The Pierre Shale, a member of the Montana Group, is extensively exposed throughout the Northern Great Plains and is well exposed along the Missouri River Trench in central South Dakota. Currently, the Pierre Shale is of formational rank, but herein it is elevated to group status and should be termed the Pierre Shale Group. Most current members of the Pierre Shale should be elevated to formational rank, as they are of distinctive lithology and are mappable throughout the Missouri River area and elsewhere. The name Montana Group should be abandoned because of its relatively limited lithostratigraphic utility. Extensive geological and paleontological investigations of the lower Missouri River Trench indicate that a number of previously described units should be subdivided. In particular, the lowest described unit of the Pierre Shale along the Missouri River, the Sharon Springs, exhibits three distinct disconformity-bounded lithostratigraphic units that are newly designated as members. The lowermost unit is distinguished by bentonites, and the upper two units can be observed in the type area of the Sharon Springs in western Kansas. The lowermost unit is characterized by numerous bentonite beds similar to the Ardmore bentonitic succession in the southern Black Hills, is normally disconformably superjacent to the Niobrara Formation, may be absent where degraded, and is named the Burning Brule Member. The overlying middle unit within the Sharon Springs consists of a siliceous shale that weathers vertically and is termed the Boyer Bay Member, whereas the upper unit is a bentonitic shale characterized by gypsiferous phosphatic concretions and is named the Nicholas Creek Member. These three units are herein regarded as new members of a hierarchically elevated Sharon Springs Formation. Other currently recognized members of the Pierre Shale in central South Dakota should likewise be elevated to formation-rank units except for the Crow Creek Member, a relatively thin tan siltstone. Because this unit is not mappable at recognized scales, it should be retained as a member of the DeGrey Formation. Inclusion of the Crow Creek Member within the DeGrey Formation is based upon another sporadically occurring tan siltstone that lies stratigraphically lower, and the intervening shales are similar to those of the lower DeGrey Formation. The Pierre Shale members should also be elevated to formational status in North Dakota, Kansas, and around the Black Hills in eastern Wyoming and western South Dakota. The elevation of units should probably be made throughout the Pierre Shale depositional area, but those decisions should be made by local investigators.

The osteichthyan paleofauna of the upper Pierre Shale Group has yielded range extensions for taxa known from its Sharon Springs Formation and the underlying Niobrara Formation, notably with the genera Protosphyraena and Pachyrhizodus . Links are now more apparent with Campanian and Maastrichtian (Late Cretaceous) paleofaunas from the Atlantic and Gulf Coasts of the United States, as with Cylindracanthus and Enchodus . Common elements are to be expected, considering the similarities in their geologic ages and environments of deposition.

Elasmosaurid plesiosaur remains containing gastroliths have been recently recovered from the Late Cretaceous Cape Lamb Member of the Snow Hill Island Formation, Antarctic Peninsula, as well as from the Sharon Springs Formation of the Pierre Shale Group, South Dakota. The significance of the Antarctic specimens lies in the large number and relatively small size of the stones, whereas those from South Dakota and elsewhere have larger stones but relatively small numbers. All specimens compared are of relatively large elasmosaurids, and rib diameters indicate similar overall size. One testable hypothesis was whether or not the weight of the gastroliths might be similar among similarly sized individuals. However, comparisons between the gastroliths recovered from an Antarctic specimen and those recovered from the Pierre Shale show great differences in both the size and number of gastroliths. The total mass of the stones collected in the specimen from Antarctica was 3.0 kg, whereas those recovered from the South Dakota specimens totaled 2.2 kg and 8.3 kg, respectively. Perhaps not all stones originally within each plesiosaur were found, but efforts were designed to recover every stone. The number of stones recovered from the Antarctic plesiosaur was exceptionally large (2626) and appears to represent the most gastroliths recovered from a single plesiosaur. Those recovered from South Dakota elasmosaurids totaled only 124 and 253, respectively. Therefore, neither weight nor number of stones corresponds among these large individuals, perhaps mirroring the temporal or behavioral differences. Lack of correspondence among individuals furthers questions concerning the utilization of gastroliths for neutral buoyancy, ballast, or as an aid in digestion. Many parameters remain unexplored, and questions arise not only as to the utilization of the gastroliths but also as to whether physiology of the Antarctic plesiosaurs differs from those at lower latitudes.

The Cretaceous Pierre Shale Group along the Missouri River has produced numerous mosasaur specimens since the western fossil discoveries of Lewis and Clark in 1804 that included a 45-foot “fish.” Many of these marine reptile specimens represent the largest of mosasaurs, the tylosaurines. In 1990 the largest mosasaur heretofore recorded along the Missouri River was discovered near Nicholas Creek, Lyman County, central South Dakota. The specimen was recovered from a lag deposit representing an intra–Pierre Shale Group unconformity and consists of vertebrae, ribs, paddle elements, and a partial skull. The partial skeleton is referable to the subfamily Tylosaurinae, cf. Tylosaurus sp., based on large size, tooth structure, and long pre-dental rostrum. Further identification must await resolution of the taxonomy of the Tylosaurinae. A lower jaw measures 1.6 m, indicating a projected body length of 11.5 m. Therefore, the large “fish” described by Lewis and Clark may have been a tylosaurine mosasaur.

The Missouri River and its tributaries in central South Dakota have eroded into the Pierre Shale, exposing a vertical sequence of late Campanian strata, from older to younger: the Crow Creek Member of the DeGrey Formation, upper DeGrey Formation, and Verendrye Formation, deposited during the transgressive phase of the Bearpaw cyclothem. The DeGrey Formation ( Baculites compressus Ammonite Range Zone) is predominantly gray bentonitic shale with interbeds of thin and relatively pure cream-colored bentonite and bioturbated, fossiliferous black manganese-iron carbonate concretions. Fossils are molds, casts, and fragmented shell material from a diverse epifauna of marine bivalve mollusks dominated by byssate, cemented, and unattached inoce-ramids, and ostreids. Inoceramids include Inoceramus convexus Hall and Meek, I. perplexus Whitfield, I. pertenuis Meek and Hayden, I. proximus Tuomey, I. sagensis Whitfield, I. tenuilineatus Hall and Meek, Platyceramus vanuxemi (Meek and Hayden), and Endocostea barabini (Morton). The larger specimens are commonly encrusted with the oyster Pseudoperna congesta (Conrad). Ostrea patina Meek and Hayden, a solitary oyster, is also present. External casts of articulated valves of Pholadomya hodgii Meek, a burrower-borer, are abundant as are those of Pteria ( Pseudopteria ) sublevis Whitfield, P. linguaeformis (Evans and Shumard), and P . sp. B. Anomia subtrigonalis Meek and Hayden and A. oblique Meek and Hayden complete the bivalve fauna. Less abundant are the gastropods Anisomyon borealis Morton, A. shumardi Meek and Hayden, Ampullina ? paludinaeformis Sohl, and Acmaea occidentalis (Hall and Meek). The Late Campanian ammonite index fossils Baculites compressus Say, and Jeletzkytes nodosus (Owen) and J. brevis (Meek), are abundant. Bivalves of the DeGrey Formation lived on a low-energy substrate where bentonite-rich clay was accumulating. Abundant epifauna and extensive bioturbation of the manganese-iron carbonate concretions suggest aerobic conditions. Adjacent deeper, anoxic basin waters periodically transported iron and manganese upward into the oxic zone, where it was precipitated with carbonate. The fragmentary nature of most of the bivalve shells, disruption of the sediment, and abundance of possible coprolitic material suggest predation by sediment-probing and shell-crushing predators.