Top 10 Scariest Prehistoric Sea Monsters

The modern ocean is a scary place, filled with barracuda, sharks, super-squids, and possibly Cthulhu. However, no matter what we find in the depths these days, none of them seem to come close to the giant terrors that ruled the seas in Earth’s past; giant sea-lizards, monster sharks and even “hypercarnivorous” whales. For most of these things, humans would barely qualify as a snack. Here are 10 of the scariest prehistoric sea monsters to ever call the ocean home in prehistory.


Megalodon is probably one of the best-known sea monsters in the list; it’s hard to keep the idea of a shark the size of a school bus out of pop culture. Plus, science-minded entertainment sources like the Discovery Channel love creatures that could pass for a movie monster. Despite the popular idea that Megalodon coexisted with dinosaurs, they lived from 25 to 1.5 million years ago, meaning that at best they missed the last dinosaur by 40 million years. On the other hand, this meant they might have still been around for the first humans. Eek.


If Jurassic Park had an aquarium scene, and actually featured more animals from the Jurassic period, liopleurodon probably would have been in it. Although the actual length of these beasts is contested (some scientists have claimed lengths in excess of 50’), most agree that it was around 20 feet in length, with a full fifth of that being pointy-toothed head. When the mouth of the “smaller” estimate is still plenty large to eat you whole, I think that is perfectly huge enough.


Despite the name and appearance, that is not a reptile, but actually a whale (and not even the most fearsome on the list!) Basilosaurs were predatory ancestors of modern whales, and could be 50 to 85 feet long! It is described as being the closest a whale has ever come to being a snake because of how long and sinuous it was. Imagine swimming in the ocean with an 80+ foot long alligator-snake-whale. Now imagine being afraid to even take a bath ever again.

Physical evidence suggests that basilosaurus did not have the cognitive ability of modern whales, nor the ability to echolocate, and could only navigate in 2 dimensions (so no deep diving or breaching). So at least this monster whale was dumber than a bag of prehistoric hammers and could not chase you if you dove or scrambled out on dry land, probably forever.

Sea Scorpion

Nothing about the words “sea scorpion” are comforting to begin with, so this should not come off as too awful: this was one of the two largest arthropods to have ever lived, reaching a length of over 8 feet of armored, clawed horror. Most of us freak out at the thought of inch-long ants and foot wide spiders, so it’s easy to imagine screaming like a little child if you ever stumbled across one of these sea monsters, alive and kicking.


Mauisaurus was named after the Maori god Maui, who pulled the islands of New Zealand up from the sea floor with a fish hook, so already you know this thing is going to be enormous. The neck of these sea monsters measured up to 49 feet long; the longest proportionate (and really, “actual”) neck of any living thing aside from some sauropod dinosaurs. Their overall length was about 66 feet, and that ridiculously long neck had plenty of vertebrae, implying that it was flexible. Imagine a snake strung through a sea turtle with no shell, and you have an approximate idea of what this thing looked like.


Dunkleosteus was a 30 foot long carnivorous tank. It was outlasted by sharks, but  it is small consolation for the variety of creatures this beast ate. Instead of teeth, it had bony ridges, like a turtle. It has been calculated that they had a bite force of 8,000 pounds per square inch, putting it on par with crocodiles and T-Rex in terms of being history’s strongest biters. They also believe, based on the evidence in the skull regarding its musculature, that it could have opened its mouth in one fiftieth of a second, meaning it vacuumed food into its guillotine of a mouth.


Kronosaurus is another short-necked pliosaur (like Liopleurodon up at number 9), and like Liopleurodon, its overall length has been contested. It was a “mere” 30 feet long and the longest teeth in its massive mouth were up to 11 inches long. This is why it was named after Kronos, the king of the old Greek Titans.

The head was up to 9 feet long. They could eat an entire modern man whole, and still have room left over for half of another. It has also been suggested that since their flippers are so similar in design to those of modern sea turtles, that they may have crawled out onto land to lay eggs. You can be sure no one was digging up these thing’s nests to get at the eggs.


These sea monsters grew to be about 15 feet long, and had a lower jaw that was made of a “tooth whorl”. It looks like a cross between a circular saw and a shark, and when you mix apex predators with power tools, the world quakes in fear.

Helicoprion’s teeth were serrated, implying that they were definitely carnivores, but there is some debate as to whether their teeth were in the front of the mouth, as shown in the picture, or if they were farther back, which would suggest a softer diet, like jellyfish. However it was arranged, it clearly worked; Helicoprion survived the Permian Triassic extinction, which means they may have been smart enough to create bomb shelters. Or maybe they just lived in the deep sea.


Imagine a cross between an orca and a sperm whale. Livyatan melvillei, best known as Leviathan, was a whale that ate other whales. It had the largest teeth of any animal to ever use their teeth to eat (elephant tusks are bigger, but they just look impressive and help them smash things; they don’t eat with them) topping out at 1.18 feet. these sea monster lived in the same oceans and ate the same food as the Megalodon, so this whale actually had to compete with the largest predatory shark ever.


If the Liopleurodon was huge, than Mosasaurus was colossal. Fossil evidence suggests that Mosasaurus could reach as much as 50 feet (15m) in length, making it one of the largest marine predators of the Cretaceous period. The Mosasaurus’s head was like that of a crocodile, lined with hundreds of razor sharp teeth which could kill even the most well-armored enemies.

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