DINOSAUR SKELETON OCCURRENCE IN THE LATEST CRETACEOUS (LATE MAASTRICHTIAN) HELL CREEK FORMATION OF SOUTHWESTERN NORTH DAKOTA AND SOUTHEASTERN MONTANA

The Cretaceous/Paleogene (K/Pg) mass extinction event is one of the best-studied mass extinction events. However there is still debate regarding the mechanism and tempo of the extinction for terrestrial vertebrates, particularly whether nonavian dinosaurs went extinct gradually or catastrophically. Here we use a high resolution GPS survey and GIS to create a geostatistically interpolated model for the K/Pg boundary across a ~475 square mile study area along the Cedar Creek anticline in southwestern North Dakota and southeastern Montana and plot the stratigraphic position and depositional environment for over 100 dinosaur skeletons (three or more associated bones from the same taxon). Skeletons were recovered for nine of the eighteen dinosaur species recognized from the latest Cretaceous Hell Creek Formation. Our data suggest that the ecosystem’s sole large carnivore, Tyrannosaurus rex was found throughout the Hell Creek and was found equally in both riverine and associated floodplain environments. The small ornithopod Thescelosaurus is only found in the bottom third of the Hell Creek in the study area and is preferentially found in sandstone riverine depositional environments by an 8:1 (riverine vs. floodplain). The large duckbill dinosaur Edmontosaurus is found throughout the Hell Creek and are preferentially found in sandstone riverine environments by a 16:1 margin. Ceratopsians (Triceratops and Torosaurus) dominate the Hell Creek in overall number and are preferentially found in mudstone overbank deposits by a 2:1 margin. However, they are rare in the lower and middle portion of the Hell Creek and only completely dominate the assemblage in the upper third. Their dominance is correlated with the appearance of the Hell Creek III megaflora. Finally, partial skeletons of the three most common dinosaurs (Edmontosaurus, Triceratops, and Tyrannosaurus) occur within 3 meters of the palynologically calibrated K/Pg boundary suggesting that the diversity of nonavian dinosaurs was not gradually waning throughout the latest Maastrichtian. Skeletons from the two most common dinosaurs in the study area, Triceratops and Edmontosaurus, were recovered from the Fort Union Formation of Cretaceous age, suggesting that the facies change at the formation contact is not the cause for the disappearance of the dinosaurs.