The qualities of a good index fossil are that it belongs to a relatively abundant and widely dispersed species that nevertheless existed for only a brief time on Earth. Marine animals best suit these parameters as sea currents carry them to distant shores where the tide gradually layers upon them the sediments that later form solid, sedimentary rock. The most common index fossils are the pre-historic mollusks called ammonites.
An index fossil is a fossil used to determine the age of the sediments in which it is found. When quickly approximating the age of new finds, an archaeologist looks at the rocks in which it is buried, as those sediments would have been on the surface at the time of the death of the pre-historic creature. However, different conditions can affect the appearance of sedimentary rock, making accurate impressions of age difficult to conclude.The age of the sediment, and of other fossils, can be estimated by comparing them to any index fossils uncovered in the same excavation. For this purpose, it is critical that the species of the index fossil is known to have not been long-lived, on a geological time frame, surviving no more than a few hundred thousand years.