MEGAFLORA FOSSILS FROM A DINOSAUR EXCAVATION SITE IN THE LATE CRETACEOUS HELL CREEK FORMATION OF WESTERN NORTH DAKOTA

Megaflora fossils found throughout the Hell Creek Formation of western North Dakota and across the K-Pg boundary between the Hell Creek and Fort Union Formations provide insight into the changing landscape during this turbulent period of Earth’s history. This study characterizes megaflora fossils collected at a vertebrate fossil excavation on a small butte in western North Dakota to better understand the landscape, the depositional environment, and the relative stratigraphic location of the site within the formation. We collected all potentially identifiable fossils of leaves and other plant materials from a 25 m x 6 m area along the edge of the butte, adjacent to the main vertebrate fossil collection site. In this area, fossils were mostly preserved as impressions in sandstone and ironstone that represents the base of a fluvial channel sand. Leaf identification was based on the morphotypes established by Johnson (2002). After identifying the megaflora fossil material, we calculated the relative abundance of each type at the site. Together, the angiosperms Dryophyllum and Leepierceia accounted for more than 80% of identifiable leaves. Leaf and cone fossils from the gymnosperm Metasequoia were the third most abundant group. Several types of woody material were also preserved. The relative abundance of leaf morphotypes at the site suggests the presence of a mixed angiosperm and gymnosperm forest near the channel of a meandering river that lies within the HCI or HCIIa megafloral zones established by Johnson and Hickey (1990), in the lower 50 – 75 m of the formation.