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