From Practical Fishkeeping:
Fossil fish brain studied
French and American scientists have described the first images of the world’s oldest fossil brain in a recent issue of the Proceedings of the National Academy of Sciences.
Alan Pradel and coauthors were studying some remarkably well-preserved skulls of Sibyrhynchus denisoni, an iniopterygian fish that lived about 300 million years ago, from Oklahoma and Kansas when they made this discovery.
They possess an unusual morphology characterized by enlarged pectoral fins mounted high on their bodies, with numerous denticles (teeth) on the front edge of the fins and club-like tails.
See also here.
Superfoetative viviparity in a Carboniferous chondrichthyan and reproduction in early gnathostomes: here.