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Abstract
Cortical variation in mammals and other terrestrial vertebrates, re-examined by current
comparative methodology (out-group analysis), indicates that separate lateral (olfactory),
dorsal and medial (hippocampal) pallial or cortical formations arose with the origin
of vertebrates. Although the exact origin of mammalian isocortex (so-called neocortex)
is still disputed, it appears that the earliest mammals already had a six-layered
isocortex with ten to 20 functional subdivisions. Among placental mammals, at least,
isocortex has expanded numerous times, producing additional cortical subdivisions.
Because these expansions were independent transformations of a simpler cortex, they
produced subdivisions that are not homologous.