Index fossils have several important characteristics, such as wide geographic abundance, easy identification and a relatively short geological existence. These characteristics make the fossils useful for identifying the age of the geologic formations where they are found.
One widely known set of index fossils is the ammonites, an extinct marine member of the cephalopods. The ammonites emerged in the middle Devonian period, approximately 400 million years ago. They died out after the Cretaceous-Palogene extinction event roughly 66 million years ago. They are excellent index fossils because of this relatively narrow time of existence, as well as the fact that their spiral-shell fossils are widely found throughout the world and easily identified.