Certain fossil types are found solely in rock of a particular age. These kinds of fossils are known as index fossils. Index fossils are particularly useful to scientists because discovery of these fossils in undated rock layers allows scientists to determine the absolute age of these rocks.

In order to be considered an index fossil, an organism must meet specific criteria. Species that form index fossils must:

• be distributed over a wide geographical area


• be clearly recognizable with distinguishable characteristics


• be abundant


• have lived for a short span of geologic time

The trilobite, for example, is a widely used index fossil. Each species of trilobite lived for a fairly short and distinct time period before it went extinct.