What is Gastrolith?
[[File:Gastroliths.jpg|thumb|[[Plesiosaur]] gastroliths from the [[Tropic, Utah|Tropic]] [[Shale]].]] [[File:Psittacosaurus stomach stones.jpg|thumb|230px|''[[Psittacosaurus]]'' fossil with gastroliths in its stomach region, American Museum of Natural History]] '''Gastroliths''', or '''gizzard stones''', are small [[Rock (geology)|rocks]] kept by an animal inside its [[stomach]] or at an early part of the [[gastrointestinal tract|alimentary canal]]. The function of these is to mash up the food so [[digestion]] takes place faster. Typically, animals which do this do not have suitable [[teeth]] for [[wikt:grinding|grinding]], and the gastroliths are a good substitute for this. A second function, in [[aquatic]] animals, is that gastroliths can be used for [[ballast]] to make diving easier. ==Identification== Several pieces of evidence are needed to show that a rock was used by a fossil animal 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 gizzard any genuine gastrolith would grind against other stones and fibrous materials, like the action of a [[rock tumbler]]. Lastly, the stone must be found with the [[fossil]]s of the animal which ingested it. == Occurrence == Among living [[vertebrate]]s, gastroliths are common among [[crocodile]]s, [[alligator]]s, [[herbivore|herbivorous]] [[bird]]s, [[earless seal|seals]] and [[sea lion]]s. Domestic [[fowl]] need access to ''grit''. Stones swallowed by [[ostriches]] can exceed a length of {{Convert|10|cm}}. Amphibians such as the [[Axolotl]] ingest rocks that are presumed to be gastroliths.<ref>Kulbisky et al [http://www.umbjournal.org/article/S0301-5629(99)00040-X/abstract The axolotl as an animal model for the comparison of 3-D ultrasound with plain film radiography] rrr in Medicine and Biology, July 1999 Volume 25, Issue 6, Pages 969–975</ref><ref>Wings, O [http://www.dinosaurhunter.org/files/app-2007-wings-gastrolith_function_classification.pdf A revie