Tristychius is an extinct genus of shark from the Early Carboniferous era. Superficially, it looked like a modern dogfish. It was a hybodont shark. It's fossils have been found in Scotland, and measured approximately 2 feet(60 cm) long.
It was a small shark with a well-developed caudal fin that was aimed upwards, similar to that of many modern sharks. Tristychius also had spikes attached to the bases of its dorsal fins, probably for protection against predators. The pectoral and pelvic fins had much narrower bases, and were therefore more flexible swimming appendages that the broad-based, rigid fins of earlier sharks, such as Cladoselache. The upper obe of teh tail was developed into the powerful, propulsive, upturned fin seen in modern-day pelagic sharks. It was related to the larger Hybodus.