Coprolites – fossil excrement from animals – are numerous in Mazon Creek concretions. Sizes
range from tiny pellets to thick masses and are composed of calcium- and phosphate-rich
material (hydroxyapatite). They often include bone fragments, scales, ostracodes, portions of
bivalves, bony fish, crustaceans, and sometimes plant material. Most coprolites cannot be
matched with the animal that produced them, although those with a spiral shape are considered
to be from sharks.