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.
A fossil is an index fossil when it is distinctive, abundant, and limited in geological areas. A good index fossil lived for a short period of time so that the range of ages of the strata in which the index fossil is located is small. A good index fossil is found in a wide geological range.
A fossil is an index fossil when animals or plants preserved in the rock record of the Earth is characteristic by a particular span of geologic time or environment.
