The 10 Deadliest Marine Reptiles

Kronosaurus queenslandicus dives into the ocean depths.
Stocktrek Images / Getty Images

Today, the most dangerous creatures in the sea are sharks, along with the some whales and fish--but that wasn't the case tens of millions of years ago, when the oceans were dominated by pliosaurs, ichthyosaurs, mosasaurs, and the occasional snake, turtle and crocodile. On the following slides, you'll meet some marine reptiles that could practically swallow a great white shark whole--and other, more petite predators next to which hungry piranhas seem like a cloud of pesky mosquitoes.

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Kronosaurus. Wikimedia Commons

Named after Cronus--the ancient Greek god who tried to eat his own children--Kronosaurus may have been the most fearsome pliosaur that ever lived. True, at 33 feet long and seven tons, it didn't approach the bulk of its close relative Liopleurodon (see next slide), but it was more sleekly built and possibly faster as well. Befitting vertebrates at the top of the early Cretaceous food chain, pliosaurs like Kronosaurus ate pretty much everything that happened across their paths, ranging from meek jellyfish to respectably sized sharks to other marine reptiles.

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Liopleurodon (Wikimedia Commons).

A few years ago, the BBC TV show Walking with Dinosaurs depicted a 75-foot-long, 100-ton Liopleurodon lunging out of the sea and swallowing a passing Eustreptospondylus whole. Well, there's no reason to exaggerate: in real life, Liopleurodon measured "only" about 40 feet from head to tail and tipped the scales at 25 tons, max. Not that this mattered to the unfortunate fish and squids this voracious pliosaur vacuumed up, like so many Jujubes and Raisinets, over 150 million years ago, during the late Jurassic period.

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Dakosaurus (Dmitri Bogdanov).

It sounds like something out of a science-fiction movie: a team of paleontologists unearths the skull of a vicious marine reptile high up in the Andes mountains, and are so terrified by the fossil that they nickname it "Godzilla." That's exactly what transpired with Dakosaurus, a one-ton marine crocodile of the early Cretaceous period possessing a dinosaur-like head and a crude set of flippers. Clearly, Dakosaurus wasn't the speediest reptile ever to ply the Mesozoic seas, but it feasted on its fair share of ichthyosaurs and pliosaurs, possibly including some of the other ocean denizens on this list.

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Shonisaurus (Nobu Tamura).

Sometimes, all a marine reptile needs to attain "most wanted" status is its sheer, enormous bulk. With only a few teeth mounted on the front end of its narrow snout, Shonisaurus can't really be described as a killing machine; what made this ichthyosaur ("fish lizard") truly dangerous was its 30-ton weight and almost comically thick trunk. Imagine this late Triassic predator plowing through a school of Saurichthys, swallowing every ninth or tenth fish and leaving the rest splattered in its wake, and you'll have a good idea why we've included it on this list.

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Archelon (Wikimedia Commons).

One doesn't normally use the word "turtle" and "deadly" in the same sentence, but in the case of Archelon, you might want to make an exception. This 12-foot-long, two-ton prehistoric turtle plied the Western Interior Sea (a shallow body of water covering the modern-day American west) at the end of the Cretaceous period, crushing squids and crustaceans in its massive beak. What rendered Archelon especially dangerous was its soft, flexible shell and unusually wide flippers, which might have made it nearly as fast and agile as a contemporary mosasaur.

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Cryptoclidus (Wikimedia Commons).

One of the Mesozoic Era's biggest plesiosaurs--the long-necked, sleek-trunked contemporaries of the more compact and deadly pliosaurs--Cryptoclidus was an especially fearsome apex predator of the shallow seas bordering western Europe. What lends this marine reptile an extra air of menace is its sinister-sounding name, which actually refers to an obscure anatomical feature ("well-hidden collarbone," if you have to know). The fish and crustaceans of the late Jurassic period had another name for it, which translates roughly as "oh, crap--run!"

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Clidastes (Wikimedia Commons).

Mosasaurs--sleek, hydrodynamic predators that terrorized the world's oceans during the late Cretaceous period--represented the pinnacle of marine reptile evolution, virtually driving contemporary pliosaurs and plesiosaurs into extinction. As mosasaurs go, Clidastes was fairly small--only about 10 feet long and 100 pounds--but it compensated for its lack of heft with its agility and numerous sharp teeth. We don't know much about how Clidastes hunted, but if it plied the Western Interior Sea in packs, it would have been hundreds of times more deadly than a school of piranha!

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Plotosaurus (Flickr).

Clidastes (see previous slide) was one of the smallest mosasaurs of the Cretaceous period; Plotosaurus ("floating lizard") was one of the biggest, measuring about 40 feet from head to tail and tipping the scales at five tons. This marine reptile's narrow trunk, flexible tail, razor-sharp teeth and unusually large eyes made it a true killing machine; you need only take one look at it to understand why the mosasaurs had rendered other marine reptiles (including ichthyosaurs, pliosaurs and plesiosaurs) completely extinct by the end of the Cretaceous period.

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Nothosaurus (Berlin Natural History Museum).

Nothosaurus is one of those marine reptiles that gives paleontologists fits; it wasn't quite a pliosaur or plesiosaur, and it was only distantly related to the contemporary ichthyosaurs that plied the seas of the Triassic period. What we do know is that this sleek, web-footed, long-snouted "false lizard" must have been a formidable predator for its 200-pound weight. Judging by its superficial similarity to modern seals, paleontologists speculate that Nothosaurus spent at least part of its time on land, where it was presumably less dangerous to the surrounding wildlife.

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Pachyrachis (Karen Carr).

Pachyrhachis is the odd reptile out on this list: not an ichthyosaur, plesiosaur or pliosaur, not even a turtle or crocodile, but a plain, old-fashioned prehistoric snake. And by "old-fashioned," we mean really old-fashioned: the three-foot-long Pachyrhachis was equipped with two vestigial hind feet near its anus, at the other end of its slender body from its python-like head. Does Pachyrhachis really deserve the appellation "deadly?" Well, if you were an early Cretaceous fish encountering a marine snake for the first time, that might be the word you used, too!

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Strauss, Bob. "The 10 Deadliest Marine Reptiles." ThoughtCo, Sep. 8, 2021, Strauss, Bob. (2021, September 8). The 10 Deadliest Marine Reptiles. Retrieved from Strauss, Bob. "The 10 Deadliest Marine Reptiles." ThoughtCo. (accessed June 4, 2023).