fossil vs belemnite

fossil and belemnite both are nouns.

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

As nouns, belemnite is a hyponym of fossil; that is, belemnite 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
belemnite: a conical calcareous fossil tapering to a point at one end and with a conical cavity at the other end containing (when unbroken) a small chambered phragmocone from the shell of any of numerous extinct cephalopods of the family Belemnitidae

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

fossil (noun)	belemnite (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 conical calcareous fossil tapering to a point at one end and with a conical cavity at the other end containing (when unbroken) a small chambered phragmocone from the shell of any of numerous extinct cephalopods of the family Belemnitidae
someone whose style is out of fashion

fossil (adjective)	belemnite (adjective)
characteristic of a fossil