Tristychius is an extinct genus of shark from the Carboniferous period. Fossils have been found in Scotland. Tristychius was a small shark, about 60 centimetres (2 ft) 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.