gastrolith

Gastroliths are smooth, rounded stones that are swallowed and retained in the digestive tracts of a variety of animals. In modern birds, gastroliths reside in the gizzard and act as grinding agents to mechanically break down food, particularly tough seeds and plant matter. In other animals, including some reptiles and several extinct groups, gastroliths have been found in association with the digestive system and are interpreted as serving different roles.

Distribution and examples across groups include birds in which gastroliths are common, some reptiles that swallow stones, and numerous extinct marine reptiles and dinosaurs. In many marine reptiles such as plesiosaurs and ichthyosaurs, gastroliths are often interpreted as ballast to aid buoyancy and stability in water. In sauropod dinosaurs, gastroliths are commonly found with fossils and are thought to have aided digestion, though their exact function is debated. Some researchers also consider that gastroliths could have contributed to gut fermentation or helped regulate the digestion of large plant matter in terrestrial species.

Evidence and study of gastroliths rely on geological and paleontological findings. In living animals, the stone composition and placement in the digestive tract confirm their function as grinders. In fossils, recovered stones, their wear patterns, and their location relative to the skeleton help infer whether they served digestion, buoyancy, or other roles. Caution is needed, as not all stones found with fossils were necessarily swallowed during life; some may be geologic or post-medial artifacts.