Coprolites, distinct from paleofaeces, are fossilized animal dung. Like other fossils, coprolites have had much of their original composition replaced by mineral deposits such as silicates and calcium carbonates. Paleofaeces, on the other hand, retain much of their original organic composition and can be reconstituted to determine their original chemical properties, though in practice the term coprolite is also used for ancient human faecal material in archaeological contexts. In the same context, there are the urolites, erosions caused by evacuation of liquid wastes and nonliquid urinary secretions.
The fossil hunter Mary Anning had noticed that "bezoar stones" were often found in the abdominal region of ichthyosaur skeletons found in the Lias formation at Lyme Regis. She also noted that if such stones were broken open they often contained fossilized fish bones and scales as well as sometimes bones from smaller ichthyosaurs. It was these observations by Anning that led the geologist William Buckland to propose in 1829 that the stones were fossilized feces and named them coprolites. Buckland also suspected that the spiral markings on the fossils indicated that ichthyosaurs had spiral ridges in their intestines similar to those of modern sharks, and that some of these coprolites were black with ink from swallowed belemnites.
By examining coprolites, paleontologists are able to find information about the diet of the animal (if bones or other food remains are present), such as whether it was a herbivorous or carnivorous, and the taphonomy of the coprolites, although the producer is rarely identified unambiguously, especially with more ancient examples. In one example these fossils can be analyzed for certain minerals that are known to exist in trace amounts in certain species of plant that can still be detected millions of years later. In another example, the existence of human proteins in coprolites can be used to pinpoint the existence of cannibalistic behavior in an ancient culture. Parasite remains found in human and animal coprolites have also shed new light on questions of human migratory patterns, the diseases which plagued ancient civilizations, and animal domestication practices in the past (see archaeoparasitology and paleoparasitology).
The recognition of coprolites is aided by their structural patterns, such as spiral or annular markings, by their content, such as undigested food fragments, and by associated fossil remains. The smallest coprolites are often difficult to distinguish from inorganic pellets or from eggs. Most coprolites are composed chiefly of calcium phosphate, along with minor quantities of organic matter. By analyzing coprolites, it is possible to infer the diet of the animal which produced them.
Coprolites have been recorded in deposits ranging in age from the Cambrian period to recent times and are found worldwide. Some of them are useful as index fossils, such as Favreina from the Jurassic period of Haute-Savoie in France.