Members of the genus had a long spine growing from the back of their skull and a very long dorsal fin, which ran all along its back giving it an eel-like appearance. About 260 million years ago, Orthacanthus was the terror of freshwater swamps and bayous in Europe and North America. Its body reached nearly 10 feet in length and its powerful jaws were lined with double-fanged teeth. They first appeared almost 400 million years ago in the Devonian, and became extinct just before the Mesozoic, about 225 million years ago.