You will feel a lot more comfortable about going out for a swim or an afternoon boat ride knowing that such behemoths are extinct today!The Dunkleosteus is school bus-sized armored fish with razor sharp bladed jaws for cutting through anything in its path. The mighty Liopleurodon is a true monster, largest and most powerful of the Pliosaurs. The deadly Mosasaurs where the real sea serpents of their day, and some where large enough to dispatch an adult tyrannosaur easily.