A SHALLOW MARINE FAUNA FROM THE LATE CAMPANIAN DEGREY MEMBER OF THE PIERRE SHALE ALONG THE MISSOURI RIVER, CENTRAL SOUTH DAKOTA

The DeGrey Member of the Pierre Shale crops out along the Missouri River and its tributaries in central South Dakota. Here, gray bentonitic shale beds a few inches to several feet thick are interbedded with thin and relatively pure cream-colored bentonites and black manganese-iron-carbonate nodules and concretionary layers. Fossils in the upper DeGrey Member are typically preserved as molds and casts in these extensively bioturbated manganese-iron beds. Shallow marine bivalve molluscs of the following taxa are abundant: Anomia (2 species) (inner shelf), Ostrea patina (inner to middle shelf), Pseudoperna congesta (middle to outer shelf) usually cemented to Inoceramus (8 species), Pholadomya hodgii (inner shelf), and Pteria (3 species). Anisomyon(2 species), Ampullina, and Acmaea gastropods are also present. Index fossils Baculites compressus, Didymoceras cheyennense, and Jeletzkytes nodosus indicate a late Campanian age. Bones of several species of mosasaur are also present within the DeGrey Member.