Orthacanthus buxieri fossil spike. Orthacanthus is an extinct genus of freshwater shark-like cartilaginous fish. The spike that rose up from the back of the head of Orthacanthus seems to have been a defensive feature to stop other predators from clamping their jaws onto its head. Members of the genus also had a long spine growing from the back of their skull and a very long dorsal fin, which ran all along its back, making the shark look more like an eel. This fossil dates to the Lower Permian period, 290 million years ago. Found in Buxieres-les-Mines, Allier, France.