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index fossil

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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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Origin of index fossil

First recorded in 1895–1900
Also called guide fossil.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for index fossil

index fossil

noun
  1. a fossil species that characterizes and is used to delimit a geological zoneAlso called: zone fossil
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

index fossil in Science

index fossil

[ĭndĕks′]
  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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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.