index fossil vs guide fossil

index fossil and guide fossil both are nouns.

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

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

index fossil and guide 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

index fossil (noun)	guide 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