fossil vs fucoid

fossil and fucoid both are nouns.

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

As nouns, fucoid is a hyponym of fossil; that is, fucoid 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
fucoid: a fossilized cast or impression of algae of the order Fucales

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

fossil (noun)	fucoid (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	any of various algae of the family Fucaceae
someone whose style is out of fashion	a fossilized cast or impression of algae of the order Fucales

fossil (adjective)	fucoid (adjective)
characteristic of a fossil