Elasmobranch Central Nervous System Organization and Its Possible Evolutionary Significance

Examination of shark brain:body ratios reveals that these taxapossess relative brain volumes in a range overlapping thoseof bony fish as well as birds and mammals. Much of the variationis due to relative development of the telencephalon and cerebellum.Telencephalic weights vary from 24% in Squalus to 52% in Sphyrna.Analysis of the cytoarchitectonics of the shark brains revealsat least two patterns of development. Squalomorph sharks possesslow brain:body ratios, and the telencephalon of these taxa possesswell developed lateral ventricles and poorly developed pallialareas. The diencephalon is characterized by prominent periventricularlaminae, and the cerebellum lacks foliation. The lamniform andcarcharhiniform sharks are characterized by high brain: bodyratios, and there is marked hypertrophy of the telencephalon.The roof (pallial) regions, as well as the diencephalon, arecharacterized by extensive cellular migrations. The cerebellaof these forms possess extensive complex foliation. These brain patterns are compared with the brain organizationof Holocephali, and I conclude that the holocephalans are asister radiation of the elasmobranchs. Comparisons with bonyfish and land vertebrates suggest that elasmobranchs have independentlydeveloped complex pallial fields and cerebellar foliation asa result of parallel evolutionary trends.