Brain variation and phylogenetic trends in elasmobranch fishes.

A cladistic analysis utilizing the out-group criterion to establish the phylogenetic polarity of neural features among living chondrichthyans leads to the conclusion that the earliest chondrichthyans possessed relatively small brains. Relative brain size appears to have increased independently in some myliobatiforms and in the common ancestor of heterodontid and galeomorph sharks. Furthermore, the brains of the earliest chondrichthyans must have exhibited a wide rhomboid fossa of the medulla, an unfoliated cerebellar corpus divided into anterior and posterior lobes, a bilobed and laminated optic tectum with the majority of the cell bodies located periventricularly, a moderately developed midbrain octavolateralis complex, and a telencephalon consisting of sessile olfactory bulbs and paired cerebral hemispheres with extensive lateral ventricles and thin walls. The corpus of the cerebellum has increased dramatically in size and has become complexly foliated independently in some batoids and galeomorph sharks. Similarly, the majority of tectal neurons have migrated away from a periventricular zone in all living elasmobranchs resulting in two distinctly derived patterns, compared to that in holocephalimorphs, but the polarity of these patterns among elasmobranchs is uncertain. The midbrain octavolateralis complex is uniquely derived in all living batoids, but two patterns appear to occur in other chondrichthyans, and, again, their polarity is uncertain. Different portions of the telencephalic roof appear to have hypertrophied in holocephalimorphs, in some batoids, and in some galeomorphs. An examination of pertinent details of chondrichthyan general biology reveals that the biological significance of most chondrichthyan neural trends is unknown.