thecodont reptile vs archosaur

thecodont reptile and archosaur both are nouns.

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

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

thecodont reptile: presumably in the common ancestral line to dinosaurs and crocodiles and birds
archosaur: extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts

Other hypernyms of thecodont reptile include archosaurian, archosaurian reptile.

thecodont reptile (noun)	archosaur (noun)
presumably in the common ancestral line to dinosaurs and crocodiles and birds	extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts