A gastrolith, also called a stomach stone or gizzard stones, is a rock held inside a gastrointestinal tract. Gastroliths are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. The grain size depends upon the size of the animal and the gastrolith's role in digestion. Other species use gastroliths as ballast. Particles ranging in size from sand to cobbles have been documented.
Gastrolith comes from the Greek gastro meaning stomach and lithos meaning stone.
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. While some fossil gastroliths are rounded and polished, many stones in living birds are not polished at all. Gastroliths associated with dinosaur fossils can weigh several kilograms. Stones swallowed by ostriches can exceed a length of 10 centimetres (3.9 in).
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 fossils 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. Christopher H. 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.
None of the gastroliths examined in a 2001 study of Cedarosaurus gastroliths had the "soapy" texture popularly used to distinguish gastroliths from other types of clast. The researchers dismissed using a soapy texture to identify gastroliths as "unreliable". Gastroliths tended to be universally dull, although the colors represented were varied including black, dark brown, purplish red and grey-blue. Reflectance values greater than 50% are very diagnostic for identifying gastroliths. Clasts from beaches and streams tended to have reflectance values of less than 35%. Less than ten percent of beach clasts have reflectance values lying between 50 and 80%.
The American Museum of Natural History Photograph # 311488 demonstrates an articulated skeleton of a Psittacosaurus mongoliensis, 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.
Geologic distribution 
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. Most known instances of preserved sauropod gastroliths are from Jurassic animals.
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.
Sauropod gastroliths 
Cedarosaurus weiskopfae 
In 2001 Frank Sanders, Kim Manley, and Kenneth Carpenter published a study on 115 clasts discovered in association with a Cedarosaurus specimen. These clasts were the first discovery of in situ gastroliths from the Cedar Mountain Formation. The clasts were "partially matrix supported" and there were many contacts between clasts and bones and between the clasts themselves. The clasts were identified as gastroliths on the basis of their tight spatial distribution, partial matrix support and an edge-on orientation indicative of their being deposited while the carcass still had soft tissue. Their high surface reflectance values are consistent with other known dinosaur gastroliths. The clasts were generally of dull coloration, suggesting that color was not a major factor for the sauropod's decision making. All but three of the Cedarosaurus gastroliths were found within a .06 m volume of space. This space was located within the gut area. No other clasts were found within the quarry, which at the time had a volume of about 11 m cubed. The set of gastroliths is believed complete due to their being discovered in a single pocket deep in the quarry. The skeletal position suggests that the skeleton came to rest on its belly.
The total mass of the gastroliths was 7 kilograms (15 lb), total volume 2,703 millilitres (95.1 imp fl oz; 91.4 US fl oz) and the total surface area 4,410 square centimetres (684 sq in). A majority, 67 of the 115 gastroliths, were less than 10 millilitres (0.35 imp fl oz; 0.34 US fl oz) in volume. Individual clasts ranges from .04 to 270 millilitres (0.0014 to 9.5 imp fl oz; 0.0014 to 9.1 US fl oz) in volume. The least massive clast was .1 grams (0.0035 oz) and the most was 715 grams (25.2 oz). Most of the gastroliths tended to be small. The clasts tended to be close to spherical in shape, with the largest specimens being the most irregular. 43% were oblate spheroids, 34% spheroids, 16% prolate spheroids, and 7% ellipsoidal. The largest gastroliths contributed the most to the total surface area of the set. Since some of the most irregular gastroliths are also the largest, it is unlikely that they were ingested by accident. Cedarosaurus may have found irregular clasts to be attractive potential gastroliths or was not selective about shape. Some gastroliths were so large and irregularly shaped that they may have been difficult to swallow. The gastroliths includes chert, sandstone, siltstone, and quartzite clasts. Some of the chert clasts actually contained fossils. The sandstone clasts tended to be fragile and some broke in the process of collection. 62% were chert, 31 percent were sandstone and siltstone, 7% were quartzite.
The most reflective gastroliths were composed of chert. Some of the gastroliths couldn't be tested for reflectance due to a confounding metallic coating, which may have been hematite. Expansion and contraction of the supporting mudstone around the inflexible clasts actually left series of parallel scratches in the coating. The metallic coating "probably originated from the iron rich mudstone" surround the fossils. The sandstone gastroliths may have been rendered fragile after deposition by loss of cement caused by the external chemical environment. If the clasts had been that fragile while the animal was alive, they probably rolled and tumbled in the digestive tract. If they were more robust, they could have served as part of a ball-mill system. The high surface area to volume ratio of the largest clasts "also supports a grinding or crushing model."
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 behaviour of dinosaurs.
See also 
- Darby and Ojakangas (1980).
- "Description," Sanders et al. (2001). Page 176.
- "Description," Sanders et al. (2001). Pp. 176-177.
- "Description," Sanders et al. (2001). Page 177.
- "Occurrence of Gastroliths in Mesozoic Taxa," Sanders et al. (2001). Page 168.
- "Abstract," Sanders et al. (2001). Page 166.
- "Occurrence in Cedarosaurus," Sanders et al. (2001). Page 169.
- "Depositional Setting," Sanders et al. (2001). Page 169.
- "Table 12.2," Sanders et al. (2001). Page 171.
- "Description," Sanders et al. (2001). Page 172.
- "Table 12.3," Sanders et al. (2001). Page 174.
- "Description," Sanders et al. (2001). Page 174.
- "Conclusion," Sanders et al. (2001). Page 177.
- Darby, D.G. and Ojakangas, J. (1980). Gastroliths from an Upper Cretaceous Plesiosaur. J. of Paleontology 54:3
- Whittle, C. (1989). On the Origins of Gastroliths: Determining the Weathering Environment of Rounded and Polished Stones by Scanning Electron Microscope Analysis. Geological Society of America Bulletin 51:5.
- Whittle, C. (1988). On the Origins of Gastroliths. Journal of Vertebrate Paleontology, Supplement to 3:28.
- Wings, Oliver (2003): Observations on the Release of Gastroliths from Ostrich Chick Carcasses in Terrestrial and Aquatic Environments. Journal of Taphonomy 1(2): 97-103. PDF fulltext
- Wings, Oliver (2004): Identification, distribution, and function of gastroliths in dinosaurs and extant birds with emphasis on ostriches (Struthio camelus). Ph.D. Thesis, The University of Bonn, Bonn, Germany, 187 pp. URN: urn:nbn:de:hbz:5N-04626 PDF fulltext
- Wings, Oliver (2007): A review of gastrolith function with implications for fossil vertebrates and a revised classification. Acta Palaeontologica Polonica 52(1): 1-16. PDF fulltext
- Wings, Oliver & Sander, P.M. (2007): No gastric mill in sauropod dinosaurs: new evidence from analysis of gastrolith mass and function in ostriches. Proc. R. Soc. B 274(1610): 635–640. doi:10.1098/rspb.2006.3763 PMID 17254987 PDF fulltext