Yes, PLESIOSAUR is a Scrabble word!

Letter values in Scrabble US: P 3, L 1, E 1, S 1, I 1, O 1, S 1, A 1, U 1, R 1.

Is it a valid word?

PLESIOSAUR is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

extinct marine reptile with a small head on a long neck a short tail and four paddle-shaped limbs; of the Jurassic and Cretaceous

Not playable in Scrabble US: PLESIOSAURUS.