Orthacanthus is a genus of prehistoric freshwater shark that lived during the Permian Age. It was also a member of a group of ancient sharks called xenacanths, all of which were freshwater sharks with long fins down their backs and a spine at the start of the fin. It was the largest of the known xenacanths at 3 meters (10 feet) and 45 kg (100 lbs).
About 260 million years ago, Orthacanthus was the apex predator of freshwater swamps and bayous in Europe and North America. It owned a peculiar set of double fanged teeth. They first appeared about 400 million years ago in the Devonian, and became extinct just before the Mesozoic period, about 299 million years ago.