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A gastrolith--also called a stomach or gizzard stone--is a rock within a gastrointestinal tract. Gastroliths are used to grind food in animals lacking grinding teeth: birds, crocs, alligators, seals, sea lions. The size depends on the animal's size and its role in digestion. Some species use gastroliths as ballast: plesiosaurs. Certain sauropod dinosaurs used stones to grind plant matter. Some fossil gastroliths are rounded and polished.