fossil vs index fossil

fossil and index fossil both are nouns.

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

As nouns, index fossil is a hyponym of fossil; that is, index fossil is a word with a more specific, narrower meaning than fossil:

fossil: the remains (or an impression) of a plant or animal that existed in a past geological age and that has been excavated from the soil
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

Other hyponyms of fossil include fucoid, belemnite, ammonite, ammonoid, guide fossil, microfossil, wormcast.

fossil (noun)	index fossil (noun)
the remains (or an impression) of a plant or animal that existed in a past geological age and that has been excavated from the soil	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
someone whose style is out of fashion

fossil (adjective)	index fossil (adjective)
characteristic of a fossil