First sauropod (Dinosauria: Saurischia) remains from the Guichَn Formation, Late Cretaceous of Uruguay

We report the first sauropod remains recorded in the Guichَn Formation, western Uruguay. The materials belong to a middle-sized sauropod represented by more than one individual, and among other remains include more than fifty caudal centra. Close to the bones, several eggshell fragments resembling Sphaerovum erbeni Mones, 1980 were found. We discuss the biostratigraphic implications of these findings, which for the first time allow us to confidently refer the Guichَn Formation to the Late Cretaceous. The combination of several synapomorphies such as a biconvex first caudal centrum, strongly procoelous middle and distal caudal centra, and a pyramidal astragalus suggests that the sauropod remains belong to a derived lithostrotian, probably related to Pellegrinisaurus powelli, Baurutitan britoi and Alamosaurus sanjuanensis. A few isolated teeth (now lost) referred by Frederich von Huene in 1934 to ornithomimid theropods and ornithischians are herein reinterpreted as belonging to indeterminate theropods and basal iguanodontians.