All About Prehistoric Marine Reptiles: The Monsters of the Sea


The dinosaurs took rule of the land, these reptiles swarmed the seas. For millions and millions (and a couple more million) of years, all kinds of reptiles inhabited the whole Earth. Many of those who dwelled on land were dinosaurs, but no dinosaur swam the deep waters. The ocean had its own reptile repertoire––and most of them were apex predators. Imagine an ocean filled with great whites and killer whales, that’s how the waters were before.


Some of them looked like your everyday dolphins, swimming incredibly fast underwater, while some had long necks, flippers, and were as big as a bus. Either way, there is so much to learn about these aquatic creatures that will surely raise an eyebrow or two. Here’s everything you need to know about prehistoric marine reptiles!


Description and Appearance


If you didn’t know, marine reptiles also breathed air but just chose to live underwater. Their ancestors were reptiles found on land from the Permian and Triassic periods. Some marine reptiles were closely related to the reptilian evolutionary tree while some were very close relatives of lizards and snakes (I find this even scarier). For example, today’s crocodiles and turtles hold their breaths underwater––just like our prehistoric friends.


Long before Sir Richard Owen coined the term “dinosaur,” marine reptiles were already found in Europe. By the time it was coined, there finally was an umbrella term for the likes of the mosasaurus, ichthyosaurs, nothosaurs, placodonts, marine crocodiles, marine turtles, and especially the plesiosaurs. In many different ways, the discovery and the study of these prehistoric marine reptiles paved the way for vertebrate paleontology or, rather, the study of extinct animals with backbones.


Placodonts were, at most, medium-sized reptiles that loved feeding on bottom-dwellers. If walruses had armor, they would resemble the placodonts very well. Nothosaurs were seal-like predators that moved swiftly underwater and boasted long necks and tails to aid their powerful swimming strokes.


Plesiosaurs were probably the most renowned prehistoric marine reptile of them all. Their necks varied in size but they were violent predators of the ocean. The long-necked plesiosaurs operated quickly to catch fish with the help of their long necks, of course. On the other hand, short-necked plesiosaurs were beasts underwater––scaring fish with their gaping jaws and were considered as one of the top predators underwater. If you’re looking for anything that resembled the Loch Ness Monster, well, today’s your lucky day, my friend.


Gage Beasley’s Plesiosaurus Dinosaur Stuffed Plush Toy is the perfect reincarnation of our marine reptile friend. It does have its mouth wide open, possibly ready to devour some of your flesh, but it’s the cuddliest of them all. It might’ve been one of the top predators of the ocean, but it’s the softest friend elsewhere.


Gage Beasley’s Plesiosaurus Dinosaur Stuffed Plush Toy


Come to think of it, the diet of land-dwelling dinosaurs was what you expected of them. It was either each other or leaves from the treetop. As for prehistoric marine reptiles, it varies a little bit more between themselves. Their diets included fish, cephalopods, hard-shelled invertebrates, and their fellow vertebrates. Unsurprisingly, there have been no accounts of prehistoric marine reptiles that are herbivores.


Some marine reptiles would look thoroughly for fish, cephalopods, and other reptiles. Some, however, were satisfied with benthic (the bottom of the body of water) prey like hard-shelled invertebrates––maybe even some soft-bodied invertebrates along the way. The latter was only speculated as prehistoric marine reptile food because they were rarely preserved and left inside and as fossils.



Just like the reproduction of land dinosaurs, prehistoric marine reptile reproduction is a partly-known topic. However, thanks to fossil evidence, it has been known that these sea creatures gave live birth. For example, sauropterygians and the mosasaurus were evidence of this breakthrough. Although, this might not be the case for every prehistoric marine reptile. Information to work through this subject was limited and it couldn’t have been prevalent between different reptile species.


That wasn’t the only breakthrough the mosasaurus spearheaded, it also broke into Gage Beasley’s dinosaur collection as the Mosasaurus Soft Stuffed Plush Toy! It isn’t the prettiest sight compared to unicorns and butterflies, but it’s fiercer than anyone and anything you can imagine. It looks like a furious crocodile-manatee combination that’s prepared to lash out on the monsters under the bed.


Gage Beasley’s Mosasaurus Soft Stuffed Plush Toy


Live birth is the best bet as it has met certain scientific conditions and evidence. It was most probably a useful feature for these reptiles, but not at all necessary to happen underwater. Some marine reptiles lay their eggs on land. In fact, there is little to no evidence to suggest that most bottom-dwelling reptiles gave live birth except solely for the mosasaurus. In contrast, pelagic marine reptiles are more prone to exhibit live birthing. Sea turtles, however, inhabit pelagic environments and use the setting to lay their eggs––so it might not have been as necessary as we thought.


Facts About Prehistoric Marine Reptiles

  1. The Liopleurodon was the biggest prehistoric marine reptile––managing to go over 50 feet. However, it could only meet half the size of the humungous blue whale.
  2. The Deinosuchus was a massive prehistoric crocodile. Its bite could’ve easily beaten that of a Tyrannosaurus rex and weighed eight times as much as the crocodiles of the modern day. Suddenly, I no longer fear crocodiles.
  3. The only dinosaur known to have spent time living in the water was the Spinosaurus. It could have definitely dominated the waters with its webbed feet while looking for prey.
  4. The Ceratosaurus could have also been a possible dinosaur known to have lived in the water as it also boasted the ability and features to catch aquatic prey like various fish and even crocodiles.
  5. The Nothosaurus is the first large ocean reptile. Its name literally meant “false reptile.” Today, it’s most commonly attributed to the seal.
  6. Among the Plesiosaurus species, the Pilosaurus was the biggest and had over 30 centimeter-long teeth with a bite four times stronger than the T-rex. Wow, is everybody stronger than the T-rex underwater?
  7. The Elasmosaurus is another Plesiosaurus species that were mostly known for having necks that were too long. They didn’t have the power to lift them above the water not unless the waters were shallow enough––which seems to be impossible for these huge creatures.
  8. The Elasmosaurus, because of its huge and unproportionate body, was one of the slowest swimming marine reptiles that were only capable of following small schools of fish for food.
  9. They’re referred to as marine reptiles and not “dinosaurs” because of where their limbs were. Normally, dinosaurs would have limbs sticking out directly under their bodies, but these reptiles had limbs sticking out from the side.



Though they aren’t called “dinosaurs,” scientifically, these prehistoric marine reptiles were on a world of their own. The water is where they excelled––they didn’t need to prove their might on land. They varied in size and physical attributes, but they were all something to be feared underwater. Some had long necks, some had stronger jaws, and some dwelled at the bottom of the ocean. It’s only fair to say that, if there were enough evidence of how they lived, prehistoric marine reptiles might have had a more exciting history compared to our lovely dinosaurs.


That’s not to belittle the Tyrannosaurus rex or the Velociraptor in any way, but the Plesiosaurus and the Mosasaurus were both deadly and fascinating in their own special ways. These marine reptiles’ diets are interesting, too. From your common fish to their fellow reptiles and cephalopods on the deep part of the ocean, their diets didn’t involve plants underwater. Unlike dinosaurs who were big on herbivores and omnivores, our prehistoric friends were predators. They moved graciously underwater, assumingly so, but they weren’t the best of friends you would’ve asked for. Either way, these are your prehistoric marine reptiles in a nutshell––or should I say fossil?




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