plesiosaurus vs plesiosaur

plesiosaurus and plesiosaur both are nouns.

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

As nouns, plesiosaurus and plesiosaur are synonyms defined as:

plesiosaurus and plesiosaur: 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

plesiosaurus (noun)	plesiosaur (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