Gastroliths (stomach stones or gizzard stones) are rocks which are or have been held inside the digestive tract of an animal. Among living vertebrates, gastroliths are common among herbivorous birds, crocodiles, and seals. Some extinct animals, such as bird-like theropod dinosaurs, appear to have used them to grind tough plant matter. Gastroliths do only rarely occur in sauropod dinosaurs and a trituration of their food with the stones is not plausible. Aquatic animals, such as plesiosaurs, may have used them as ballast to help balance themselves or decrease their buoyancy. More research is needed to understand the function of the stones in aquatic animals. While some fossil gastroliths are rounded and polished, many stones in living birds are not polished at all. Gastroliths associated with dinosaur fossils can be several kilogram in weight. Stones swallowed by ostriches can also reach a length of more than 10 cm.
Geologists usually require several pieces of evidence before they will accept that a rock was used by a dinosaur 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 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 . Lastly, the stone must be found with the bones of the dinosaur which ingested it. It is this last criterion that causes trouble in identification, as smooth stones found without context can be dismissed as having been polished by water or wind.
The Early Cretaceous Cedar Mountain Formation of Central Utah is full of highly polished red and black cherts which may partly represent gastroliths. Interestingly, the cherts may themselves contain fossils of ancient animals, such as corals. These stones do not appear to be associated with stream deposits, and are rarely more than than fist-sized, which is consistent with the idea that they are gastroliths.
Paleontologists are researching new methods of identifying gastroliths dissassociated from animal remains because of the important information they can provide. If the validity of such gastroliths can be verified, it may be possible to trace gastrolithic rocks back to their original sources. This may provide important information on how dinosaurs migrated. Because the number of suspected gastroliths is large, they could provide significant new insights into the lives and behavior of dinosaurs.
