Index fossil

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

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".

References and Links

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  • index fossil — n. any fossil of wide geographical distribution and a short range in time, used to correlate and date rock strata and their associated fossils …   English World dictionary

  • index fossil — Geol., Paleontol. a widely distributed fossil, of narrow range in time, regarded as characteristic of a given geological formation, used esp. in determining the age of related formations. Also called guide fossil. [1895 1900] * * *       any… …   Universalium

  • index fossil — noun a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found • Syn: ↑guide fossil • Hypernyms: ↑fossil …   Useful english dictionary

  • index fossil — noun Date: 1900 a fossil usually with a narrow time range and wide spatial distribution that is used in the identification of related geologic formations …   New Collegiate Dictionary

  • index fossil — in′dex fos sil n. gel pal a widely distributed fossil, of narrow range in time, regarded as characteristic of a given geological formation and used esp. in determining the age of related formations • Etymology: 1895–1900 …   From formal English to slang

  • index fossil — noun Any ancient remains of a plant or animal that lived during a specific geological period and that can be used to date the containing rocks …   Wiktionary

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