Coprolites are fossilized dung.

While they aren't found very often in the field, they are more commonly found for sale. You will likely find turtle coprolites the most, although there is debate over whether these are true coprolites or concretions. Dinosaur coprolites are occasionally found. The largest single coprolite piece as yet confirmed, is that of a Tyrannosaurus rex, the size of a fire extinguisher.