Conservative Features of Neocortical Evolution in Dolphin Brain

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Abstract
A Golgi survey of the convexity cortex in the brain of the dolphin, Tursiops truncatus, has revealed many cellular characteristics which may be indicative of conservative cortical evolution. These include a high degree of pyramidalization, and an accentuation of layer II. The presence of an accentuated layer II in convexity cortex is a protoneocortical characteristic found in more ''primitive'' cortical arrangements. The growth ring concepts of cortical development outward in concentric waves from archicortical and paleocortical origins are discussed. In that context we have not been able to identify cores of hyperspecialization in the dolphin cortex corresponding to koniocortex and gigantopyramidal areas. This leads us to suggest that the cortex of the dolphin reflects a condition of the paralimbic-parinsular stage of evolutionary development. Thus, the dolphin may serve as a model of the theoretical mammalian archetype brain and its study may shed light on the organization of the brains of the initial ancestors of modern mammals.

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