In many cases, tracks of animals are the only record of their existence. For example, in many places, dinosaur tracks are much more abundant than dinosaur bones. During its lifetime, a single dinosaur makes millions of tracks, but leaves only one skeleton, which may or may not be preserved. Trace fossils include tracks, trails, burrows, and borings. Dinosaur tracks, Morrison Formation, Dinosaur Ridge near Denver, Colorado.
Fossil species appear and disappear throughout the stratigraphic record.
The Geologic Time Scale is based on these appearances and disappearances. Each of the Eras ends with a mass extinction. Period boundaries coincide with smaller extinction events, followed by appearances of new species. Fossils can be used to recognize the approximate age of a unit and its place in the stratigraphic column. They can also be used to correlate strata from place to place.