Gastrolith
A rare example of this is the Early Cretaceous theropod Caudipteryx zoui from northeastern China, which was discovered with a series of small stones, interpreted as gastroliths, in the area of its skeleton that would have corresponded with its abdominal region.
[4][5][6] In 1906, George Reber Weiland reported the presence of worn and polished quartz pebbles associated with the remains of plesiosaurs and sauropod dinosaurs and interpreted these stones as gastroliths.
[7] In 1907, Barnum Brown found gravel in close association with the fossil remains of the duck-billed hadrosaur Claosaurus and interpreted it as gastroliths.
In 1932, Friedrich von Huene found stones in Late Triassic sediments, in association with the fossil remains of the prosauropod Sellosaurus and interpreted them as gastroliths.
In 1942, William Lee Stokes recognized the presence of gastroliths in the remains of sauropod dinosaurs recovered from Late Jurassic strata.
First, it should be rounded on all edges (and some are 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.
[citation needed] 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.
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.
Finally, highly polished gastroliths often show long microscopic rilles, defined by Whittle as "longitudinal gashes, deeper than one millimeter and longer than one centimeter at a magnification of 50 times".
[11] Gastroliths tended to be universally dull, although the colors represented were varied including black, dark brown, purplish red and grey-blue.
[13] 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.
[14] The Early Cretaceous Cedar Mountain Formation of Central Utah is full of highly polished red and black cherts, and other rounded quartzose clasts, which may partly represent gastroliths.
[15] In 2001 Frank Sanders, Kim Manley, and Kenneth Carpenter published a study on 115 gastroliths discovered in association with a Cedarosaurus specimen.
[16] The stones 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.
[16] The high surface area to volume ratio of the largest clasts suggests that the gastroliths may have broken down ingested plant material by grinding or crushing it.
[13] Paleontologists and geologists are researching new methods of identifying gastroliths that have been found disassociated from animal remains, because of the important information they can provide, if indeed they are trace fossils.
