Index fossils (or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil. If the species concerned were short-lived (in geological terms, lasting a few hundred thousand years) then you can be sure that the sediments in question were deposited within that narrow time period. The shorter the lifespan of a species, the more precisely you can correlate different sediments, and so rapidly evolving types of fossils are particularly valuable. The best index fossils are common, easy-to-identify at species level, and have a broad distribution—otherwise the likelihood of finding and recognising one in the two sediments is low.

Ammonites fit these demands well, and are the best-known fossils that have been widely used for this. Other important groups that provide index fossils are the corals, graptolites, brachiopods, trilobites, and echinoids (sea urchins). Conodonts may be identified by experts using light microscopy such that they can be used to index a given sample with good resolution. Terrestrial fossils, such as the teeth of mammals, have also been used.

Geologists use both large fossils (called macrofossils ) and microscopic fossils (called microfossils) for this process, known as biostratigraphy. Macrofossils have the advantage of being easy to see in the field, but they are rarer, and microfossils are very commonly used by oil prospectors and other industries interested in mineral resources when accurate knowledge of the age of the rocks being looked at is needed.