Yes, STEGOSAUR is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, E 1, G 2, O 1, S 1, A 1, U 1, R 1.

Is it a valid word?

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

herbivorous ornithischian dinosaur with a row of bony plates along its back and a spiked tail probably used as a weapon

Also: stegosaur stenops.