COMPARISON OF GASTROLITHS WITHIN PLESIOSAURS FROM THE LATE CRETACEOUS MARINE DEPOSITS OF VEGA ISLAND, ANTARCTIC PENINSULA, AND THE MISSOURI RIVER AREA, SOUTH DAKOTA

Plesiosaur remains containing gastroliths have been recovered from the Late Cretaceous Cape Lamb Member of the Lopez de Bertodano Formation, Antarctic Peninsula, as well as the Sharon Springs Member of the Pierre Shale, South Dakota. The significance of the Antarctic specimen lies in the large number and relatively small size of the stones. All specimens compared are of relatively large elasmosaurids, and rib diameters indicate similar overall size of the three specimens. One testable hypothesis was whether or not the weight of the gastroliths might be similar among similarly sized individuals. However, comparison between the gastroliths recovered from the Antarctic specimen and those recovered from the Pierre Shale show a large degree of difference in both the size and number of gastroliths. The total mass of the stones collected in the specimen from Antarctica was 3.02 kg., whereas those recovered from the South Dakota specimens totaled .92 kg and 9.3kg, 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 (2,626), and may represent the most gastroliths ever recovered from a single plesiosaur specimen. However, those recovered from South Dakota totaled only 42 and 253, respectively. Therefore, neither weight nor number of stones corresponds among these large individuals. 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 Antarctic research was funded through a grant from the Office of Polar Programs, National Science Foundation (OPP #0087972), and the US Corps of Engineers provided partial funding for the South Dakota investigations.