Gastroliths (stomach stones or gizzard stones) are rocks, which are or have been detained inside the digestive tract of an animal. Among living vertebrates, gastroliths are ordinary among herbivorous birds, crocodiles, and seals. Some extinct animals, such as bird-like theropod dinosaurs, appear to have used them to grind rough 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 sense of balance themselves or decrease their buoyancy.
More research is needed to appreciate 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 kilograms in weight. Stones swallowed by ostriches can also reach a length of more than 10 cm.
Geologists usually require more than a few pieces of evidence before they will accept that a dinosaur to aid its digestion used a rock. First, the stone must be unlike the rock established in its geological vicinity. Secondly, it should be rounded and polished, because inside a dinosaur's gizzard other stones and fibrous materials in a process similar to the action of a rock tumbler would have acted upon any genuine gastrolith. 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 refined red and black charts, 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 fist-sized, which is consistent with the idea that they are gastroliths.
Paleontologists are researching new methods of identifying gastroliths disassociated from animal leftovers 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 unique sources. This may offer important information on how dinosaurs migrated. Because the number of suspected gastroliths is large, they could provide important new insights into the lives and behavior of dinosaurs.