CONTEXTUALIZING A NEW, LARGE ALLOSAURUS SKELETON FROM WYOMING INTO STRATIGRAPHIC PERSPECTIVE WITHIN THE LATE JURASSIC MORRISON FORMATION

A partially-articulated Allosaurus skeleton was excavated in 2004 by a Concordia College summer research team southwest of Shell, WY. Roughly 40% of the skeleton was found, including numerous cranial elements, most presacral vertebrae and girdle elements, and the distal half of the right femur. The skeleton exhibits a unique combination of large body size with subadult characteristics. We interpret the remains as belonging to a subadult due to a prevalence of unfused elements, numerous alveoli, and based on histologic analyses of the femur. At the time, the specimen was not put into a stratigraphic context and sedimentological and taphonomic studies were not undertaken. Therefore, we returned to document the stratigraphy in this area and the location of the Allosaurus specimen in comparison to the base of the Morrison Formation, the top of the Morrison, and the “clay change” within the middle of the Morrison. Based on regional stratigraphic marker horizons, we documented the Morrison Formation in this area to be ~50 m thick, in general agreement with the few prior studies undertaken in this area of the Bighorn Basin. Morrison strata in this area consist of primarily variegated white, gray, and red mudstones. The skeleton was preserved in a blocky, light green-gray mudstone, which is characterized by fine dark-gray blocks at the bottom with lighter-colored coarse blocks at the top of the bed. This mudstone exhibits occasional orange-brown coloration on fracture surfaces, as well as fine gypsum growth, both probable products of recent diagenesis. Based on the position of regional marker beds, comparisons with prior stratigraphic studies in the region, and field observations of the character of mudstones, we determined that the skeleton derives from the lower portion of the Brushy Basin Member. X-ray diffraction analyses are ongoing to test our tentative field assignment of the stratigraphic position of the mid-Formation “clay change” from illitic to smectite-dominated clays. Defining the stratigraphic position of this new Allosaurus will help us constrain the specific age of the specimen so that anatomical comparisons can be drawn with the most relevant specimens from across the Western Interior, which will in turn help clarify the morphologic diversity of Allosaurus through space and time across the western U.S.