plesiosaur vs plesiosaurus

plesiosaur and plesiosaurus both are nouns.

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

As nouns, plesiosaur and plesiosaurus are synonyms defined as:

plesiosaur and 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

plesiosaur (noun)	plesiosaurus (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 marine reptile with a small head on a long neck and a short tail and four paddle-shaped limbs; of the Jurassic and Cretaceous