index fossil

American  

noun

Geology, Paleontology.

1. a widely distributed fossil, of narrow range in time, regarded as characteristic of a given geological formation, used especially in determining the age of related formations.

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index fossil British  

noun

1. Also called: zone fossil.  a fossil species that characterizes and is used to delimit a geological zone

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index fossil Scientific  

/ ĭn′dĕks′ /

1. 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. The best type of index fossils are usually those of swimming or floating organisms that evolved quickly (and therefore did not cover a long span of geologic history) and were able to spread over large areas. Ammonites and graptolites are good index fossils.

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Etymology

Origin of index fossil

First recorded in 1895–1900

Example Sentences

So distinctive is this fossil that paleontologists often use it as an index fossil to determine the age of rock layers.

From Science Daily • Nov. 12, 2025

A good index fossil should be specific to a time period but widespread enough to be found in many places.

From Textbooks • Jan. 1, 2018

The grounds were often more interesting than the matches, the All England Lawn Tennis and Croquet Club being such an index fossil from the nineteenth century.

From The New Yorker • Mar. 2, 2015

There is no specific limit on how short the time span has to be to qualify as an index fossil.

From Textbooks • Jan. 1, 2015