Bivalves may bore into wood, rock, or other hard materials. Sponges also produce borings, often riddling shells with numerous tiny holes. Coprolites may contain fragments of undigested food, and thus provide valuable information about the feeding habits of fossil organisms. Coprolites smaller than 1 mm are called fecal pellets . Fecal pellets may be extraordinarily abundant in some environments, and are the dominant allochem in pelleted limestones.
Some fecal pellets have a high phosphate content.
Gastroliths or gizzard stones were probably used to grind food in the stomach of the animal. Root marks are trace fossils produced by plants. In some areas, roots may also be associated with caliche or sedimentary iron stones. There are about 1.5 million known species of living plants and animals. In all, there may be as many as 4.5 million living species.