Phylogenetic analysis with x-ray computed tomography of fossilized and
recent crania implicates differential growth of the neocortex in the
evolution and development of the mammalian middle ear, In premammalian
tetrapods, the middle ear evolved as a chain of bones attached to the
mandible and cranium, but in adult mammals the chain is detached from
the mandible and lies behind it, The neocortex evolved concurrently w
ith detachment of the chain. In mammalian development the auditory cha
in arises connected to the mandible but later detaches, recapitulating
the phylogenetic transformation. In modern didelphid development, the
auditory chain reaches mature size by the third week after birth and
is then separated from the jaw and displaced caudally as the neocortex
grows for another 9 weeks.