osteostracan vs jawless fish

osteostracan and jawless fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
osteostracan	Yes	No	No	No
jawless fish	Yes	No	No	No

As nouns, jawless fish is a hypernym of osteostracan; that is, jawless fish is a word with a broader meaning than osteostracan:

osteostracan: extinct jawless fish of the Devonian with armored head
jawless fish: eel-shaped vertebrate without jaws or paired appendages including the cyclostomes and some extinct forms

Other hypernyms of osteostracan include agnathan, jawless vertebrate.

osteostracan (noun)	jawless fish (noun)
extinct jawless fish of the Devonian with armored head	eel-shaped vertebrate without jaws or paired appendages including the cyclostomes and some extinct forms