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Index fossil

Index fossils (also known as guide fossils, indicator 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 minor.

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.

List of Common Index Fossils [1]
Fossil Scientific Name Time Period Million Years Ago

Calico Scallop
Pecten gibbus
Argopecten gibbus
Quaternary Period 1.8 million years ago
Neptunea tabulata Quaternary Period 1.8 million years ago
Viviparus glacialis Tiglian (Early Pleistocene) 0.5 million years ago
Calyptraphorus velatus Tertiary Period
Venericardia planicosta Eocene

Scaphites hippocrepis Cretaceous Period 145.5 to 65.5 million years ago

Inoceramus labiatus Cretaceous Period
Perisphinctes Perisphinctes tiziani Jurassic Period
Nerinea trinodosa Jurassic Period
Tropites subbullatus Triassic Period
Monotis subcircularis Triassic Period
Leptodus americanus Permian Period
Parafusulina Parafusulina bosei Permian Period
Dictyoclostus americanus Pennsylvanian Period
Lophophyllidium proliferum Pennsylvanian Period
Cactocrinus multibrachiatus Mississippian Period
Prolecanites gurleyi Mississippian Period

Mucrospirifer mucronatus Devonian Period 416 to 359 million years ago
Palmatolepus unicornis Devonian Period
Silurian Period
Tetragraptus fructicosus Ordovician Period
Paradoxides pinus Cambrian Period

See list of trilobites Cambrian Period 540 million years ago
Billingselia corrugata Cambrian Period
Archeocyathids Lower Cambrian


1. ^ Index Fossils, from the US Geological Survey. Updated July 28, 1997.

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