Living things are made up of chemical compounds, most of which are organic compounds. Organic compounds consist mainly of carbon, oxygen, and hydrogen. After a plant or animal dies, it decomposes. As organisms decompose, their organic compounds change into simpler compounds like carbon dioxide and water. Decomposition is fastest when the organisms are in water that contains dissolved oxygen. Organisms can also decompose even without oxygen. Some kinds of bacteria feed on plant and animal tissues even though there is no oxygen. These are called anaerobic ("no air") bacteria.

A fossil is any evidence of past life. Fossils formed from animal bodies or their imprints are called body fossils. When people think about fossils, they usually think about body fossils. Trace fossils are another kind of fossil. A trace fossil is any evidence of the life activity of an animal that lived in the past. Burrows, tracks, trails, feeding marks, and resting marks are all examples of trace fossils. It is usually hard to figure out exactly which kind of animal made a particular trace fossil.