guide fossil vs index fossil

guide fossil and index fossil both are nouns.

Word	Noun	Adjective	Verb	Adverb
guide fossil	Yes	No	No	No
index fossil	Yes	No	No	No

As nouns, guide fossil and index fossil are synonyms defined as:

guide fossil and index fossil: 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

guide fossil (noun)	index fossil (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	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