A study of the Pierre Shale was undertaken, focusing on two measured sections along the eastern side of the Missouri River near Fort Thompson, South Dakota. The members of the Pierre Shale represented within these sections are, successively, the Gregory, Crow Creek, DeGrey, and Verendrye. Micropaleontologic samples were analyzed, with factor analysis performed to classify the assemblages. Four foraminiferal biofacies are recognized: two predominantly agglutinated, Boreal in origin, one shallower and the other deeper; one composed of calcareous benthic foraminifers, Tethyan in origin; and a fourth, which represents a mixed assemblage, transitional between the others. 87 Strontium/ 86 Strontium age estimation yielded 12 usable results. Samples from near the Baculites compressus Biozone yielded an age of ca. 72.43 ± 2 Ma. An average 87 Sr/ 86 Sr date of 72.41 ± 2 Ma was determined from fossil shells near a Globidens sp. excavation site within the upper DeGrey Member. More equivocal dates of 74.58, 74.63, and 75.09 ± 2 Ma were determined for the basal Crow Creek Member, a calcareous sandy siltstone within the upper Gregory Member, and the B. gregoryensis Biozone at the base of the section, respectively. Based on this study, the Gregory Member is interpreted as a highstand systems tract but as a part of the regressive pulse of the Claggett depositional cycle; the Crow Creek and DeGrey Members are the result of the Bearpaw transgression (a transgressive systems tract), and the boundary between the DeGrey and Verendrye Members is interpreted as a maximum flooding surface; and the Verendrye Member is a highstand systems tract. The distinct, calcareous sandy siltstone unit in the upper Gregory Member, herein named the Fort Thompson Beds, is interpreted to represent a storm-dominated deposit.

Biogenic apatite crystals in living organisms contain relatively high concentrations of carbonate, sodium, and other species, making the crystallite relatively soluble and reactive. During fossilization, apatite composition changes from a metastable carbonate hydroxylapatite to a thermodynamically more stable fluorapatite. Calcium, sodium, carbonate, and hydroxyl are replaced by fluoride, REE, and trace elements during diagenesis. The total REE concentrations in osteological material are generally <20 ppm; in fossil bones, however, the concentrations may be >1000–10,000 ppm. More than 95% of REE in fossil bone is incorporated during diagenesis. The fossilization process occurs within a few thousand years, and thus the chemical composition of the fossils records the composition of early diagenetic fluids. Apatite becomes a “flight recorder” for the environment of early diagenesis and can be used to interpret the condition of the paleoenvironment. REE were analyzed from marine reptile (Mosasauridae) bones collected from five superposed formations (Sharon Springs, Gregory, Crow Creek, DeGrey, and Verendrye) of the Upper Cretaceous Pierre Shale Group at localities along the Missouri River in central South Dakota. Fossil vertebrates from each lithostratigraphic unit sampled of the Pierre Shale Group have different REE signatures. Fossils from the Sharon Springs Formation have distinctive REE signatures that may be further subdivided into three superposed members that correspond with the upper, middle, and lower Sharon Springs Formation. REE signatures are distinctive from each stratigraphic unit; therefore, fossils eroded from their stratigraphic context may be assigned to their proper depositional unit on the basis of REE signature comparisons. Differences in REE compositions of fossil bones among lithostratigraphic subdivisions appear to have resulted 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 decreased for the middle and upper Sharon Springs, and the overlying Gregory and Crow Creek units were deposited in even more shallow water. Finally, the overlying DeGrey and Verendrye Formations were deposited in progressively deeper marine waters, but not as deep as for the lower Sharon Springs. These interpretations are generally consistent with those based on faunal diversity and eustatic sea level curves.

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.

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.

One of the rarest of marine reptiles is the mosasaur genus Globidens , characterized by a massive, bulbous dentition. The rarity of the taxon, coupled with the bulbous dentition, resulted in various theories concerning life habits. Although a consensus indicates that the dentition was adapted for crushing resistant elements, hypotheses have varied concerning prey, ranging from turtles or bivalves to scavenging. Finally, a partial skeleton of Globidens has been recovered from the Big Bend area of the Missouri River in central South Dakota. The specimen was discovered in the upperDeGrey Formation (upperCampanian) of the Pierre Shale Group. During analysis, bivalve fragments were found packed within the rib-cage region of the skeleton. In the field, bivalve concentrations did not occur laterally or above or below the skeleton, indicating that they were the stomach contents of the mosasaur. Associated within the stomach area are a number of bivalve taxa, including oysters and small bivalves with lamellar shells, probably of the genus Anomia . The most common specimens within the stomach area are bivalves that exhibit a prismatic shell microstructure typical of inoceramids. Four inoceramid shell morphotypes were recovered, including a coarse-ribbed morphotype, a fine-ribbed morphotype, one with a thickened umbo, and a large, flat, thin-shelled morphotype. Because of their position in the mosasaur, their fragmented condition, limited taxonomic diversity, and absence from surrounding sediments, the bivalves are considered stomach contents. Some smaller, complete shells of Anomia escaped breakage, whereas larger inoceramids were invariably crushed. Chondrichthyan teeth were found associated but are interpreted to be the result of scavenging. This specimen of Globidens appears to have had a preference forthe large, flat, relatively thin-shelled inoceramids that contained a large, fleshy visceral mass.

The most complete specimen of the unusual mosasaur Globidens has been discovered in central South Dakota along the Missouri River. The specimen consists of the anterior half of a skeleton, including crushed skull, pectoral girdle, partial paddle, and vertebral series. One humerus and ulna exhibit pathologies. The partial skeleton was found at the top of the DeGrey Formation (upper Campanian) of the Pierre Shale Group and is therefore the youngest skeleton from North America. The specimen exhibits apomorphies that correspond with the late occurrence and indicate a new species. Increased dental hypsodonty, lack of a posterior buttress on teeth, great size, and massive build indicate a derived intermediate species of a clade that first occurred with rounded teeth in the early Campanian in North America and Europe ( Globidens alabamaensis and Globidens dakotensis ) and culminated with high-crowned teeth with posterior buttresses in the Maastrichtian of Africa, the Middle East, and South America ( Globidens phosphaticus ). Associated with the specimen are bivalves that are interpreted as stomach contents and shark teeth that are interpreted as the result of scavenging.