STOMACH STONES FOR FEEDING OR BUOYANCY - THE OCCURRENCE AND FUNCTION OF GASTROLITHS IN MARINE TETRAPODS

Gastroliths or 'stomach stones' occur frequently in some, but not all, groups of fossil and living marine tetrapods. Comparative analysis of gastrolith distribution suggests a role in buoyancy control rather th an food processing. Once accidental ingestion by bottom-feeding animal s is excluded, gastroliths occur in most tetrapods which 'fly' underwa ter with hydrofoil limbs, including plesiosaurs, penguins, and otariid pinnipeds, but not the marine chelonians. They do not usually occur i n cetaceans, ichthyosaurs, mosasaurs, and odobenid and phocid pinniped s, which swim with a caudal fin or the equivalent. Occurrence in amphi bious forms is variable; crocodilians often have gastroliths, but noth osaurs and placodonts do not. The correlation of gastroliths and under water flight is corroborated by a comparative analysis which takes phy logenetic factors into account. There is no correlation with diet. Con sideration of function and occurrence in terrestrial forms suggests th at the use of gastroliths in digestion would not be useful, and might even be harmful, to a carnivorous marine tetrapod. Gastroliths are mor e efficient than skeletal bone (as in pachyostosis) in terms of sinkin g force per unit of added mass or volume. As well as driftwood and ice , marine tetrapods should be considered as a potential source of errat ic stones in freshwater and marine sediments. Gastroliths may have evo lved by the accidental ingestion of stones, the retention into adultho od of stones used by juveniles to process insect or plant food, or as a compensatory replacement for dense bones habitually filling the stom ach. Their presence or absence should be more carefully recorded and f urther studies should be carried out on their function.