A reappraisal of the Late Cretaceous Argentinean sauropod dinosaur Argyrosaurus superbus, with a description of a new titanosaur genus

ABSTRACT

Argyrosaurus superbus is one of the earliest-named Argentinean dinosaurs. The holotype comprises a complete forelimb, probably from the upper member of the Bajo Barreal Formation (Late Cretaceous), Chubut Province. Numerous remains have been referred to Argyrosaurus from Argentina and Uruguay; however, the type specimen has not been adequately diagnosed and referrals have predominantly been based upon their large size. Here we redescribe Argyrosaurus, demonstrating it to be a valid titanosaur genus based on five autapomorphies, as well as an unique character combination. The exact placement of Argyrosaurus within Titanosauria is uncertain, although the probable presence of carpal bones, otherwise unknown in titanosaurs, may indicate a basal position. None of the referred remains can be attributed to Argyrosaurus, and most should be regarded as indeterminate titanosauriforms. The exception to this is a partial skeleton from the lower member of the Bajo Barreal Formation (early Late Cretaceous), Chubut Province, comprising dorsal and caudal vertebrae, as well as numerous appendicular elements. This specimen is distinct from Argyrosaurus and can also be differentiated from other sauropods based on an unusual character combination (including plesiomorphic tarsus), plus one autapomorphy. Elaltitan lilloi, gen. et sp. nov., displays numerous titanosaur characters and shares several features with derived taxa such as Neuquensaurus, Opisthocoelicaudia, Rapetosaurus, Saltasaurus, and Trigonosaurus. Elaltitan can be referred to Lithostrotia; however, its precise position within this clade must await future phylogenetic analysis. The revision and description of the titanosaurs Argyrosaurus and Elaltitan provides new information on this diverse but still poorly understood clade.

ACKNOWLEDGMENTS

We are extremely grateful to all who allowed us to study specimens in their care and/or provided assistance, in particular S. Apesteguía, J. O. Calvo, I. A. Cerda, R. A. Coria, L. Ivy, A. G. Kramarz, J. E. Powell, L. Salgado, and W. F. Simpson. M. C. Lamanna is thanked for providing detailed information on the geographic and stratigraphic provenance of Argyrosaurus and Elaltitan. M. D. D’Emic and J. A. Wilson kindly provided additional photographs of Elaltitan. Reviews and constructive comments by M. D. D’Emic, M. C. Lamanna, and an anonymous referee greatly improved earlier versions of this work. We also acknowledge the work of J. F. Bonaparte, who discovered, collected, and prepared the holotype of Elaltitan. English translations of many of the foreign-language papers used in this study are available at The Polyglot Paleontologist Web site (www.paleoglot.org). P.D.M.'s research was supported by a University College London NERC studentship (NER/S/A/2006/14347), an Alexander von Humboldt Research Fellowship, an Imperial College London Junior Research Fellowship, and a Jurassic Foundation grant which enabled travel to South America to study sauropod specimens. A.O.'s research was supported by the Consejo Nacional de Investigaciones Científicas y Técnicas. The restoration of the Argyrosaurus holotype was supported by a separate Jurassic Foundation grant, awarded to A.O.

Handling editor: Patrick O’Connor.