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 lower Pierre Shale consists primarily of the Sharon Springs Formation, which has been correlated regionally throughout Kansas, Colorado, Nebraska, Wyoming, South Dakota, and North Dakota. The unit represents distal sedimentation in a tectonically active foreland basin. Correlation of the lower Pierre Shale Group is complicated by the application of a single name, the Sharon Springs, to a wide range of facies. Bentonite correlation provides an independent framework for verifying the age equivalence of various facies of the Sharon Springs Formation. Bentonite correlation involves using a variety of unique chemical characteristics to differentiate individual beds. A combination of whole rock rare-earth-element geochemistry, phenocryst composition, biotite geochemistry, and stratigraphic position has been used to correlate bentonites of the lower Pierre Shale and equivalent units across the Western Interior of the United States.

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 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.

The Campanian Pierre Shale Group contains a diverse assemblage of marine reptiles, including the sea turtle Toxochelys . Twenty-one specimens from South Dakota include cranial or mandibular material useful for taxonomic and stratigraphic analyses. Twenty of the toxochelyids were collected from three formations of the lower to middle Campanian Pierre Shale in western South Dakota: 1 from the basal Gammon Ferruginous Formation, 15 from the overlying Sharon Springs Formation, and 4 from the Mitten Black Shale Formation. Examination of the specimens suggests identification as Toxochelys sp. cf. T. browni . These toxochelyids reveal previously undescribed characters and are defined by snout emargination, a hooked beak of the mandible, and labial and lingual ridges of the dentary. One toxochelyid was recovered from the DeGrey Formation of the upper Campanian Pierre Shale Group in central South Dakota and is identified as Toxochelys sp. A. This toxochelyid is distinguished by a smooth snout without emargination, a posterolateral margin of the pterygoid that extends to the base of the condylus mandibularis, and absence of the depressor mandibulae muscle attachment site. Qualitative and quantitative stratigraphic analyses of the South Dakota toxochelyids indicate that Toxochelys sp. cf. T. browni could serve as the basis for a bio-stratigraphic local range taxon zone, spanning the time from the late early Campanian to the middle Campanian, between 81 and 78 Ma.

A plesiosaur skeleton from the base of the Greenhorn Limestone (lower upper Cenomanian), western South Dakota, represents a new taxon tentatively referred to the Polycotylidae. The specimen possesses a number of features in common with the Polycotylidae, including an exceptionally elongate muzzle and mandibular symphysis, lack of a parietal foramen, homodont dentition, and faint striae only on the medial side of teeth. However, the palate and limb morphologies differ with respect to Polycotylidae as currently known. The pterygoids are united along their midline anterior to the parasphenoid, lacking an anterior interpterygoid vacuity. The preserved parasphenoid is relatively robust, possesses a strong ventral keel, and is sutured to the dorsal surface of the pterygoids. The paddles possess many plesiomorphic features, including only relatively minor posterodistal expansion of the propodials, epipodials that are longer than wide, a distinct antebrachial foramen, and relatively elongate phalanges. Derived features make it a plausible representative sister taxon to Polycotylidae, a prospect that is complemented by its stratigraphic position. This hypothesis is hampered by the palate morphology, which suggests a separate lineage of short-necked plesiosaurs far removed from genera classically assigned to Polycotylidae ( Dolichorhynchops , Polycotylus , Trinacromerum ). Nevertheless, the specimen represents the persistence of plesiomorphic limb traits and possession of a closed palate within an early polycotylid lineage until at least the early late Cenomanian in the Midcontinent of North America.

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 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.

A partial skull of a large mosasaur was discovered eroding from the upper Campanian shales of the DeGrey Formation of the Pierre Shale Group from Gregory County, South Dakota. Although the skull is not complete, sufficient characters exist to indicate that the specimen represents the first undisputed North American occurrence of the large tylosaurine mosasaur Hainosaurus , otherwise best known from the Maastrichtian of Europe. Diagnostic characters include relatively laterally compressed, symmetrical teeth with finely serrated carinae, an interlocking premaxillary-maxillary suture, a very large alar opening of the quadrate, a distinctly laterally deflected quadrate shaft, a nontriangular centrum outline of anterior caudal vertebrae, and, in particular, a pineal opening bordered by the frontal and parietal. Features of the quadrate indicate that the South Dakota specimen represents a new species of Hainosaurus , which is described herein.

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 Cretaceous Pierre Shale Group in eastern South Dakota and western Wyoming contains numerous fossils, including the flying reptile Pteranodon , 19 specimens of which are described herein. Pteranodon specimens have been found in the two lower formations of the Pierre Shale Group: 4 in the basal Gammon Ferruginous Formation, and 15 in the overlying Sharon Springs Formation. Of these specimens, 64% are associated forelimb elements. Two factors, or a combination thereof, explain the abundance of associated forelimb elements: (1) predatory preference of the muscle mass in the chest over the wing membrane, which may not have offered much nutrient to the predators; and (2) the strong wing membrane, which would have secured the wing elements while the muscle mass would have fallen away. In either case, the wing membrane may have served as a protective layer over the forelimb bones until burial. Two Pteranodon specimens contain vertebrae of the fish Enchodus , which appear to be stomach contents, the first documented Pteranodon stomach contents from South Dakota.

Fossil birds are relatively rare in Cretaceous deposits of the Northern Great Plains, so the discovery of a large, new diving bird was unexpected. From marine deposits of the Niobrara Formation in Kansas a small diversity of birds was known, but until now the large diving bird, Hesperornis , was the only bird taxon known from the Pierre Shale Group of South Dakota. The new discovery, a partial skeleton of another diving bird, Baptornis , was secured from the Sharon Springs Formation (lower middle Campanian) of the Pierre Shale Group in Fall River County, South Dakota. The specimen is represented by vertebrae, pelvic fragments, and lower leg elements that are similar to but much more robust than Baptornis advenus from the subjacent Niobrara Formation. The new taxon is nearly twice the size of the Niobrara species, principally in robustness rather than in length of elements. Overall, the specimen represents the first occurrence of Baptornis from the Pierre Shale Group, represents a new species, and indicates a greater diversity of birds from the Pierre Shale Group than was previously known.

Although vertebrate fossils, except for fish, are not common in the Maastrichtian Fox Hills Formation, amphibian, reptilian, and avian remains have been recovered at several localities in south-central North Dakota from shoreline facies of the retreating Pierre–Fox Hills seaway. This mixed fauna of aquatic, terrestrial, and marine taxa provides insight into the composition of coastal communities and habitats at the interface between the Hell Creek delta and the Western Interior Seaway. The delta-platform aquatic paleocommunity is represented by the efficient swimming salamanders Opisthotrition kayi and Lisserpeton bairdi , the carnivorous soft-shelled turtle “ Aspideretes ” sensu lato, the underwater piscivorous predator Champsosaurus laramiensis , and the large, predatory crocodile ? Borealosuchus . Terrestrial areas were inhabited by the tortoise-like Basilemys and the predatory dinosaurs Tyrannosaurus and cf. Saurornitholestes . Birds occupied niches in the warm-temperate to subtropical, forested delta platform and shoreline areas. These nonmarine taxa in the Fox Hills Formation indicate that the geographic range of these animals extended to shoreline areas of the Western Interior Seaway. The toxochelyid turtle Lophochelys and the ambush predators Mosasaurus dekayi and ? Plioplatecarpus resided in the shallow marine and estuarine habitats. These taxa and marine fish taxa reported earlier indicate that normal marine conditions in south-central North Dakota persisted into the latest Late Cretaceous in comparison with coeval Hell Creek Formation sites more distal from the Western Interior Seaway.