nothosaur vs archosaur

nothosaur and archosaur both are nouns.

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

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

nothosaur: extinct marine reptile with longer more slender limbs than plesiosaurs and less completely modified for swimming
archosaur: extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts

Other hypernyms of nothosaur include archosaurian, archosaurian reptile.

nothosaur (noun)	archosaur (noun)
extinct marine reptile with longer more slender limbs than plesiosaurs and less completely modified for swimming	extinct reptiles including: dinosaurs, plesiosaurs, pterosaurs, ichthyosaurs, thecodonts