ichthyosaur vs archosaur

ichthyosaur and archosaur both are nouns.

Word	Noun	Adjective	Verb	Adverb
ichthyosaur	Yes	No	No	No
archosaur	Yes	No	No	No

As nouns, archosaur is a hypernym of ichthyosaur; that is, archosaur is a word with a broader meaning than ichthyosaur:

ichthyosaur: any of several marine reptiles of the Mesozoic having a body like a porpoise with dorsal and tail fins and paddle-shaped limbs
archosaur: extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts

Other hypernyms of ichthyosaur include archosaurian, archosaurian reptile.

ichthyosaur (noun)	archosaur (noun)
any of several marine reptiles of the Mesozoic having a body like a porpoise with dorsal and tail fins and paddle-shaped limbs	extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts