crookedindifference:

Stethacanthus

It’s a type of prehistoric shark whose nickname was ‘ironing board shark’. This name was given to it because males had a strong fin on their heads. They probably served as a defence against predators or, due to their lack of presence in females, as a sexual courtship. 

They lived both in saltwater and freshwater; most of their fossils have been found in Europe and North America. They moved quickly under the water and ate fish, although they also were preys of bigger animals such as Dunkleosterus.