"index fossil." YourDictionary, n.d. Web. 17 December 2018. <https://www.yourdictionary.com/index-fossil>.
index fossil. (n.d.). Retrieved December 17th, 2018, from https://www.yourdictionary.com/index-fossil
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.