Yes, MAMMALIANS is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, M 3, M 3, A 1, L 1, I 1, A 1, N 1, S 1.

Is it a valid word?

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

any warm-blooded vertebrate having the skin more or less covered with hair; young are born alive except for the small subclass of monotremes and nourished with milk