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The world has changed since the extinction of the dinosaurs. Now, the most terrifying and dangerous beasts that control the oceans are for example sharks. Tens of millions of years ago, we can be certain that a shark would not have been able to compete with a marine dinosaur such as a Plesiosaurus, Ichtyosaurus or Mosasaurus.
In this article we will present a list of marine dinosaurs and some of them could practically swallow a whole great white shark as well as other current predators that would appear much smaller next to such monsters. Even a swarm of hungry piranhas would look like a simple cloud of mosquitoes. These marine animals will clearly surprise you.
This marine dinosaur was named after Cronus, the ancient Greek god who tried to eat his own children. Kronosaurus was perhaps the most formidable plesiosaurus that ever lived. Although it was 10 meters long and weighed seven tons, it did not get too close to its close relative the Liopleurodon, which was much larger (see below), but it was certainly thinner and therefore faster too.
Like most vertebrates at the top of the food chain in the Lower Cretaceous period, Plesiosaurus like Kronosaurus ate just about everything in their path, from sweet jellyfish to sharks of respectable size to other marine reptiles.
A few years ago, the BBC program Walking with Dinosaurs (see video below) showed a Liopleurodon, 23 meters long and 100 tons, coming out of the sea and swallowing a whole Eustreptospondylus (a carnivorous dinosaur).
In reality, this was a bit of an exaggeration because other research has shown that the Liopleurodon was "only" 12 meters long from head to tail and tilted the scales at a maximum of 25 tons. This was of no importance for the unfortunate fish and squid that this voracious fish sucked up more than 150 million years ago towards the end of the Jurassic.
If you were told that a team of paleontologists has unearthed the skull of a marine dinosaur from the top of a mountain in the Andes Cordillera. And that they were so terrified of the fossil that they nicknamed it "Godzilla". You would probably think that this sounds like the beginning of a science fiction movie. Well, it's true, because that's exactly how the Dakosaurus - another name for Dakosaurus, an early Cretaceous one-ton marine crocodile with a dinosaur head and sparsely formed fins - was found.
Clearly, when one studies its skeleton, Dakosaurus was not the fastest reptile to roam the Mesozoic seas, but it must have feasted on its share of ichthyosaurus and plesiosaurus, perhaps including some of the other ocean dwellers on this list.
Sometimes all a marine dinosaur needs to make its place on this list is its huge mass. With only a few teeth mounted on the front of its narrow snout, the Shonisaurus cannot really be described as a killing machine.
However, what made this Ichtyosaurus ("fish lizard") really dangerous was its weight of 3 tons and its really thick trunk in relation to the rest of its anatomy. Imagine this Upper Triassic predator crossing a school of Saurichthys, swallowing one fish out of nine or ten and leaving the rest in its wake, and you will have a good idea of why we have included it in this list.
We do not normally use the words "turtle" and "dangerous" in the same sentence, but in the case of Archelon, we make an exception. This prehistoric turtle, less than 4 meters long and weighing less than two tons, crossed the Western Interior Sea (a shallow body of water covering today's American West) at the end of the Cretaceous, crushing squid and crustaceans in its massive beak.
What made the Archelon particularly dangerous was its soft shell and unusually wide fins, which could have made it almost as fast and agile as the Mosasaurs - one of the largest families of carnivorous reptiles that dominated the world during the Cretaceous period.
In the family of the Plesiosaurs that lived during the Mesozoic era, the Cryptoclide was one of the most deadly marine dinosaurs. This predator was particularly formidable in shallow seas, especially those bordering Western Europe. What gives this marine reptile an additional air of threat is its sinister name, which actually refers to an obscure anatomical feature and means "well hidden clavicle".
The fish and crustaceans of the late Jurassic period must have had another name for this marine reptile, which perhaps translated as "Oh, here it comes, let's disperse!".
The Clidaste belongs to the Mosasaur family which is generally described as predators whose anatomy allows them to go very fast in the water. They would have largely dominated the oceans of the planet at the end of the Cretaceous period. They simply represent the summit of the evolution of marine reptiles and it is assumed that they pushed the plesiosaurs to disappear.
A branch of the Mosasaurs, the Clidate was quite small - only about 3 meters long and 45 kilos - but it compensated for its lack of weight with exceptional agility and many sharp teeth. We don't know much yet about how the Clidastes hunted, but in a pack, they would have been hundreds of times more deadly than a shoal of piranhas !
The Plotosaurus, another reptile from the Mosasaurus family and whose name means "floating lizard", was one of the largest, measuring about 12 meters from head to tail and tipping the scales at five tons. Its narrow trunk, flexible tail, razor-sharp teeth and unusually large eyes made it a real killing machine.
One need only look at it to understand why the Mosasaurs had wiped out other marine reptiles (including Ichtyosaurs, Pliosaurs, and Plesiosaurus) before the end of the Cretaceous.
Nothosaurus is one of those marine reptiles that give paleontologists a hard time; it was not exactly a Plesiosaurus, and is by far related to the Ichthyosaurs that roamed the Triassic oceans. What we do know is that this "lizard" with webbed feet and long crow's feet must have been a fearsome predator for its relatively low weight of a hundred kilos.
Judging by its superficial similarity to modern seals, paleontologists believe that the Nothosaurus spent at least part of its time on land, where it was probably less dangerous to the surrounding fauna.
Fossils Teeth of Nothosaurus
Pachyrhachis is the weird reptile on this list: not an Ichtyosaurus, a Plesiosaurus, not even a turtle or a crocodile, but a simple and deformed prehistoric snake. And by "deformed" we mean really deformed: the five-meter long Pachyrhachis had two hind legs near its anus, at the other end of its slender body, from its head which is python-shaped.
Does Pachyrhachis really deserve the name "dangerous"? Well, if you were one of the first Cretaceous fish to encounter a sea snake for the first time, maybe that's the word you would use too!
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