Miss Scarlett

Hi, just to let you know, I'm new to this site. Anyway my question is, in Macbeth, he says to the ghost "Thy bones are marrowless". I've read one critic who says that this is proof that the ghost is Duncan, seeing as how Banquo is only recently dead. My question is, is this true? Will the marrow decompose quite quickly after death leaving behind just the hard mineral shell of the bone?