Glossary

A **gastrolith** is a hard, non-caloric object—typically a stone—voluntarily ingested and retained within the gastrointestinal tract of an animal. Gastroliths are best documented in living birds, where the muscular gizzard (ventriculus) contracts rhythmically around ingested grit to mechanically triturate and mix food, effectively compensating for the absence of teeth. They are also reported in crocodilians, pinnipeds (seals and sea lions), cetaceans, and numerous extinct taxa including non-avian dinosaurs and marine reptiles such as plesiosaurs. The most widely accepted function of gastroliths is the **mechanical breakdown of plant material** in herbivorous animals. In birds, gastrolith mass consistently approaches approximately 1% of body mass, a ratio that is maintained across species spanning four orders of magnitude in body size. Alternative functional hypotheses include hydrostatic ballast for buoyancy control in aquatic animals, mineral supplementation (particularly calcium from limestone), stomach cleaning (especially in raptors), and stimulation of digestive secretions. However, the degree of empirical support varies widely among these proposals. Gastroliths are significant in paleontology as indirect evidence of diet, digestive physiology, and even migratory behavior in extinct animals. The discovery of bird-like gastrolith clusters in derived theropods such as *Caudipteryx* and ornithomimosaurs indicates that the avian gastric mill evolved deep within the theropod stem lineage, well before the origin of crown-group birds. Provenance analysis of gastrolith lithologies has also been used to infer long-distance dinosaur migrations spanning hundreds of kilometers.