The Axial Skeleton of Rinconsaurus caudamirus (Sauropoda: Titanosauria) from the Late Cretaceous of Patagonia, Argentina

Titanosaurs were the predominant herbivores during the Late Cretaceous, inhabiting all continents. This clade was especially diverse in South America with some of the largest and smallest sauropod species known to date. Despite its diversity, the evolution of this clade is far from being well-known, although some recent analyses have begun to find some consensus in their results. Rinconsauria (defined as the least inclusive clade containing Rinconsaurus and Muyelensaurus) includes small titanosaurs and is considered as closely related to the lineage of giant titanosaurs, Lognkosauria (defined as the least inclusive clade containing Futalognkosaurus and Mendozasaurus), both being part of the recently named clade Colossosauria. The titanosaur Rinconsaurus caudamirus, from the Bajo de la Carpa Formation (Santonian), Río Negro, Argentina, is represented by several axial and appendicular elements from at least four specimens. This taxon was only briefly described when it was named and most of its originally proposed autapomorphies are now recognized as having a more widespread distribution amongst titanosaurs. Herein we present a detailed osteological description of the axial skeleton and a revised diagnosis for this taxon that firmly establishes its validity. Based on comparisons with other titanosaurs, we found three new possible autapomorphies for the axial skeleton of Rinconsaurus, which added to its original combination of characters, endorsing this taxon as a valid genus. Besides, this revision of Rinconsaurus provides additional osteological data that will contribute to a better resolution of titanosaur phylogeny, contributing at the same time to our understanding of the clade Rinconsauria.