flying reptile vs archosaur

flying reptile and archosaur both are nouns.

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

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

flying reptile: an extinct reptile of the Jurassic and Cretaceous having a bird-like beak and membranous wings supported by the very long fourth digit of each forelimb
archosaur: extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts

Other hypernyms of flying reptile include archosaurian, archosaurian reptile.

flying reptile (noun)	archosaur (noun)
an extinct reptile of the Jurassic and Cretaceous having a bird-like beak and membranous wings supported by the very long fourth digit of each forelimb	extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts