Index fossil

Index fossils (also known as guide fossils or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil. If the species concerned were short-lived (in geological terms, lasting a few hundred thousand years), then it is certain that the sediments in question were deposited within that narrow time period. The shorter the lifespan of a species, the more precisely different sediments can be correlated, and so rapidly evolving types of fossils are particularly valuable. The best index fossils are common, easy-to-identify at species level, and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is low.

Ammonites fit these demands well, and are the best-known fossils that have been widely used for this. Other important groups that provide index fossils are the corals, graptolites, brachiopods, trilobites, and echinoids (sea urchins). Conodonts may be identified by experts using light microscopy such that they can be used to index a given sample with good resolution. Fossilized teeth of mammals, have also been used.

Geologists use both large fossils (called macrofossils) and microscopic fossils (called microfossils) for this process, known as biostratigraphy. Macrofossils have the advantage of being easy to see in the field, but they are rarer, and microfossils are very commonly used by oil prospectors and other industries interested in mineral resources when accurate knowledge of the age of the rocks being looked at is needed.

Common Index Fossils

List of Common Index Fossils

Fossil	Scientific Name	Time Period	Million Years Ago
File:Calico scallop 02.jpg Calico Scallop	Pecten gibbus Argopectin gibbus	Quaternary Period	1.8 MYA
	Neptunea tabulata	Quaternary Period	1.8 MYA
	Calyptatraphorus velatus	Tertiary Period
	Venericardia planicosta	Tertiary Period
File:SmallScaphites.jpg Scaphites	Scaphites hippocrepis	Cretaceous Period	144 to 66.4 MYA
Inoceramus	Inoceramus labiatus	Cretaceous Period
Perisphinctes	Perisphinctes tiziani	Jurassic Period
	Nerinea trinodosa	Jurassic Period
	Trophites subbullatus	Triassic Period
	Monotis subcircularis	Triassic Period
	Leptodus americanus	Permian Period
Parafusulina	Parafusulina bosei	Permian Period
	Dictyocostus americanus	Pennsylvanian Period
	Lophophyllidium proliferum	Pennsylvanian Period
	Cactocrinus multibrachiatus	Mississippian Period
	Prolecanites gurleyi	Mississippian Period
File:MucrospiriferSp.jpg Mucrospirifer	Mucrospirifer mucronatus	Devonian Period	416 to 359 MYA
	Palmatolepus unicornis	Devonian Period
File:Ammonite Jeletzkytes.jpg Ammonite	Ammonite jeletzkytes	Late Silurian to Early Devonian	400 MYA
	Cystiphyllum niagarense	Silurian Period
	Hexamoceras hertzeri	Silurian Period
Trilobite	Bathyurus extans	Ordovician Period
	Tetragraptus fructicosus	Ordovician Period
	Paradoxides pinus	Cambrian period
File:Asaphiscuswheelerii.jpg Trilobite	See list of trilobites	Cambrian Period	540 MYA
	Billingselia corrugata	Cambrian Period

In Popular Culture

Musical Group Bad Religion have a song titled "Part IV (The Index Fossil)" on their 1987 album Suffer. The song suggests that one day humanity will be "an index fossil buried in [its] own debris".