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Rock held inside the intestinal tract

A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In other species the rocks are ingested and pass through the digestive system and are frequently replaced. The grain size depends upon the size of the animal and the gastrolith's role in digestion. Other species use gastroliths as ballast. Particles ranging in size from sand to cobble have been documented.

Gastrolith comes from the Greek γαστήρ (gastēr), meaning "stomach", and λίθος (lithos), meaning "stone".

Among living vertebrates, gastroliths are common among crocodiles, alligators, herbivorous birds, seals and sea lions. Domestic fowl require access to grit. Stones swallowed by ostriches can exceed a length of Apparent microgastroliths have also been found in frog tadpoles. Ingestion of silt and gravel by tadpoles of various anuran (frog) species has been observed to improve buoyancy control.

In 1906, George Reber Weiland reported the presence of worn and polished quartz pebbles associated with the remains of plesiosaurs and sauropod dinosaurs and interpreted these stones as gastroliths. In 1907, Barnum Brown found gravel in close association with the fossil remains of the duck billed hadrosaur Claosaurus and interpreted it as gastroliths. Brown was among the first paleontologists to recognize that dinosaurs used gastroliths in their digestive systems to aid in the grinding of food. Other paleontologists over the years were unconvinced. In 1932, Friedrich von Huene found stones in Late Triassic sediments, in association with the fossil remains of the prosauropod Sellosaurus and interpreted them as gastroliths. In 1934, the Howe Quarry, a fossil location in northwestern Wyoming also yielded dinosaur bones with their associated gastroliths. In 1942, William Lee Stokes recognized the presence of gastroliths in the remains of sauropod dinosaurs recovered from Late Jurassic strata.

Geologists usually require several pieces of evidence before they will accept that a rock was used by a dinosaur to aid its digestion. First, it should be rounded on all edges (and some are polished) because inside a dinosaur's gizzard any genuine gastrolith would have been acted upon by other stones and fibrous materials in a process similar to the action of a rock tumbler. Second, the stone must be unlike the rock found in its geological vicinity, i. e., its geologic context. Many gastroliths have been found in fine grained lake, mud, and swamp deposits. These environs are calm water deposits and could not carry pebbles and cobbles (unlike a river or beach). Oliver Wings also argues that the stone must be found with the fossils of the dinosaur which ingested it. It is this last criterion that causes trouble in identification, as smooth stones found without context can (possibly erroneously in some cases) be dismissed as having been polished by water or wind. Christopher H. Whittle (1988,9) pioneered scanning electron microscope analysis of wear patterns on gastroliths. Wings (2003) found that ostrich gastroliths would be deposited outside the skeleton if the carcass was deposited in an aquatic environment for as little as a few days following death. He concludes that this is likely to hold true for all birds (with the possible exception of moa) due to their air filled bones which would cause a carcass deposited in water to float for the time it needs to rot sufficiently to allow gastroliths to escape. Gastroliths can be distinguished from stream or beach rounded rocks by several criteria: gastroliths are highly polished on the higher surfaces, with little or no polish in depressions or crevices, often strongly resembling the surface of worn animal teeth. Stream or beach worn rocks, particularly in a high impact environment, show less polishing on higher surfaces, often with many small pits or cracks on these higher surfaces. Finally, highly polished gastroliths often show long microscopic rilles, presumably caused by contact with stomach acid. Since most gastroliths were scattered when the animal died and many entered a stream or beach environment, some gastroliths show a mixture of these wear features. Others were undoubtedly swallowed by other dinosaurs and highly polished gastroliths may have been swallowed repeatedly. None of the gastroliths examined in a 2001 study of Cedarosaurus gastroliths had the "soapy" texture popularly used to distinguish gastroliths from other types of clast.

In 2001 Frank Sanders, Kim Manley, and Kenneth Carpenter published a study on 115 gastroliths discovered in association with a Cedarosaurus specimen.