plesiosaurus vs archosaur

plesiosaurus and archosaur both are nouns.

Word	Noun	Adjective	Verb	Adverb
plesiosaurus	Yes	No	No	No
archosaur	Yes	No	No	No

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

plesiosaurus: extinct marine reptile with a small head on a long neck and a short tail and four paddle-shaped limbs; of the Jurassic and Cretaceous
archosaur: extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts

Other hypernyms of plesiosaurus include archosaurian, archosaurian reptile.

plesiosaurus (noun)	archosaur (noun)
extinct marine reptile with a small head on a long neck and a short tail and four paddle-shaped limbs; of the Jurassic and Cretaceous	extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts