LATE CRETACEOUS THEROPOD DINOSAUR PALEOECOLOGY, BIG BEND NATIONAL PARK, TEXAS

Late Cretaceous theropod dinosaurs are not well known from southern North America, especially compared to northern areas like Wyoming, Montana, and Alberta. Collection of microvertebrate sites from the upper Aguja and lower Javelina Formations (late Campanian to late Maastrichtian) in Big Bend National Park, Texas has produced some of the southernmost records of theropods in North America. Taxa include Saurornitholestes, Richardoestesia isosceles, "Paronychodon", ornithomimids, tyrannosaurids, birds, and two indeterminate taxa. Notably absent from the assemblage are Dromaeosaurus and Troodon, common in many contemporaneous northern assemblages. Teeth of hatchlings or juveniles demonstrate that dinosaurs nested in the area. Evidence of semi-arid conditions is from abundant soil horizons containing caliche nodules and from possible flash flood deposits. This southern theropod assemblage was distinct and possibly less diverse than contemporaneous northern areas and reflects the climatic and environmental differences of this area. This work provides an important and new geographic perspective on theropod diversity during the final ten million years of the Cretaceous.