Not all fossils are hardened bones and teeth, or molds and casts. There are also fossils that are merely evidence that creatures once crawled, walked, hopped, burrowed, or ran across the land. Trace fossils are just that: the traces of a creature left behind, usually in soft sediment like sand or mud. For example, small animals bored branching tunnels in the mud of a lake bed in search of food; and dinosaurs hunted for meals along a river bank, leaving their footprints in the soft sand. Similar to the fossil formation of hard parts, the footprints and tunnels were filled in by sediment, then buried by layers of more sediment over millions of years, eventually solidifying. Today we see the results of this long-ago activity as trace fossils. Many originators of trace fossils are unidentifiable—in other words, there are no hard fossils of the creatures left in the area, just their tracks. Some of the most famous trace fossils are those of dinosaurs tracks (for example, in Culpepper, Virginia, and near Golden, Colorado), and human-like footprints (for example, in east Africa), which were all found in hardened sediment.

There is a difference between tracks and trails, too: tracks are generally the traces of distinct footprints, whereas trails may have been produced by an animal dragging its feet or some other appendage as it moved. Tracks, therefore, are more distinctive, and different animals can be distinguished by their own particular footprints. Trails can seldom be associated with a particular animal.