Top 10 Incredible Prehistoric Sea Monsters

You wouldn’t want to meet these beasts in a dark alley, never mind the bottom of the ocean. Welcome to WatchMojo.com, and today we’re counting down our picks for the Top 10 Terrifying Prehistoric Sea Monsters.

For this list, we’ll be ranking the most fearsome and frightening creatures that once roamed the depths of the prehistoric seas. “Jaws” may have made you afraid to get back in the water, but these giant extinct beasts could give you nightmares for weeks.

The first sea monster on our list hails from the family of plesiosaurs, marine reptiles that were known for their wide bodies and long, extended necks. The Elasmosaurus was an extremely large variation of a plesiosaur that lived primarily during the late Cretaceous period of evolution. So long was the neck on this variation, that an Elasmosaurus was rarely able to raise its head above water for any extended period of time. Instead, this slow swimming sea monster preferred to stalk its prey from below, often springing upon schools of fish while concealed beneath the deep and dark ocean waves.

If you’re looking for a clue as to what the temperament of the Dakosaurus was like when this extinct carnivore roamed the Jurassic and Cretaceous oceans, then look just look at what its name means in English: “biter lizard.” The Dakosaurus possessed rows of sharp and serrated teeth that made good upon this title, giving this beast a jaw structure similar to that of a killer whale. As a result, many marine biologists believe the Dakosaurus to be one of the apex predators of its day, ripping and tearing chunks out of just about anything which was unfortunate enough to be caught within its jaws.

Look out mako sharks and back off hammerheads, for this prehistoric shark was one of the biggest and baddest beasts during the Cretaceous period. Fossils of Cretoxyrinha were first discovered during the mid-1800s, and provide images of a fearsome and frightening species of a shark that earned its nickname of “Ginsu Shark,” thanks to its mouth full of razor-sharp teeth. Cretoxyrinha was one of, if not the largest predatory shark of its time, with very few natural predators to its name. Indeed, giant marine reptiles and other sea creatures of all kinds fell victim to this efficient killing machine, essentially placing Cretoxyrinha at the very top of the food chain during its time on earth.

Have you ever wondered which species of marine lizard was the biggest of the big? Well, then look no further than the Shastasaurus, which exists today as the largest discovered marine reptile on record. The Shastasaurus hails from the family of reptiles known as ichthyosaurs, and were toothless beasts who rarely delved into extremely deep water, preferring instead to feed closer to the surface. One can only imagine what it was like to behold the sight of this creature in the flesh, as the Shastasaurus ranged from 25 to 69 feet long, depending on the species. In other words: watch out!

Although the suffix to this next creature implies that the Basilosaurus was a reptile, further research has actually proven it to be a prehistoric species of whale. Nevertheless, the name stuck, and probably for the best, as Basilosaurus possesses a frightening look far different than many of its modern cousins. Maybe it’s the creature’s long snout full of sharp teeth, or the slithering, eel-esque quality to its physique. Or maybe it’s the fact that Basilosaurus was fond of chomping down upon the skulls of its prey with amazingly impressive force, crushing bone and chewing up flesh with the greatest of ease. Whichever reason you choose, Basilosaurus wins almost every time.

It was during the twilight years of the Cretaceous period when a marine lizard species known as the Mosasaur took hold as one of the most common hunters of the era. Tylosaurus serves as one of the most frightening examples of these mosasaurs, thanks to its incredible size and fiercely aggressive nature. The creature’s long, rounded snout was often used as a battering ram of sorts during battle, with the Tylosaurus serving as the sort of equal opportunity predator which fed upon birds, fish and sharks alike. They typically hunted in shallow waters, and weren’t beyond leaping above the waves to catch a low flying or flightless bird or two, with the Tylosaurus’ toothy grin likely serving as that prey’s last, frightful sight before death.

2006 signified the fossilized discovery of a new species of marine lizard, one that was nicknamed “Predator X.” The remains placed Predator X in the family of pliosaurs, extremely large predatory lizards that thrived in European and South American waters during the late Jurassic period. Given the taxonomic name Pliosaurus funkei in 2012, it was one of the largest pliosaurs on record, with artist renderings painting a frightening portrait of a large-headed, razor-toothed monster that likely prowled the icy waters off the coasts of Scandinavia. Although the remains of Predator X are partial, and don’t create a complete portrait of what this monster looked like, the images of this muscular sea killer remain a scary sight indeed.

We’ve already mentioned the Tylosaurus as one of the scariest examples from the prehistoric Mosasaur family. But what about its even bigger and badder cousin, the Mosasaurus? The beast’s appearance almost looks like something out of “Jurassic Park,” with its large eyes, pointed snout and huge teeth giving it a nightmarish and monstrous appearance. The Mosasaurus didn’t possess incredible eyesight, however, and didn’t often delve too deep when looking for food. Instead, these creatures took to shallow waters for their prey, attacking smaller fish, turtles and mollusks with ruthless and relentless precision.

The next sea monster on our list may possess a name that translates to “smooth-sided teeth,” but there’s nothing easy or relaxed about the Liopleurodon. Indeed, there was little that could stand against this apex predator that prowled European waters during the Jurassic period thanks to Liopleurodon’s no-nonsense capacity for killing. The Liopleurodon was a strong, propulsive swimmer, and likely pounced upon its prey with an ambush-type approach, probably striking before any form of prey knew what hit them. One specific species of Liopleurodon, known as L. Ferox, has even been measured at over twenty feet, making this sea monster the stuff of which all bad dreams are made.

Before we haunt those dreams with our number one sea creature, here are a few honorable mentions!
- Dunkleosteus
- Xiphactinus

Its name means “big tooth,” and with good reason, as the prehistoric sea monster known as the Megalodon was one of the most iconic and frightening species of shark to ever roam the earth. Likely a historical counterpoint for the “Sharknado” movie series, this beast was feared for its near mythological level of kill potential. So strong and impactful was Megalodon upon its local aquatic communities, that entire ecosystems could be affected by how this apex predator chose to hunt its food. Indeed, both the Great White and Mako sharks owe this primordial beast a debt for their own fearsome pedigree, with the Megalodon serving as the OG of prehistoric shark badasses.