fossil vs ammonoid

fossil and ammonoid both are nouns.

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

As nouns, ammonoid is a hyponym of fossil; that is, ammonoid 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
ammonoid: one of the coiled chambered fossil shells of extinct mollusks

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

fossil (noun)	ammonoid (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	one of the coiled chambered fossil shells of extinct mollusks
someone whose style is out of fashion

fossil (adjective)	ammonoid (adjective)
characteristic of a fossil