Sapient Xenacanthus

Xenacanthus is a genus of freshwater shark from the Devonian to Triassic periods worldwide. It was about a meter (three feet) in length and had a spine on the back of its head. Its long, ribbon-like dorsal-fin made it look very similar to an eel, giving it and its close relatives, such as Orthacanthus, the nickname of "eel sharks"