what is an index fossil

An index fossil is a fossil that is used to define and identify geologic periods or faunal stages. These fossils must have a short vertical range, wide geographic distribution, and rapid evolutionary trends. They are distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time. Index fossils are the basis for defining boundaries in the geologic time scale and for the correlation of strata. They are commonly used in marine strata, where single-celled Protista with hard body parts and larger forms such as ammonoids are used. In terrestrial sediments of the Cenozoic Era, mammals are widely used to date deposits. Index fossils are also called guide fossils or indicator fossils.