Morganucodon
Brain size is another factor. It is certain that early mammals did have enlarged brains relative to their size. Reconstructions of the brain size has shown this. Apparently, brain size in Mesozoic mammals lay within the lower range of brain size for living mammals. This is an overall increase of four or more times the volume of basal amniote brains. Very likely this involved the development of the neocortex, "one of the most striking of all mammalian characteristics".p120