linkage Grant (LP100100339, awarded to B. Kear) and hosted at Uppsala University, Sweden. S.F.P. would also like to thank the Winston Churchill Memorial Trust for a Churchill Fellowship (awarded in 2017) that enabled him to examine several Argentinian sauropod specimens, and The Paleontological Society for an Arthur James Boucot Research Grant (awarded in 2017) that funded a trip to Winton to study Savannasaurus at first hand. P.D.M.’s research is supported by a Royal Society University Research Fellowship (UF160216). P.U.’s contribution was facilitated by a Leverhulme Trust Research Grant (RPG-129).