archosaur vs archosaurian reptile

archosaur and archosaurian reptile both are nouns.

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

As nouns, archosaur and archosaurian reptile are synonyms defined as:

archosaur and archosaurian reptile: extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts

Other synonyms of archosaur include archosaurian.

archosaur (noun)	archosaurian reptile (noun)
extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts	extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts