Stratigraphic Reassessment of the Mexican Chasmosaurine Coahuilaceratops magnacuerna as the First Diagnostic Dinosaur Remains from the Cerro Huerta Formation (Lower Maastrichtian) Supporting the Southern Origin of the Triceratopsini

Very few remains of ceratopsid dinosaurs have been recovered so far from the Difunta Group of Coahuila, Mexico. The enigmatic chasmosaurine Coahuilaceratops magnacuerna was previously described on the basis of two partial skulls purportedly derived from the Cerro del Pueblo Formation (~73–72.5 Ma?). On the basis of a new measured section and lithological identification of the host rock, we reassign Coahuilaceratops to the overlying Cerro Huerta Formation (~71.5–70.5 Ma?). Thus, we formally assign the first dinosaur taxon to the Cerro Huerta Formation. This reassignment is more consistent with the relatively derived phylogenetic position of Coahuilaceratops, with implications for the southern Laramidia hypothesis concerning the origin of the Triceratopsini.