The Pliosaur

That gigantic shark-chomping aquatic hell gator from ancient Earth's past is supposed to be a Mosasaur, a smaller, more agile version of his older, tougher, more badass cousins, the Pliosaurs.  

Pliosaurs maliciously lurked the deepest, most miserable depths of Earth's oceans during the Jurassic period roughly 160 to 145 million years ago, give or take a few centuries.  Pliosaur means "MORE LIZARD", which is awesome, and these impossibly-gigantic evil underwater croc-mouthed murder monsters are the largest and most fucked-up aquatic reptiles to ever stalk the scariest depths of our planet's waters.  They combine the quad-flipper propulsion of a big-ass sea turtle with the terrifying spike-filled mouth of a saltwater crocodile, only instead of just wolfing down bullshit creatures like two-ton wildebeest this thing ate Loch Ness Monsters by crushing their bodies with a wrecking ball-strength jaw clamp augmented by serrated rows of razor-sharp curved teeth the size of your foot.

The most well-known species of these gigantic gator-mouthed monstrosities is Liopleurodon ferox, mostly because some British TV show lost its shit and told everyone that Liopleurodon grew to be like 80 feet long and could bench press skyscsrapers. In reality, Liopleurodon was only like 5-7 meters tops, which makes it just a couple feet bigger than a 22-foot gator:

So you're probably safe as long as you can swim faster than like 30 knots or so over a short distance.  Just for a fun Pliosaur factoid, Liopleurodon means "smooth sided teeth," which is a direct reference to his impeccable dental hygiene and also at his skill of plesiosauring all the ladies.

Among the biggest Pliosaurs ever found was the awesomely-nicknamed PREDATOR X, which was a fucking obscene travesty of sanity and reality that measured forty-three feet in length, weight almost 40 tons (the biggest whales on earth today weigh around 3 tons), had a goddamn crocodile jaw filled with 12-inch teeth, and rocked two difference sizes of flippers that gave him an insane short-distance sprinting speed underwater.  The thing's skull was almost seven feet long, with three to four times the cross-sectional space for bite muscles as your typical alligator.  

In case you forgot, alligators have the strongest bite in all of the animal kingdom.

Rocking a bite four times the strength of a T-Rex and with teeth twice the size of that sucker, Predator X was the biggest and most badass apex predator of Jurassic Earth.  Discovered on some Norwegian island over by the North Pole in 2009, Predator X was eventually given the genus-specie Pliosaurus Funkei because it was MORE FUNKY LIZARD and also because the guy who found it was named Bjorn Funke, which sounds like the name of a Swedish 1970s revival band specializing in George Clinton cover jams.  

Pliosaurs rocked big, frightening, horrific jaws lined with teeth the size of Bowie knives, which were designed to grab, crush, and rip, and which were curved backwards so they'd lock in to the face of an unsuspecting Plesiosaur and keep it from trying to punk out and swim away.  It had the similar brain size and shape of a Great White, meaning it was also probably single-mindedly interested in murdering and devouring every living thing in the ocean at all times like a sick demented freak, and it was an excellent tracker because its nasal cavity was set up so it could receive different smells in each nostril, helping it not only to detect the enemy but also to triangulate the position of a wounded bleeding animal by smell alone.  It's big-ass flippers allowed it to accelerate rapidly, possibly even jumping like a whale, and some people speculate it might have been able to move around like a turtle on land as well.  

Oh yeah, did I mention it breathed air?  So it was always up near the surface ready to leap out and lock in to shit?

One problem with Pliosaurs is that it's hard to find any good present-day predators to compare them too, since these things seem to be a weird-o mix of gators, turtles, and T-Rexes, only they're the size of Megalodons, had the bite strength to rip a modern-day automobile in half, weighed the same as a fully-loaded dump truck and primarily ate creatures that don't exist anymore.  If their attack was anything like an alligators, it would swallow most smaller prey whole, use its powerful jaws of steel to brutally crush medium prey into an easily-digestible organic paste of blood-covered misery, and with bigger prey it could bite and then do that awesome badass "death roll" where it spins around in a circle and pulls parts of it's enemy off like a living razor-toothed food processor.

Pliosaurs are typically described as "ambush predators", meaning they liked to lay completely still and come flying in out of nowhere with a short burst of speed like your typical croc or gator.  They could go super fast through the water over short spans, and everything they bit almost immediately died a gruesome horrible death.

Scientists believe they ate Krakens, whales, ichthyosaurs, giant squid, Loch Ness monsters, and probably sharks and fish and other shit.

http://news.bbc.co.uk/2/hi/science/nature/7264856.stm

http://scitechdaily.com/predator-x-officially-named-pliosaurus-funkei/

http://www.bbc.co.uk/science/seamonsters/factfiles/liopleurodon.shtml

http://dinosaurs.wikia.com/wiki/Liopleurodon