The last of the oldies: a basal rebbachisaurid (Sauropoda, Diplodocoidea) from the early Late Cretaceous (Cenomanian–Turonian) of Patagonia, Argentina

Rebbachisauridae is mainly recorded during the early Late Cretaceous in Gondwana, and in South America in particular. Sidersaura marae gen. et sp. nov. was found in rocks of the Huincul Formation (upper Cenomanian–Turonian) of Neuquén Province, Argentina. Sidersaura has notable characteristics that allow recognising it as a new species and defining its phylogenetic relationships. First, it has a frontoparietal foramen, as in dicraeosauridae. It bears fourteen caudal vertebrae with neural arches displaced closer to the anterior margin of the centrum and a ventral longitudinal hollow on middle of the centra, both characters previously considered as titanosaurian synapomorphies, and here recognised in these diplodocoid sauropods. Sidersaura shows a peculiar tarsal condition with an unusual calcaneum morphology which resembles that of basal sauropods. The haemal arches have a stellate morphology with two sets of projections comparable to the specimen MMCh-PV 47 from the Candeleros Formation (Cenomanian), previously described as a titanosaurian. The phylogenetic analysis retrieves Sidersaura as a basal Rebbachisauridae more closely related to Zapalasaurus than to Limaysaurinae. The presence of a basal taxon at Cenomanian–Turonian times, so close to the extinction of the group, implies that the evolutionary history of rebbachisauridae was more complex than previously thought.