Wikipedia
Tristychius
Tristychius is an extinct genus of shark from the Carboniferous period. Fossils have been found in Scotland.
Tristychius was a small shark, about long. It had a well-developed upturned caudal fin, similar to that of many modern sharks. Physically it may have resembled a modern dogfish. Tristychius also had spikes attached to the bases of its dorsal fins, probably for protection against predators.