thumb|[[Plesiosaur gastroliths from Tropic Shale]]

A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In other species the rocks are ingested and pass through the digestive system and are frequently replaced. 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 cobble have been documented. The ingestion of gastroliths is known as lithophagy.

Etymology

Gastrolith comes from the Ancient Greek γαστήρ (gastēr), meaning "stomach", and λίθος (lithos), meaning "stone".

Occurrence

thumb|Gastroliths from Jurassic strata near Starr Springs, Utah

Among living vertebrates, gastroliths are common among crocodiles, alligators, herbivorous birds, seals and sea lions. Domestic fowl require access to grit. Stones swallowed by ostriches can exceed a length of . Apparent microgastroliths have also been found in frog tadpoles. Ingestion of silt and gravel by tadpoles of various anuran (frog) species has been observed to improve buoyancy control.

Paleontology

History of discovery

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. In 1907, Barnum Brown found gravel in close association with the fossil remains of the duck-billed hadrosaur Claosaurus and interpreted it as gastroliths. Brown was among the first paleontologists to recognize that dinosaurs used gastroliths in their digestive systems to aid in the grinding of food. Other paleontologists over the years were unconvinced. 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 1934, the Howe Quarry, a fossil location in northwestern Wyoming also yielded dinosaur bones with their associated gastroliths. As a graduate student, William Lee Stokes was unconvinced of the presence of gastroliths recovered from Late Jurassic and Cretaceous strata. However, during his career he walked those same rocks many more times, and changed his mind, writing a paper about the ubiquity of dinosaurian gastroliths in 1987.

Identification

thumb|A [[diplodocid ingesting gastroliths entangled in vegetation it is consuming|left]]

Geologists usually require several pieces of evidence before they will accept that a rock was used by a dinosaur to aid its digestion. 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. Second, the stone must be unlike the rock found in its geological vicinity, i.e., its geologic context. Many gastroliths have been found in fine grained lake, mud, and swamp deposits. These environs are calm water deposits and could not carry pebbles and cobbles (unlike a river or beach). Oliver Wings also argues that 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 rilles, defined by Whittle as "longitudinal gashes, deeper than one millimeter and longer than one centimeter at a magnification of 50 times".

Cedarosaurus weiskopfae

In 2001 Frank Sanders, Kim Manley, and Kenneth Carpenter published a study on 115 gastroliths discovered in association with a Cedarosaurus specimen.

References

  • Darby, D.G. and Ojakangas, J. (1980). Gastroliths from an Upper Cretaceous Plesiosaur. Journal 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
  • Stokes, W. L. 1987. Dinosaur gastroliths revisited. Journal of Paleontology 61: 1242–1246.
  • Schmeisser, R.L. and Gilette, D.D. (2009). Unusual occurrence of gastroliths in a polycotylid plesiosaur from the Upper Cretaceous Tropic Shale, southern Utah. "PALAIOS",24(7):453-459. [https://doi.org/10.2110/palo.2008.p08-085r]