New anatomical information on Rhoetosaurus brownei Longman, 1926, a gravisaurian sauropodomorph dinosaur from the Middle Jurassic of Queensland, Australia

ABSTRACT

Rhoetosaurus brownei is the only known named pre-Cretaceous sauropod from Australia. It is therefore a potentially important taxon for understanding global paleobiogeographic and phylogenetic patterns among early sauropods. Despite its obvious significance, Rhoetosaurus has been too poorly understood to be included in most recent analyses of early sauropod evolution. With this in mind, we evaluated the osteology and phylogeny of undescribed materials of Rhoetosaurus, in order to attempt to close the gap in this understanding. The lower hind limb of Rhoetosaurus highlights a plethora of differences from other sauropods, supporting the distinctiveness of Rhoetosaurus even in the absence of other materials. Some unique traits include prominent crests and sulci on the tibia medially, a narrow metatarsal articular bridge, and pedal claws with an accessory groove or fossa. The pes plesiomorphically retains four claws where most sauropods have three, and bears superficial similarity to that of Shunosaurus. Preliminary cladistic analysis confirms that Rhoetosaurus is a non-neosauropod gravisaurian, although weak support for the most parsimonious topology suggests further findings are required to improve upon incompleteness in the character data. Examination of alternative phylogenetic hypotheses rules out a close relationship between Rhoetosaurus and East Asian Jurassic sauropods, and indicates a closer examination of the potential relationships between Rhoetosaurus and other contemporaneous Middle Jurassic Gondwanan sauropods is necessary.

ACKNOWLEDGMENTS

This research was not possible without the efforts of local paleontologists who contributed to recovery of Rhoetosaurus specimens in the 1970s–1990s. Amongst these we thank Drs. A. Bartholomai, R. Molnar, T. Rich, A. Rozefleds, T. Thulborn, A. Warren, and X. Zhao. In particular, the late Dr. Mary Wade was instrumental for her role in the recovery of the pes. For access to specimens in their care at Queensland Museum, we thank K. Spring and S. Hocknull. D. Lewis produced moulds of the Rhoetosaurus pes, which we cast with the assistance of M. Herne and R. Berrell. T. Rich provided unpublished diary notes pertaining to the type locality. For use of photographs or other data, or for relevant discussions, we thank A. Bartholomai, L. Beirne, M. Herne, T. Ikejiri, J. McKellar, R. Molnar, P. Rose, and the late Norman Timms (who provided a recollection of the original discovery). We appreciate the feedback given by the anonymous reviewers, M. D’Emic, and co-senior editor P.M. Barrett. This research was funded in part by the Australian Research Council (LP0347332 and LP0776851) and The University of Queensland (to S.W.S.), in association with Longreach Regional Council, Winton Shire Council, Land Rover Australia, the Queensland Museum, and a Rea Postdoctoral Fellowship (to S.W.S.) at Carnegie Museum of Natural History.

Handling editor: Hailu You