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 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.