Paleoenvironmental interpretations of rare earth element signatures in mosasaurs (reptilia) from the upper Cretaceous Pierre Shale, central South Dakota, USA

Abstract

Rare earth elements were analyzed from fossil marine reptile (Mosasauridae) bones collected from five superposed members (Sharon Springs, Gregory, Crow Creek, DeGrey, and Verendrye) of the upper Cretaceous Pierre Shale at localities along the Missouri River in Brule, Buffalo, Hughes, and Hyde counties in central South Dakota. Fossil bones from each member of the Pierre Shale have different rare earth element (REE) signatures that may be distinctive over wide geographic areas. Fossils from the Sharon Springs Member have distinctive REE signatures that may be further subdivided into three superposed groups that correspond with the upper, middle, and lower Sharon Springs Member. REE signatures are distinctive from each stratigraphic unit; therefore, fossils eroded from their stratigraphic context may be assigned to their proper depositional unit based on REE signature comparisons. Differences in REE compositions of bones among members appear to result from differential mixing of oxygenated and anoxic seawaters. If differences in mixing are interpreted as depth differences, the lower Sharon Springs Member was deposited in deep, anoxic water; water depths decrease in the middle and upper Sharon Springs, and the overlying Gregory and Crow Creek Members were deposited in even more shallow water. Finally, according to this interpretation, the overlying DeGrey and Verendrye members were deposited in progressively deeper marine waters, but not as deep as the lower Sharon Springs. These interpretations are generally consistent with those based on faunal diversity and eustatic sea level curves.