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index fossil (index species, zone fossil) 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.
index fossil (zone fossil) An animal fossil of a group that existed continuously during a particular span of geological time and can therefore be used to date the rock in which it is found. Index fossils are found chiefly in sedimentary rocks. They are an essential tool in stratigraphy for comparing the geological ages of sedimentary rock formations. Examples are ammonites and graptolites.