Yes, DEVONIAN is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, V 4, O 1, N 1, I 1, A 1, N 1.

Is it a valid word?

DEVONIAN is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

from 405 million to 345 million years ago; preponderance of fishes and appearance of amphibians and ammonites

Also: devonian period, age of fishes.