FAUNAL COMPOSITION OF THE LOWER TWO MEDICINE FORMATION (CAMPANIAN), MONTANA

The Two Medicine Formation of northwestern Montana represents an extensive record of the evolution of terrestrial ecosystems in the Late Cretaceous of Laramidia. Whereas numerous non-avian dinosaur taxa have been reported in the upper half of the formation, the faunal composition of the lower half of the Two Medicine is more enigmatic. Radiometric studies suggest that the lower portion of the Two Medicine precedes the fossiliferous Dinosaur Park and Kaiparowits Formations and is coeval with the Foremost Formation of Alberta as well as the Wahweap Formation of southern Laramidia. The Wahweap has recently produced the basal tyrannosaurid Lythronax and the basal ceratopsid Diabloceratops. The brachylophosaurine Acristavus is present in both the Wahweap and the lower half of the Two Medicine, suggesting that there may be further overlap in the faunal composition of these units. As such, the lower Two Medicine represents a critical window into the evolution and biogeography of Cretaceous dinosaurs. Here we present a survey of the fauna of the lower half of the Two Medicine Formation as represented by material curated at the Museum of the Rockies. Dinosaur taxa present include the hadrosaurids Acristavus and Gryposaurus as well as the leptoceratopsid Cerasinops. Fragmentary material indicating the presence of tyrannosauridae, hadrosauridae, and ceratopsidae is also present. At least two morphotypes of small theropod teeth referable to Paronychodon and Saurornitholestes are recorded in this survey. Teeth referable to Saurornitholestes suggest an extensive stratigraphic range for this taxon and highlight potential systematic complications due to ontogenetic change. Non-dinosaurian material includes the turtles Aspidertoides and Naomichelys, crocodylians, gar, batoid fishes, and freshwater gastropods. Localities surveyed have produced juvenile dinosaur material and are rich in fossil eggshell, with at least three morphotypes being represented including previously unrecognized morphologies. Further study of the fauna of the lower Two Medicine Formation will provide tests of hypotheses regarding dinosaur biogeography and the development of Cretaceous ecosystems.