archosaur vs diapsid

archosaur and diapsid both are nouns.

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

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

archosaur: extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts
diapsid: reptile having a pair of openings in the skull behind each eye

Other hypernyms of archosaur include diapsid reptile.

archosaur (noun)	diapsid (noun)
extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts	reptile having a pair of openings in the skull behind each eye