One of the byproducts of digestion is waste material, which is conveniently excreted from the body at an end of the body opposite from the mouth region in most animals. This waste material can be in the form of a gas, liquid, or solid, but when preserved as a solid is politely called feces. A socially acceptable but often giggle-inducing colloquial synonym for fecal material used by modern trackers is "scat," and these same people will refer to the study of scat as "scatology." Coprolites are fossilized feces, and dinosaurs were no different from other animals in leaving such deposits after digesting their meals.
Despite their common association with bathroom humor, dinosaur coprolites are valuable trace fossils for figuring out the paleodiets (or feeding behavior) of dinosaurs. Coprolites can show either body fossils of plant material (indicating an herbivorous diet) or bones (indicating a carnivorous diet). Modern grizzly bears of North America (Ursus horribilis) will leave scat containing both plant and animal matter (indicating an omnivorous diet), but no dinosaur coprolites have demonstrated an omnivorous diet by choice for dinosaurs (as far as I know). Nevertheless, herbivorous dinosaurs almost certainly would have ingested plant-dwelling insects and other arthropods while feeding, thus making them "accidental" insectivores.
Coprolites also provide information about habitats and the presence of dinosaurs in areas otherwise lacking dinosaur body fossils or other trace fossils (such as tracks). Preservation of coprolites is dependent on their original organic content, water content, where they were deposited, and method of burial. For example, coprolites made by carnivorous dinosaurs (theropods) were more likely to be preserved than those made by herbivores because of the high mineral content provided by bone material of the consumed prey animals. An good preservational environment would have been a floodplain associated with rivers, where the feces deposited on a dry part of the floodplain dehydrated slightly before rapid burial by a river flood. Other environments where coprolites were likely to have been preserved include "watering holes" (ponds), swamps, streams, and muddy areas associated with estuaries or lakes.
This coprolite is most likely from a sauropod, owing mainly to its large size (about 40 cm diameter), and age (Jurassic); it is from the Morrison Formation in eastern Utah and the specimen was in the Löwentor Museum of Stuttgart, Germany. A coprolite this large probably does not represent one single pellet but an amalgamation of several pellets that merged together (indicating an originally high fluid content to the pellets). Individual dinosaur coprolites actually can be quite small (< 10 cm length) compared to the body size of the tracemakers. An example of this seemingly anomalous correlation can be observed in modern mule deer (Odocoileus hemionus) and elks (Cervus canadensis) of North America, animals that can weigh more than 100 kg but leave many individual pellets less than 1 cm in diameter.
Thulborn, R. A. 1991. Morphology, preservation and palaeobiological significance of dinosaur coprolites. Palaeogeography, Palaeoclimatology, Palaeoecology, 83:341-366.
Wright, K. 1996. What the dinosaurs left us. Discover Magazine (June, 1996 issue), p. 58-65.
Emory University Dinosaur Trace Fossil Page