Editing Microfossil (section)

==Index fossils==
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[[Index fossils]], also known as guide fossils, indicator fossils or dating fossils, are the fossilized remains or traces of particular plants or animals that are characteristic of a particular span of geologic time or environment, and can be used to identify and date the containing rocks. To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends. Rock formations separated by great distances but containing the same index fossil species are thereby known to have both formed during the limited time that the species lived.

Index fossils were originally used to define and identify geologic units, then became a basis for defining [[geologic column|geologic periods]], and then for faunal stages and zones.

Species of [[micropaleontology|microfossils]] such as [[acritarchs]], [[chitinozoa]]ns, [[conodont]]s, [[dinoflagellate]] cysts, [[ostracods]], [[pollen]], [[spores]] and [[foraminifera]]ns are amongst the many species have been identified as index fossils that are widely used in [[biostratigraphy]]. Different fossils work well for sediments of different ages. To work well, the fossils used must be widespread geographically, so that they can be found in many different places. They must also be short lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the poorer the stratigraphic precision, so fossils that evolve rapidly.

Often biostratigraphic correlations are based on a [[faunal assemblage]], rather than an individual species — this allows greater precision as the time span in which all of the species in the assemblage existed together is narrower than the time spans of any of the members. Further, if only one species is present in a sample, it can mean either that (1) the strata were formed in the known fossil range of that organism; or (2) that the fossil range of the organism was incompletely known, and the strata extend the known fossil range. If the fossil is easy to preserve and easy to identify, more precise time estimating of the [[stratigraphic layer]]s is possible.