The Chinese colossus: an evaluation of the phylogeny of Ruyangosaurus giganteus and its implications for titanosaur evolution

For many years the precise taxonomy of Titanosauria has been a puzzle, and even today only certain segments of this vast clade are well-understood. The phylogenetic positions of many titanosaurs are murky, though specimens often still await rigorous analysis. One of the largest examples is the massive Chinese titanosaur Ruyangosaurus giganteus – though largely incomplete, the holotype is distinct enough to indicate strong phylogenetic affinities with a specific subgroup of titanosaurs. A review of previous literature on Ruyangosaurus, referred tentatively to Andesauridae, shows that this classification is based on three weak, non-diagnostic characters. Ruyangosaurus differs from taxa traditionally included in Andesauridae in at least 20 characters of the torso, femur, and tibia. Several plesiomorphies of Ruyangosaurus are extremely rare in titanosauria except for the clade Lognkosauria and its close relatives. The vertebra initially described as a posterior cervical is most likely an anterior dorsal, with a strong resemblance to that of Puertasaurus. The posterior dorsal of Ruyangosaurus shares synapomorphies with Mendozasaurus and Dreadnoughtus. The femur clusters close to the femora of Malawisaurus, Traukutitan, and Pitekunsaurus. Ruyangosaurus is here recovered as a lognkosaurian, with significant implications for the distribution and evolution of that group and the paleobiology of Mid-Cretaceous China.