Speaking of Jurassic, the pliosaur was the most fearsome beast the waters saw in those days. Described by one news outlet as a "crocodile on steroids," this creature ran up to 59 feet (18 meters) in length. The roughly 155 million-year-old mammoth featured a human-sized jaw and large teeth that may have been able to take down a Dearcmhara in one bite, as well as huge, melon-sized eyes that gave the animal impressive binocular vision. In the early 2000s, fossils of the beast have been located in England and Norway [source: Morelle].
A short-necked marine reptile, the pliosaur resembles a large croc with longer, paddle-shaped limbs. That made it faster than many of its counterparts, traveling at speeds of up to 6 mph (10 kph). The creature fed mostly on fish, mollusks and other marine reptiles, but paleontologists have also found something else in pliosaur stomachs: dinosaur remains. They don't believe that these beasts actually hunted dinosaurs, but instead probably feasted on discarded remains [source: BBC].
Author's Note: 10 Extinct Exotic Sea Creatures
Don't get me wrong, I think it's pretty safe to say that I wouldn't want to encounter any of these creatures during a dip in the water. I have to say, however, that I've already come face to face with a monster every bit as depraved and fearsome: The dreaded hangover at sea. It's tough to enjoy a nice jaunt out to the ocean under a cloudless Caribbean sky when you're also tussling with Montezuma's revenge.
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Entelodonts were frightening creatures that once patrolled throughout Eurasia, North America and Africa. HowStuffWorks looks at the now-extinct beast.