Until now it was thought havøglene had fish-like, but with long narrow body without caudal fin, and they made their way into the water by meander forward, like an eel. Now scientists might reconsider sight.
It was an international research team, led by Swedish researcher at Lund University, Johan Lindgren, who discovered a clear imprint of a fish-like tail fin of the prehistoric Mosasaurs Prognathodon.

Source: Here