Fossil whose presence is chosen to denote the zone in which it occurs and after which the zone is named. Index fossils are selected for their distinctiveness and/or abundance. To be of use in biostratigraphy, ideally an index fossil should have a narrow range in time (i.e. to have undergone rapid evolutionary change) but have had a wide geographical distribution. Trilobites (Cambrian); graptolites (Ordovician and Silurian); ammonites (Jurassic); and foraminifera (Cretaceous and Cenozoic) are among the most notable index fossils.