Index fossil

From Wikipedia, the free encyclopedia

example index fossils

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.

[edit] Common Index Fossils

List of Common Index Fossils ^[1]
Fossil	Scientific Name	Time Period	Million Years Ago
Calico Scallop	Pecten gibbus Argopectin gibbus	Quaternary Period	1.8 million years ago
	Neptunea tabulata	Quaternary Period	1.8 million years ago
	Calyptatraphorus velatus	Tertiary Period
	Venericardia planicosta	Tertiary Period
Scaphites	Scaphites hippocrepis	Cretaceous Period	145.5 to 65.5 million years ago
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
Mucrospirifer	Mucrospirifer mucronatus	Devonian Period	416 to 359 million years ago
	Palmatolepus unicornis	Devonian Period
Ammonite	Ammonite jeletzkytes	Late Silurian to Early Devonian	400 million years ago
	Cystiphyllum niagarense	Silurian Period
	Hexamoceras hertzeri	Silurian Period
Trilobite	Bathyurus extans	Ordovician Period
	Tetragraptus fructicosus	Ordovician Period
	Paradoxides pinus	Cambrian period
Trilobite	See list of trilobites	Cambrian Period	540 million years ago
	Billingselia corrugata	Cambrian Period

[edit] In Popular Culture

Musical Group Bad Religion have a song titled "Part IV (The Index Fossil)" on their 1988 album Suffer. The song suggests that one day humanity will be "an index fossil buried in [its] own debris".