Top 20 Most Incredible Prehistoric Sea Monsters

Eurypterids are a type of extinct aquatic arthropods, informally referred to as “sea scorpions,” and Jaekelopterus was the king of them all. Clocking in at around 7 and a half to 8 and a half feet, Jaekelopterus is currently the largest arthropod that scientists have discovered. Living during the Devonian period, Jaekelopterus was believed to have been an apex predator of its day, and with its impressive size and formidable claws, we can see how it would be a tough opponent to fight off. We’re also incredibly thankful that bugs don’t get this big today!

This sea monster 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 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.

While not technically a “sea” monster, as it primarily inhabited freshwater bodies, Sarcosuchus was nevertheless an aquatic creature. A massive crocodile-like Sarcosuchus inhabited what are now South America and Africa during the Cretaceous period. Sarcosuchus was an opportunistic predator that fed on pretty much anything that came near it. This huge croc could reach more than 30 feet long and often weighed over 4 tons. Given its enormous size, girth, and distinctive nub on its snout, we doubt anyone would mistake this beast for a log floating down a river.

Generally regarded as one of the earliest marine reptiles, Mesosaurus was an aquatic predator native to the early Permian period. In more recent years, we’ve learned more about their habitats, like the fact that they likely existed in a hypersaline water body. Shaped roughly like a crocodile, albeit with a long neck and tail and webbed feet, Mesosaurus is believed to have hunted fish and crustaceans, and may have spent some of its time on land, as its remains have primarily been found in what would have been coastal regions. Mesosaurus also helped provide some strong evidence of continental drift, since its fossils have been discovered in South America and Africa.

A relative of Elasmosaurus, Thalassomedon was another plesiosaur. Clocking in at around the same length as its “cousin,” it was among the biggest of these marine reptiles. At around 35 feet long, 19 of which consisted of its neck, Thalassomedon lived up to its name, which translates to “sea lord.” This plesiosaur “ruled” the sea that divided what is now North America during the late Cretaceous. Like long-necked dinosaurs, Thalassomedon also ate stones to aid in digestion (or possibly ballast). You can see specimens displayed at museums around the United States.

Sharks have been around for eons and while their basic design hasn’t changed, because of how efficient they are as predators, there have been a few strange variations on body type over the epochs. One of the most bizarre appeared on Stethacanthus. This shark, which lived from the late Devonian to the late Carboniferous periods, may have been only a few feet long, but they had a strange, anvil-shaped dorsal fin; referred to as a “spine-brush complex.” There are various theories about the purpose of this odd protrusion, including to attract mates; a way to latch on to larger sea creatures; or else to ward off those same threats.

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 reptile 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 in this creature’s name suggests 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.

During the twilight years of the Cretaceous period a marine reptile 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.

Kronosaurus is a member of the pliosaur clade, a sub-group of plesiosaurs with short necks and tails, but with large heads and mouths full of teeth that make them look like flippered crocodilians. Roaming the seas during the Early Cretaceous period, Kronosaurus grew to around 30 to 36 feet long. Despite its size however, it was believed to have been an active and speedy swimmer; using its four flippers to swiftly turn about in the water to pursue its prey. Although, given that its bite force was around twice that of modern-day saltwater crocodiles, anything that entered those jaws was unlikely to live long!

Discovered in 2006, this marine reptile was nicknamed “Predator X.” The remains unearthed placed Predator X in the family of pliosaurs, extremely large predatory reptiles 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.

Plenty of plesiosaurs swam the prehistoric oceans, but the claim to fame of Mauisaurus is that it had one of the longest necks, in terms of vertebrae number, out of any of them. Although scientists first believed that Mauisaurus was much larger, recent reassessments have put it shorter than initial estimates, with its body’s total length still being a respectable 26 feet. Named for the Māori demigod, Māui, Mauisaurus lived in the seas around present day New Zealand approximately 77 million years ago. What can we say except, it’s awesome?

As we’ve already established, sharks have come in many odd varieties over the years, and easily the weirdest is Helicoprion. When scientists first discovered the strange spiral of teeth, they weren’t quite sure how they fit in with its mouth, due to the lack of a complete skeleton. While a few different positions were postulated, the real position is believed to have been on the lower jaw as a kind of buzz saw of teeth that grew inward as new teeth replaced old ones. Despite this strange feature, Helicoprion was a quite the successful predator, living through the extinction event between the Permian and Triassic Periods.

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.

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.

Many fearsome fish have swum the world’s oceans, but few were as formidable as Dunkleosteus. Living during the Devonian period, Dunkleosteus was a type of armored fish that could reach up to nearly 29 feet long, easily dwarfing many modern day aquatic predators. While they lacked teeth, Dunkleosteus had powerful jaws with bone-like beaks that it used to crush and chomp its prey. It was basically the fish equivalent of a tank, in terms of size and durability, if not attack strength. We certainly wouldn’t want to run into faces like that in a battle…or anywhere else for that matter!

#2: Megalodon

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What If Megalodon Sharks Didn't Go Extinct?

Its name means “big tooth,” and with good reason. The prehistoric sea monster known as the Megalodon was one of the most frightening species of shark to ever roam the oceans. Scientists estimate that it reached lengths of up to 59 ft! We’re just lucky it went extinct about 3.6 million years ago … at least to our knowledge! 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. The Megalodon was the OG of prehistoric shark badasses. Before we get to our top pick, here are a few honorable mentions: Odobenocetops How About Those Tusks? Hallucigenia A Worm With Legs Anomalocaris Is This an Alien Squid?!

A contemporary and competitor of Megalodon during the Miocene Epoch, Livyatan Melvillei is an extinct genus of sperm whale. Named for the Hebrew name of the Biblical Leviathan as well as Herman Melville, who wrote “Moby-Dick”, this whale would’ve been an even more terrifying foe for Captain Ahab. Ranging between 44 and 57 feet long, Livyatan was not only one of the biggest predators to ever live, it also had some of the largest sharp teeth of any animal. This leviathan was so enormous it even ate other whales! Whale song is beautiful, but we have a feeling Livyatan’s songs sounded like death metal.