Meaning of index fossil

in'dex fos"sil

Pronunciation: [key]
— Geol., Paleontol. Geol.,Paleontol.
  1. a widely distributed fossil, of narrow range in time, regarded as characteristic of a given geological formation, used esp. in determining the age of related formations.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.