A gastrolith is a stone
found in the stomach
s of many vertebrate
s. Two types of gastrolith exist.
Many birds and reptiles, as well as a few mammals, swallow small stones to aid in digestion. The stones, which take up residence in the stomachs of unfeathered creatures and in the gizzards of birds, become smooth and rounded with wear as they grind against each other and break up the animal's food. In marine animals, gastroliths probably serve as ballast as well.
Aside from birds' gizzards, gastroliths are common in crocodiles and are often found with the fossilized remains of dinosaurs. Gastroliths of the extinct moas of New Zealand are popular with collectors.
The second sort of gastrolith develops within the stomach itself, as salts of various mineral composition crystallize in layered deposits on a "seed" of some sort, somewhat similarly to the formation of a pearl. These stones, rather than assisting healthy digestion, are often indicative of a pathological condition.