NEW PALEOENVIRONMENTAL INTERPRETATIONS FOR THE LATEST CRETACEOUS HELL CREEK FORMATION, NORTH DAKOTA

The latest Cretaceous Hell Creek Formation of the northern Great Plains, USA, is famous for dinosaurs and other vertebrates and a well-documented K/P (Cretaceous/Paleogene) boundary. The geology and paleontology of the Hell Creek has been well documented, including its litho-, magneto-, and chemostratigraphy, paleobotany, palynology, and vertebrate paleontology. An important previous study on the vertebrate biostratigraphy of the Hell Creek of North Dakota recorded numerous vertebrate fossils from many sites leading up to the K/P boundary based on surface collection and excavation. However, no screen-washing of microvertebrate sites was done, potentially missing or under-sampling small specimens such as small fish and shark teeth. Does screen-washing microvertebrate sites change the paleoenvironmental interpretations for the Hell Creek in this area? We addressed this question by screen-washing 11 microvertebrate sites all tied to the K/P boundary. We used fine-mesh screens and sorted the resulting matrix with microscopes. This yielded thousands of small specimens (teeth and bones) from numerous vertebrates. Results include: 1) abundant fish, especially Lepisosteus (gar) and Myledaphus (guitar fish), and many other fish, sharks, and rays; 2) amphibians, lizards, snakes, turtles, crocodylians, and champsosaurs; 3) numerous small (hatchling) hadrosaur and ceratopsian dinosaurs; 4) theropod dinosaurs including juvenile tyrannosaurids, cf. Saurornitholestes, Richardoestesia, and “Paronychodon”; 5) birds; and 6) mammals (multituberculates: Mesodma, Meniscoessus, Cimolodon; marsupials: pediomyid and alphadontid; and eutherian: cf. Gypsonictops). This work yielded two new and important results. First, numerous dinosaur hatchlings indicate dinosaur nesting sites were in this area. Second, fish, sharks, and rays are abundant, but had been previously missed or under-sampled. This indicates the close proximity of a seaway in this area. The presence of Richardoestesia isosceles, a fish-eating theropod, and cf. Hesperornithoformes, a fish-eating bird also supports this new paleoenvironmental interpretation.