Stethacanthus

Stethacanthus was one of the first sharks to evolve.

The sharks evolved in the Devonian, and have little changed since then. They became predators, but throughout the ages they were preyed upon by bigger fish (Dunkleosteus), marine reptiles (Liopleurodon) and whales (Basilosaurus). Eventually they became top predators and produced giants (Megalodon).

Stethacanthus was unique. Its dorsal fin was like an ironing board, as it was flat at the top with hundreds of tooth-shaped scales. Scientists cannot know for sure what this strange fine was used for, but it could have been for courting and mating, or making the shark look like it had another mouth. This method would have scared off predators.