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  • Ritoban Mukherjee

These Monsters Claimed the Seas When Dinosaurs Ruled

Updated: Jan 13

Calling them dinosaurs would be a bit of a stretch, not to mention scientifically inaccurate, as true dinosaurs only ever lived on land. True dinosaurs also possessed a distinct hip structure that never saw itself replicated in other reptiles of the time. In fact, dinosaurs were only a class of archosaurs (“ruling lizards”) that inhabited the earth during the Mesozoic Era.


There were also other kinds of giant reptiles, many of which lived during the same prehistoric period, that were never dinosaurs to begin with. Fun fact, even the infamous pterodactyl, the predatory winged lizard that rained hell from the skies, wasn’t really a dinosaur.


Still, marine life during the dinosaur age was remarkably different from what it is today, marked by fanged reptiles of colossal proportions that could swallow even a great white shark whole. These reptiles, dinosaurs or no, were apex predators at the top of their own food chain, at least until the day evolution birth something even deadlier.


Generally classified as Marine Reptiles of the Mesozoic Era, these creatures swam the oceans from 248 to 65 million years ago. Today, they are all extinct, for reasons that were never quite clear, and for which you can thank your lucky stars.

The Mesozoic Era was largely divided into the Triassic, Jurassic and Cretaceous periods, each spanning several millions of years and boasting its own distinct forms of land and sea life. The first marine reptiles made their way to the ocean in the Triassic period about 237 million years ago and consisted mainly of the long-necked plesiosaurs, real-life counterparts of the mythic Loch Ness Monster; and ichthyosaurs, also known as fish lizards.


Shortly after the Great Permian Extinction decimated about 96% of all life in the ocean, new forms of aquatic life started making their way to the seas. The ichthyosaurs were the first, and they came in a variety of different species with sizes ranging from 31 inches to 72 feet. Most of them were deep ocean dwellers, carnivorous creatures that fed on fish, squid and smaller marine reptiles that lived near the ocean bottom. About 100 different species of ichthyosaurs have been discovered so far, a few significant ones being the Ophthalmosaurus, the Besanosaurus, and the Ichthyosaurus.


Closely following the ichthyosaurs came the plesiosaurs. The so-called sea serpents of the Mesozoic Era, with their broad bodies, short tails and unusually long necks, the plesiosaurs lived primarily on a diet of fish and small sea animals. They were gigantic, especially when it came to their necks, but although carnivorous, they were never quite the vicious predators as their cousins, the pliosaurs.


A classic example would be the Elasmosaurus platyurus, which, although no less than 40 feet from head to tail with a 23-foot-long neck, only fed on squid and smaller fish that lived deep underwater.


Shortly after the emergence of the plesiosaurs, a related species of marine reptiles with shorter necks and stronger jaws began to surface. Unlike the plesiosaurs, however, the pliosaurs were top predators. They preyed on pretty much anything they could tackle, including larger fish and even other marine reptiles. The most ferocious of pliosaurs was probably the Liopleurodon, an apex predator with four padded limbs and rows of razor-sharp teeth. The massive underwater monstrosity could weigh up to 3,500 pounds and had a jaw length the same as the height of an adult human.

An interesting thing to keep in mind, however, is that while the plesiosaurs swam the oceans about 205 million years ago, its cousins, the pliosaurs, didn’t arrive until several tens of millions of years later, during the Jurassic period. With the start of the Jurassic period, dinosaurs and marine reptiles grew larger and more vicious, faster and more well-adapted to the many dangers of a developing world. The ichthyosaurs, failing to adapt, began their slow decline. The plesiosaurs trudged on, albeit slowly. The pliosaurs, however, thrived. With their shorter necks, padded limbs and massive jaw structure, creatures like the Liopleurodon and the Cryptoclidus now reigned as the new lords of the deep sea.


By the late Jurassic period, the ichthyosaurs had all but died out. With the beginning of the Cretaceous period, they were completely gone. They were now replaced by a fourth group of marine reptiles, ruthless deepwater predators with lengths of up to 56 feet.


With long, snake-like bodies and jaws made for eating animals whole, the mosasaurs rose to become the topmost predators of their time. They’d eat absolutely anything in their path, sharks, whales, large fishes, marine reptiles and even other mosasaurs. Creatures like the Mosasaurus, Tylosaurus, and Hainosaurus became the absolute demons of the deep dark sea, from taking a plunge into the ocean’s bottom to stalking the surfaces for low-flying avian life, everything was fair game to them.

It is uncertain exactly what led to the mass extinction of these underwater behemoths and why there are no more of them left today. It is said that when a giant asteroid hit the earth 66 million years ago, the entire planetary ecosystem made a complete flip in a matter of a few hours. 75% of all living species were simply decimated, following which, there were long-lasting impacts on geology, climate, and the food chain.


Even the deep-sea reptiles, too far underwater to feel the asteroid’s initial impact, eventually succumbed to the planetary changes that followed. While certain species of aquatic animals were fast enough to adapt to this change, the marine reptiles, simply put, were not.

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