Stalking the everted telencephalon: comparisons of forebrain organization in basal ray-finned fishes and teleosts

Compared to land vertebrates and the other fishes, the basal ray-finned fishes and teleosts have morphologically unusual forebrains. The telencephalic pallium is everted, and its diencephalic inputs arise largely--not from what is clearly the dorsal thalamus but rather--from an enigmatic group of migrated nuclei of the basal diencephalon. The subpallia exhibit much less variation than the pallia. The polypteriforms have a long, thin pallial sheet that can be divided into four zones which recent data suggest could correspond to the ventral, lateral, dorsal, and medial pallia of tetrapods, with some caveats. In sturgeons and gars, the pallium is thicker and divisible into three or four zones. The data available for sturgeons and gars are not sufficient for the formulation of more than a barely working hypothesis, although a review of the existing literature has illuminated some basic unresolved issues in these taxa. The voluminous literature on the pallium of teleosts is briefly summarized. The thickened pallium is divided into four zones which are compared with those of the polypteriforms and those of the tetrapods. The topographical position of the primary olfactory target in the pallium is lateral, which is unexpected in an everted pallium. Several recent hypotheses have sought to explain this organization: partial eversion, caudolateral eversion and displacement, simple eversion with changed olfactory connections, and not quite so simple an eversion with preserved topology. These hypotheses are evaluated and some persisting problems are enumerated.