The first marine reptile from the American West was collected long before the currently accepted lithostratigraphic nomenclature was established. The reptile, Mosasaurus missouriensis (Harlan), was collected from the Big Bend area of the Missouri River from what is now considered central South Dakota and taken to Germany by Prince Maximilian of Weid. Parts of the same specimen were described in 1834 and 1846, but its lithostratigraphic source could be determined only as the Late Cretaceous Pierre Shale Group, although later authors suggested its source as the Virgin Creek Formation of the Pierre Shale. Recent examination of the preservation of the holotype and associated invertebrates indicated derivation from the upper concretionary portion of the DeGrey Formation of the Pierre Shale rather than the Virgin Creek Formation. However, an independent method was sought to confirm this conclusion. Rare earth element (REE) analysis of vertebrate fossils in the Pierre Shale Group has been used successfully in interpretations of original diagenetic environments, including interpretations of paleodepth, identification of fossil provenance, paleoenvironmental interpretations, and stratigraphic correlation. REE signatures and trace element concentrations in fossil vertebrates from stratigraphic units are sufficiently distinctive to allow identification of the original unit or location in which fossilization occurred. Comparative REE analysis from numerous specimens from each unit of the lower Pierre Shale Group along the Big Bend of the Missouri River confirmed the lithostratigraphic source of the mosasaur as the upper DeGrey Formation.