LITHOLOGICAL INFLUENCES ON MOSASAUR FOSSIL TAPHONOMY WITHIN THE PIERRE SHALE FORMATION OF SOUTH DAKOTA

Within a single lithological unit, the taphonomy of individual fossils is prone to variation laterally and through time. The Pierre Shale Formation is a late Cretaceous, fossiliferous unit, representing deposition of black shales in the Western Interior Seaway. Within members of the Pierre Shale, there is notable variability of vertebrate fossil preservation and association with secondary minerals, including iron oxides and gypsum. Stratigraphically higher layers of shale contain greater concentrations of iron and manganese oxides in association with preserved fossils, typically as concretions, while in the lower stratigraphic layers, fractures in the shale have been filled with gypsum. The goal of this study is to provide preliminary data regarding taphonomic trends, specifically geochemistry and weathering, of mosasaur fossils within members of the Pierre Shale Formation in South Dakota. Minerals present within each member of the Pierre Shale are expected to influence the geochemical composition of fossils, leading to differences as a function of lithology. Non-destructive X-ray fluorescence (XRF) analyses on mosasaur fossils and their surrounding shale matrix were conducted, focusing on the Gregory, Sharon Springs, and DeGrey members, along with mosasaurs from two unknown members. Concentrations of the most abundant elements (Fe, Mn, Ca, S, P, Al, Si, and K) were normalized to 100% to allow for direct comparisons between lithological units. Preliminary analyses suggest that mosasaur fossils from the Sharon Springs, Gregory, and DeGrey members have differing elemental compositions. Mosasaur fossils preserved in gypsum-rich members, such as the Sharon Springs Member, contained elevated calcium (~24.7%) and sulfur (~22.8%). Fossils found in the iron-manganese oxide members (i.e., DeGrey Member) contained ~24.1-32.1% iron and ~14.6-16.1% manganese. If differences in fossil geochemistry are supported by further analyses of specimens, trends in chemistry can be used to infer host lithology of unlabeled specimens in collections, and for the Pierre Shale, may narrow down the potential member the fossil was collected from even in the absence of field records. These results show that XRF has the potential to inform major element chemistry of fossils and sediments without the use of destructive analyses.