Elasmobranch Central Nervous System Organization and Its Possible Evolutionary Significance

Abstract

Examination of shark brain:body ratios reveals that these taxa possess relative brain volumes in a range overlapping those of bony fish as well as birds and mammals. Much of the variation is 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 reveals at least two patterns of development. Squalomorph sharks possess low brain:body ratios, and the telencephalon of these taxa possess well developed lateral ventricles and poorly developed pallial areas. The diencephalon is characterized by prominent periventricular laminae, and the cerebellum lacks foliation. The lamniform and carcharhiniform sharks are characterized by high brain: body ratios, and there is marked hypertrophy of the telencephalon. The roof (pallial) regions, as well as the diencephalon, are characterized by extensive cellular migrations. The cerebella of these forms possess extensive complex foliation. These brain patterns are compared with the brain organization of Holocephali, and I conclude that the holocephalans are a sister radiation of the elasmobranchs. Comparisons with bony fish and land vertebrates suggest that elasmobranchs have independently developed complex pallial fields and cerebellar foliation as a result of parallel evolutionary trends.