A gastrolith, also called a stomach stone or gizzard stone, is a rock held in the gastrointestinal tract of an animal to aid in digestion. The word “gastrolith” literally translates to “stomach stone.” Small gastroliths used by domestic fowl are often called grit. Their use is documented in a wide variety of animals, and they generally serve to further grind up food after it has been swallowed. This makes them especially useful to animals that lack teeth. Originally gastroliths are normal pieces of rock, ranging in size from sand to cobble depending on the size of the animal using it. In some animals, the rock will be passed out of the digestive tract in the animal’s waste after a period of time, having been worn down and smoothed.
Animals throughout Earth’s history have used gastroliths and they are well-known in paleontology. Identifying them can sometimes be difficult, but there are a few key features that scientists can use to determine whether a rock’s wear and tear is from animal use or exposure to water and wind. Additionally, gastroliths are typically found in sediment where they are rather different from the surrounding rock, having been transported far from their point of origin by animals.
They also have microscopic rilles which are believed to be caused by stomach acid, while rocks eroded by natural forces will usually be less polished on their higher surfaces. However, since gastroliths may wind up exposed to water or wind after leaving an animal’s body, they can have a combination of features.
The criteria for identifying gastroliths have been debated by scientists, and fossil gastroliths can be the hardest to identify for sure because of how long they have been exposed to the elements. However, there are a few features that can hint as to a rock’s past as a gastrolith. As it is used in an animal’s digestive tract to grind up food, it is worn down by the fibrous food items, other gastroliths, and the animal’s body. Eventually, it has a very smooth appearance, often compared to a stone that has been placed in a rock tumbler. More appropriately, they may be compared to worn-down animal teeth. They are rounded on all edges, and may be polished. Stones that are worn down by water and wind are less polished on higher surfaces and are often pitted or cracked here, whereas gastroliths are more polished on higher surfaces but not within any crevices in the rock. Fossil gastroliths are much more likely to be polished than those found in modern animals. Additionally, the most polished of gastroliths show microscopic rilles along their surfaces. It is believed that these are from stomach acids in the animal they were used by.
When they are first swallowed, though, gastroliths are no different from any other rock. Tougher stones may last longer, but most animals will use whatever types of rock they can find. The size of gastrolith stones is limited by the size of the animal using them. Smaller animals may use particles as small as sand grains, while bigger creatures can swallow pieces of cobble several inches wide. Those used by larger dinosaurs may weigh several kilograms. Rock of any composition may be used; some Jurassic-aged gastroliths are even made of petrified wood.
The main use for gastroliths is a digestion aid. Animals locate suitably-sized rocks and swallow them, storing them in the gizzard or stomach. This is common in herbivorous or omnivorous animals that feed on fibrous plant material, especially in species that lack teeth; in these cases, the gastroliths essentially function as teeth, being manipulated by the muscular gizzard to crush food. Many tougher plants are difficult to break down in the digestive tract, and even animals with teeth may have difficulty with certain types of plants. Gastroliths reduce wear on the teeth, or serve the role of teeth, in breaking down tougher fibrous plant matter and enabling its digestion. The stones are often used in a ball-mill fashion.
Aquatic animals have developed another use for gastroliths. Carnivorous species generally do not need gastroliths to grind up food, but those that live in water may swallow stones nonetheless. In these species, the purpose of the stones is not to aid in digestion, but for ballast, to counteract the animals’ natural buoyancy. The stones add weight to the creature, helping it sink into the sea in search of food. This has been documented in larger marine predators, which take stones from the seafloor, as well as in small freshwater creatures which instead swallow tiny pieces of gravel or sand. In addition to decreasing buoyancy, aquatic animals may also use gastroliths for balance.
In a few species, the gastroliths themselves are used as a resource rather than a means to extract them. For example, some species of freshwater crayfish are known to ingest small pebbles or sand. They select calcium-rich stones, since calcium is harder to come by in freshwater environments. Crustaceans and all arthropods are ecdysozoan animals, meaning they periodically molt their cuticle as they grow. In order to develop a new exoskeleton, crayfish need to obtain calcium, and the species that use gastroliths obtain it from these rocks. This can be considered a form of geophagy.
Some animals retain gastroliths until they are completely worn down, swallowing new ones to replace those that have been ground into finer particulate and excreted. Other animals replace their gastroliths more routinely, either excreting or regurgitating old gastroliths and selecting new ones to ingest.
Many species of animals make use of gastroliths. On land, these stones are known to be ingested by various types of herbivorous or omnivorous birds including chickens and ostriches, and in the water, they are utilized by crocodiles, alligators, seals, sea lions, tadpoles, and even some invertebrates such as crayfish. The stones may be selected deliberately or accidentally ingested along with plant material pulled up from the soil.
Used gastroliths are found in one of two ways: either on the ground (often in water) after being excreted or regurgitated by an animal, or within the carcass of an animal that had swallowed them before it died. Some species change out their gastroliths on a regular basis, while some retain them until they are ground into fine particulate. The size and number of gastroliths ingested varies depending on the type of animal; bigger creatures consume more gastroliths, and can hold a greater amount.
Since the advent of de-extinction, gastroliths may now be used by an increased number of animals. Like modern birds and their crocodilian relatives, many herbivorous dinosaurs consume gastroliths, including sauropods, plateosaurs, hadrosaurs, ceratopsians, and stegosaurs. Generally these are animals with limited chewing abilities, though genetic modification has enabled some of these creatures to chew when in prehistory they could not. Certain marine reptiles, such as plesiosaurs, swallow them for ballast. Since many of these de-extinct animals are large, they use equally large quantities of gizzard stones. Some dinosaurs such as Triceratops (in S/F canon) and Stegosaurus (in C/N canon) regularly discard old gastroliths and consume new stones to replace them, leaving large piles in areas they frequent. They may accidentally ingest harmful plants while foraging for gastroliths, leading to health issues in captivity.
Herbivorous animals have been using gastroliths for hundreds of millions of years, and the evidence for this is very well documented in paleontology. The oldest examples so far documented are known from Plateosaurus, which lived during the late Triassic period more than two hundred million years ago. Numerous gastroliths are associated with plesiosaur and sauropod remains, as well as the North American hadrosaur Claosaurus. These were among the first gastrolith fossils identified by paleontologists.
Because the United States of America has the resources to permit a large body of scientific research, many gastrolith fossils have been found by American paleontologists, most of them belonging to Jurassic sauropods. These include some of the largest gastroliths ever known, some of them reaching ten centimeters wide. The species Cedarosaurus weiskopfae is known for its gastroliths, with one specimen studied in 2001 containing a total of 115 individual gizzard stones. This fossil and others like it are known from the Cedar Mountain Formation. The late Jurassic Morrison Formation also yields quite a few gastroliths, to the point where they are popularly called Morrison stones.
Outside North America, gastroliths have been found as well, though less research has gone into them. In Asia, the small primitive ceratopsian Psittacosaurus mongoliensis used gastroliths, with a fossil from the Ondai Sair Formation of Mongolia containing forty stones. The omnivorous Chinese theropods Caudipteryx zoui and Sapeornis chaoyangensis, both closely related to modern birds, are known to have used gastroliths as well.
Behind the Scenes
Gastoliths played a major role in a brief subplot of Michael Crichton‘s Jurassic Park, in which a Stegosaurus was accidentally ingesting the toxic berries of Melia azedarach while foraging for gastroliths. This scene was adapted into the film of the book, but with the Stegosaurus replaced by a Triceratops. However, due to parts of the scene being cut from the final film, the discovery of the gastroliths around the plant is not verbally addressed by the characters and is just briefly shown.