archosaur vs diapsid reptile

archosaur and diapsid reptile both are nouns.

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

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

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

Other hypernyms of archosaur include diapsid.

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