archosaurian reptile vs diapsid

archosaurian reptile and diapsid both are nouns.

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

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

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

Other hypernyms of archosaurian reptile include diapsid reptile.

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