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Gastroliths, or gizzard stones, are small rocks kept by an animal inside its stomach or at an early part of the alimentary canal. The function of these is to mash up the food so digestion takes place faster. Typically, animals which do this do not have suitable teeth for grinding, and the gastroliths are a good substitute for this.
Several pieces of evidence are needed to show that a rock was used by a fossil animal to aid its digestion. First, the stone must be unlike the rock found in its geological vicinity. Secondly, it should be rounded and polished, because inside a gizzard any genuine gastrolith would grind against other stones and fibrous materials, like the action of a rock tumbler. Lastly, the stone must be found with the fossils of the animal which ingested it.