ichthyosaurus vs ichthyosaur

ichthyosaurus and ichthyosaur both are nouns.

Word	Noun	Adjective	Verb	Adverb
ichthyosaurus	Yes	No	No	No
ichthyosaur	Yes	No	No	No

As nouns, ichthyosaur is a hypernym of ichthyosaurus; that is, ichthyosaur is a word with a broader meaning than ichthyosaurus:

ichthyosaurus: ichthyosaurs of the Jurassic
ichthyosaur: any of several marine reptiles of the Mesozoic having a body like a porpoise with dorsal and tail fins and paddle-shaped limbs

ichthyosaurus (noun)	ichthyosaur (noun)
ichthyosaurs of the Jurassic	any of several marine reptiles of the Mesozoic having a body like a porpoise with dorsal and tail fins and paddle-shaped limbs