Stethacanthus
The Stethacanthus was a part of the shark-like genus Holocephalian, a primitive form of fish and early modern shark ancestor. These creatures lived between the Late Denovian period to the Early Carboniferous epoch, about 390 to 320 million years ago.
Also known as the anvil shark, the Stethacanthus had a dorsal fin that was shaped like a tool. It also had small spikes that protruded through the top. The peaks continued along the top of its head, which was formed from its scales. These relatively small creatures were approximately 2.3 feet long, and the fossils have been found in Asia, Europe, and North America. The Stethacanthus are more closely related to chimaeras, like the ratfish, spookfish, and ghost shark.