MOLLUSCS IN THE STOMACH CONTENTS OF GLOBIDENS, A SHELL-CRUSHING MOSASAUR, FROM THE LATE CRETACEOUS PIERRE SHALE, BIG BEND AREA OF THE MISSOURI RIVER, CENTRAL SOUTH DAKOTA

Globidens is one of the most rare of marine reptiles and is characterized by a massive, bulbous dentition rather than sectorial as are most other mosasaurs. Rarity of the taxon coupled with the unusual dentition has prompted a number of theories about life style. Most theories suggest the dentition was utilized for crushing resistant elements such as turtles or pelecypods or perhaps for scavenging. Finally, a partial skeleton of Globidens, secured from the upper DeGrey Member of the Pierre Shale along the shores of the Missouri River in the Big Bend area of central South Dakota, provides direct evidence. During excavation, numerous pelecypod fragments were found associated with the skeleton. Such concentrations were not found laterally nor above or below the skeleton. Once in the laboratory, the shell fragments were found associated within the ribcage area. Included within the stomach area are small (4 cm) bivalves with lamellar shells, probably of the genus Anomia. The most common pelecypods exhibit a prismatic shell microstructure typical of inoceramids. Of these, two very different morphotypes are included: a coarse-ribbed type and a large, flat, thin-shelled taxon. Because of their position in the mosasaur, their fragmented condition, limited taxonomic diversity, and absence from surrounding sediments, the pelecypods are considered stomach contents. Some smaller, complete shells of Anomia escaped breakage whereas larger inoceramids were always crushed. This specimen of Globidens appears to have had a preference for the large, flat, thin-shelled inoceramids that probably had a large, fleshy visceral mass.