Fossil vertebrates from the Cerro Fortaleza Formation (Upper Cretaceous), Santa Cruz Province, Argentina

Abstract

The Cerro Fortaleza Formation is a stratigraphic unit that crops out near the Cerro Fortaleza hill, northwestern Santa Cruz province, Argentina. This unit has been well-known since the end of the XIX century, and because of the finding of abundant dinosaur bones, it was named as the “Capas con dinosaurios” (=Dinosaur Beds). Intensive work since the end of the 1990's resulted in the finding of diverse dinosaur species and other vertebrates. The fossil assemblage recovered includes lamniform sharks, lepisosteiforms, the dipnoan Atlantoceratodus iheringi, chelid turtles, peirosaurid crocodyliforms, mosasaur teeth, and diverse dinosaur remains. Dinosaurs include a partial tail of a small-sized titanosauriform, and another, identified as Dreadnoughtus, shows derived procoelous caudals. Isolated sauropod teeth indicate the presence of a derived lithostrotian and the enigmatic Clasmodosaurus spatula Ameghino, 1899. Among theropods, abelisaurid and megaraptorids are reported. A partial ulna, manual claw and metatarsal belonging to the holotype of the enigmatic theropod Austrocheirus isasii are also described. The morphology of the new elements does not comfortably fit with abelisaurid or megaraptorid theropods, and the affinities of Austrocheirus among theropods still remain elusive. The fossil vertebrate content of the Cerro Fortaleza Formation indicates close similarities with the faunistic assemblage previously reported from the Mata Amarilla Formation (Cenomanian-Turonian), and strong differences are present with the overlying Chorrillo Formation (Maastrichtian). This may indicate that some kind of change among terrestrial vertebrates occurred at the Campanian/Maastrichtian boundary in the Austral Patagonia Basin.