Index fossil

An index fossil is a fossil used to identify and correlate rock layers of a particular age. It comes from a species that was geographically widespread and existed for a relatively short span of geological time.

Biostratigraphy: The study of fossil distributions in rock layers to establish their relative ages and correlations. Index fossils are the principal tools biostratigraphy uses to match strata.
Fossil: Preserved remains, impressions, or traces of organisms from the geological past. Index fossils are preserved biological evidence used to date rocks.
Geological correlation: The matching of rock units or geological events across different locations. Index fossils let geologists correlate separated layers that share the same fossil species.
Radiometric dating: A method of determining numerical ages by measuring radioactive decay in minerals or other materials. It can provide numerical ages where index fossils alone establish relative placement.
Fossil range: The interval of geological time during which a taxon is known from the fossil record. A short fossil range gives an index species its narrow age signal.
Stratigraphy: The study of rock layers, their relationships, and their sequence through time. Index fossils work within stratigraphic layers and their correlations.
Chronostratigraphy: The branch of stratigraphy concerned with the ages and time relations of rock strata. Index fossils help assign strata to units representing intervals of geological time.
Guide fossil: A fossil considered useful for identifying and correlating rock layers of a particular age. The phrase is often used as a synonym for index fossil, though usage varies.
Faunal succession: The orderly succession of fossil assemblages through geological strata. Recognizable changes in fossil assemblages support age correlation beyond a single species.
Sedimentary rock: Rock formed from accumulated sediment or minerals precipitated from water or air. Most fossils used for correlation occur in sedimentary rocks.

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