index fossil

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any ancient remains of a plant or animal that lived during a specific geological period and that can be used to date the containing rocks.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found