Gastrolith
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A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the Digestion, digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascula ...
. 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 Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...
depends upon the size of the animal and the gastrolith's role in digestion. Other species use gastroliths as ballast.Rondeau, et a
Larval Anurans Adjust Buoyancy in Response to Substrate Ingestion
Copeia: February 2005, Vol. 2005, No. 1, pp. 188–195.
Particles ranging in size from
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...
to cobble have been documented.


Etymology

Gastrolith comes from the
Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
γαστήρ (''gastēr''), meaning "stomach", and λίθος (''lithos''), meaning "stone".


Occurrence

Among living
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
s, gastroliths are common among crocodiles, alligators, herbivorous
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s, seals and sea lions. Domestic
fowl Fowl are birds belonging to one of two biological orders, namely the gamefowl or landfowl ( Galliformes) and the waterfowl ( Anseriformes). Anatomical and molecular similarities suggest these two groups are close evolutionary relatives; toget ...
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. Some
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
animals such as sauropod
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s appear to have used stones to grind tough plant matter. 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. Aquatic animals, such as plesiosaurs, may have used them as ballast, to help balance themselves or to decrease their buoyancy, as crocodiles do. Research indicates that the presence of gastroliths in elasmosaurid plesiosaurs differs from that of the short-necked plesiosaurs. While some
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
gastroliths are rounded and polished, many stones in living birds are not polished at all. Gastroliths associated with
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
fossils can weigh several kilograms. Certain crayfish store gastroliths in their stomachs. Crayfish living in freshwater store these gastroliths as the presence of calcium is limited in freshwater. These gastroliths serve as a calcium source for molting.


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. In 1942, William Lee Stokes recognized the presence of gastroliths in the remains of sauropod dinosaurs recovered from Late Jurassic
strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...
.


Identification

Geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...
s 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
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
s 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". These are presumably caused by contact with stomach acid. 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 The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
Period of
Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...
, showing a collection of about 40 gastroliths inside the rib cage, about midway between shoulder and pelvis.


Geologic distribution


Jurassic

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 Denver ( ) is a List of municipalities in Colorado#Consolidated city and county, consolidated city and county, the List of capitals in the United States, capital and List of municipalities in Colorado, most populous city of the U.S. state of ...
,
Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...
), a late
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
formation roughly 150 million years old. Some gastroliths are made of
petrified wood Petrified wood (from Ancient Greek meaning 'rock' or 'stone'; literally 'wood turned into stone'), is the name given to a special type of ''fossilized wood'', the fossilized remains of terrestrial plant, terrestrial vegetation. ''Petrifaction ...
. Most known instances of preserved sauropod gastroliths are from
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
animals.


Cretaceous

The
Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
Cedar Mountain Formation of Central
Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...
is full of highly polished red and black cherts, and other rounded quartzose clasts, which may partly represent gastroliths. 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.


Sauropods

Most known instances of preserved sauropod gastroliths are from
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
animals. The largest known gastroliths found in association with sauropod skeletons are approximately ten centimeters in length.Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. 560 pp. .


''Cedarosaurus weiskopfae''

In 2001 Frank Sanders, Kim Manley, and Kenneth Carpenter published a study on 115 gastroliths discovered in association with a '' Cedarosaurus'' specimen. 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. Their high surface reflectance values are consistent with other known dinosaur gastroliths. Nearly all of the ''Cedarosaurus'' gastroliths were found within a .06 m volume of space in the gut region of the skeleton. The total mass of the gastroliths themselves was . Most were less than in volume. The least massive clast was and the most was , with most of them being toward the smaller end of that range. The clasts tended to be close to spherical in shape, although the largest specimens were also the most irregular. The largest gastroliths contributed the most to the total surface area of the set. Some gastroliths were so large and irregularly shaped that they may have been difficult to swallow. The gastroliths were mostly composed of chert, with some
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
, siltstone, and quartzite clasts also included. 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. The clasts were generally of dull coloration, suggesting that color was not a major factor for the sauropod's decision making. 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. The sandstone clasts tended to be fragile and some broke in the process of collection. 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.


Migration

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. If the validity of such gastroliths can be verified, it may be possible to trace gastrolithic rocks back to their original source area where the dinosaur first swallowed the rock. This may provide important information on how dinosaurs migrated. Because the number of suspected gastroliths is substantial, they might provide significant new information and insights into the lives and behaviour of dinosaurs.


See also

*
Bezoar A bezoar stone ( ) is a mass often found trapped in the gastrointestinal system, though it can occur in other locations. A pseudobezoar is an indigestible object introduced intentionally into the digestive system. There are several varieties o ...
* Calculus (medicine) *
Coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name ...
* Enterolith * Regurgitalith


Footnotes


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 Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct vertebrates (animals with vertebrae and their descendants). It also t ...
, 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-0462
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

{{Authority control Trace fossils Stomach Digestive system