placoderm vs aquatic vertebrate

placoderm and aquatic vertebrate both are nouns.

Word	Noun	Adjective	Verb	Adverb
placoderm	Yes	No	No	No
aquatic vertebrate	Yes	No	No	No

As nouns, aquatic vertebrate is a hypernym of placoderm; that is, aquatic vertebrate is a word with a broader meaning than placoderm:

placoderm: fish-like vertebrate with bony plates on head and upper body; dominant in seas and rivers during the Devonian; considered the earliest vertebrate with jaws
aquatic vertebrate: animal living wholly or chiefly in or on water

placoderm (noun)	aquatic vertebrate (noun)
fish-like vertebrate with bony plates on head and upper body; dominant in seas and rivers during the Devonian; considered the earliest vertebrate with jaws	animal living wholly or chiefly in or on water