Gastrolith

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, alligators, seals and sea lions. Domestic fowl, for instance, require access to 'grit', for the purpose of food-grinding. Gastroliths are retained in the very muscular gizzard and serve the masticatory function of teeth, in an animal without suitable grinding teeth. The grain size of the gastrolith depends upon the size of the animal and its special needs. Particles as small as sand and stones the size of cobbles or greater have been found. Some extinct animals, such as sauropod dinosaurs, appear to have used stones to grind tough plant matter.

Gastroliths have only rarely been found in association with fossils of theropod 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 to decrease their buoyancy, as crocodiles do. 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 kilograms 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 (possibly erroneously in some cases) be dismissed as having been polished by water or wind. 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 hairline scratches, presumably caused by contact with a sharp corner of a freshly swallowed stone. 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.

The American Museum of Natural History Photograph # 311488 demonstrates an articulated skeleton of a Protiguanadon mongoliense, from the Ondai Sair Formation, Lower Cretaceous Period of Mongolia, showing a collection of about 40 gastroliths inside the rib cage, about midway between shoulder and pelvis.

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 fist-sized, which is consistent with the idea that they are gastroliths. Gastroliths have sometimes been called 'Morrison stones' because they are often found in the Morrison Formation (named after the town of Morrison, west of Denver, Colorado), a Late Jurassic formation roughly 150 million years old. Some gastroliths are made of petrified wood.

Paleontologists are researching new methods of identifying gastroliths that have been found disassociated 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.