index fossil

n.
The fossil remains of an organism that lived in a particular geologic age, used to identify or date the rock or rock layer in which it is found. Also called guide fossil.

The ancient remains and traces of a plant or animal that lived during a particular span of geologic time and that geologically dates the containing rocks. Index fossils are almost exclusively confined to sedimentary rocks which originated in such diverse environments as open oceans, tropical lagoons, coral reefs, beaches, lakes, and rivers.

The choice of a fossil as an index depends on several criteria. In general, the fossil represents a group that evolved rapidly. The greater the rate of evolution, the shorter the period of time represented by any given index fossil and the narrower the limits of relative age assigned to the rocks containing the index. Commonly, the span of geologic time during which a fossil lived is referred to as its range, and the thickness of rocks through which a particular index fossil or selected group of fossils occurs is referred to as a faunal zone. An index fossil also must be present in the rocks in sufficient numbers to be found with reasonable effort, must be relatively easy to collect or identify, and must be geographically extensive so that the zone it defines is widely applicable.

The fossil groups most useful as index fossils are generally marine and either floaters or open ocean swimmers, such as cephalopods, or bottom dwellers that had a floating or swimming stage in their life cycles, such as the medusa stage in the brachiopods. Such characteristics are necessary for rapid dispersal of newly evolved forms. On land, such mobile forms as the horses or wind-borne pollen and spores were relatively unrestricted by environmental barriers and became widely dispersed. All of these groups have provided biochronological zones of worldwide extent.

During the Cambrian Period (5.7 × 10⁸ years before present) the oldest highly developed animals appeared; among them the trilobites provide the first important group of index fossils. Small plantlike floating colonial animals called graptoliteshave proved useful in correlating Ordovician (4.75 × 10⁸ years B.P.) and Silurian (4.25 × 10⁸ years B.P.) rocks. Ammonoids are a classic example of the internationally useful index fossil and are important beginning in the Devonian Period (4.13 × 10⁸ years B.P.) and extending to the end of the Cretaceous Period (6.5 ×10⁷ years B.P.). From the Pennsylvanian Period (3.1 × 10⁸ years B.P.), fusulinids, a family of Foraminiferida, and pollen and spores from the coal forests are important indices. Small phosphatic teethlike fossils known as conodonts have been useful for detailed zonation throughout the Paleozoic Era as well as the early part of the Mesozoic. Closer to present time, the bones and teeth of vertebrate animals serve as index fossils for the Tertiary Era, while the remains of primitive humans have been used to date the Recent past. See also Brachiopoda; Cephalopoda; Conodont; Foraminiferida; Fossil; Fusulinacea; Geologic time scale; Graptolithina; Stratigraphy.

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