The Great Beasts That Roamed The Ancient Waters

"Discovering the majestic creatures that dominated the ancient waters || This is an adventure and discovery for all lovers of prehistoric reptiles."

Dinosaurs will always be a great fascination to most of us who want to learn about them as there are no creatures to date who were as majestic and awesome as they were. They ruled the earth during the Mesozoic era, which spans 165 million years and has been extinct for almost 65 years. 

We usually see the dinosaurs as land-dwellers, but some adapted to the aquatic lifestyle. We called them “water dinosaurs,” but this is not correct. According to paleontologists, the correct term is “swimming reptiles” Yes, not swimming dinosaurs but swimming reptiles; these were not dinosaurs.  

Let’s take a look at these ancient beasts that roamed the prehistoric waters.

The Great Beast Of The Past That Ruled The Underwater World

The Top Swimming Beasts That Roamed Ancient Waters  

Here’s the thing, though, we thought these ‘water dinosaurs’ stayed underwater 24/7 … not true. Although the bodies of these creatures were well built to swim, they were under the water temporarily, mostly to find food and come back ashore. All dinosaurs lived on land.

What makes a dinosaur is that they stand on their legs directly under their body because of a hole in the hip socket. Because they can stand upright, this allows them to run faster and for longer. We think other creatures are dinosaurs but reptiles like the crocodile. They are not dinosaurs because they have legs that spread out to the side, and they walk and run sideways.

Even though the water dinosaurs were not technically dinosaurs, they were still savage beasts not to be compared with today’s water predators. If we had to compare the ferociousness level of today’s underwater predators and the water creatures of the Mesozoic era, the shark would be the prey in those days. These swimming reptiles ruled the waters of ancient times.

There is a long list of sea creatures that existed long ago, but here are the names of the common ones:


 This sea creature is a genus of eight species that belong to the Pliosaurus family, whose bodily features are short necks, big heads, and broad flippers. The Pliosaurus roamed the waters during the late Jurassic period, about 150 to 145 million years ago. It could reach 40 feet (12 meters) and weigh 30 tons. He was a vicious predator for all sea creatures as his bite was estimated to be four times harsher than that of the intimidating T-Rex. The first discovery of its fossil was in Norway in the mid-19th century.

The Great Beast Of The Past That Ruled The Underwater World


The Kronosaurus is from the Pliosaurus family, and apart from a big head and short neck, it had a big jaw and a thick squarish trunk. They lived everywhere in the world, with many of their fossils discovered from America to Australia.

The Great Beast Of The Past That Ruled The Underwater World


Having lived 230 million years ago, the Nothosaurus is also part of the Pliosaurus family, according to fossil analysis done on its remains. He was not very large, as he measured 15 feet (4.5 meters) in length, but don’t underestimate this fact as even though he was smaller than other sea creatures, he was still very dangerous. He hunted mainly for fish and squids thanks to his needle-shaped teeth. Some experts claim that the Nothosaurus would snap its long head sideways to catch fish just like a crocodile does today. 

They are not related to the crocodile but have features closer to the sea lion because of the four legs for walking, similar to the sea lion. They went underwater only to find food but would mostly stay on land not. The name Nothosaurus means “false lizard” because, to this day, scientists haven’t figured out whether they laid eggs or gave birth to their young.

The Great Beast Of The Past That Ruled The Underwater World


Another member of the Pliosaurus family, the Styxosaurus, existed 80 to 75 million years ago, during the Cretaceous period. It could easily be seen as a sea snake because of its long neck. The Styxosaurus was 35 feet (10.6 meters) in length, but 16 feet was just for his neck, which is almost half the size of their bodies which wasn’t too large and weighed four tons overall. 

Razor-sharp coned-like teeth filled their mouths to catch fish. They did not have time to chew their prey thanks to gastroliths which are little stones on the lining of their stomach that helped break down the food for easy digestion.

Some scientists claim that because the Styxosaurus had easy digestion because of these 200 stones in their belly, they could dive deeper into the waters, searching for rare delicacies that you could not find close to shore.

The Great Beast Of The Past That Ruled The Underwater World


Coming from the Pliosaurus family, the Albertonectes had a small neck on a very long neck and limbs that looked like flippers to make it easy for them to navigate through water. They lived 76 to 70 million years ago, mostly around North America’s water. It reached 38 feet (11.5 meters) in total length, with 23 feet occupying the length of its neck, which consisted of 76 bones. 

No animal that has walked the earth has ever had so many vertebrae in the neck, not even the giraffe. The only possible explanation that scientists could come up with for such a long neck was to collect shellfish on the shore or capture their food, fish, and squids. Its stomach lining also had gastroliths that were five and a half-inch in diameter (14cm).

The Great Beast Of The Past That Ruled The Underwater World


Also, from the Pliosaurus family, the name Thalassomedon means “sea lord” in greek. Its total length from snout to tail reached 40 feet (12 meters), the height of a four-story building. Like the Albertonectes, they had flippers for limbs, and these could reach 7 feet (2 meters) in length; I can imagine how this would help him swim for long distances, and one slap from his limb is probably capable of knocking you out!

He had 62 vertebrae in his neck, which may equal 20 feet. Because the Thalassomedon is from the Pliosaurus family, it also had a small head, and it measured 19 inches (73 centimeters). His mouth was filled with long and sharp teeth, measuring 2 inches (5 centimeters) in length. 

The Great Beast Of The Past That Ruled The Underwater World


The Tylosaurus is from the Mosasaur family and lined most of its life in the shallow water of the North American continent 80 to 85 million years ago. It could grow up to 45 feet (13 meters) in length. He was narrow in body shape and possessed a big head that he used to ram into his prey. He had supple flippers for limbs and had a maneuvering fin at the tip of his long tail. Apart from eating turtles, fish, and small sharks, it also preyed on the mosasaurs like plesiosaurus and birds that could not fly.

The Great Beast Of The Past That Ruled The Underwater World


This ancient sea creature existed during the late Triassic period, 215 million years ago. The first discovery of Shonisaurus remains in 1920 in Nevada close to the Shoshone mountains, thus the name. He measured 50 feet (15 meters) and weighed about 30 tons; in appearance, one would say it looked like a chubby dolphin. An interesting fact about the Shonisarus is that it did not have teeth, nope, none at all. When born, he starts with a full mouth of teeth, but their teeth would fall out when they grow. 

The Great Beast Of The Past That Ruled The Underwater World


The Mosasaurus is a common name among the great predators that ruled the depths of the ancient waters, measuring up to 50 feet (15 meters) in length, making it the biggest aquatic carnivore back then. Inside his crocodile-like head were about 100 sharp teeth that occupied two rows on both the lower and bottom jaw. These teeth were a great help as he would not be able to grasp his prey without them. He also had pterygoid teeth attached to the bone of the roof of his mouth, which made the daily hunting easy.

The Great Beast Of The Past That Ruled The Underwater World


This sea reptile lived 210 million years ago in the latter part of the Triassic period; it is dubbed the biggest aquatic reptile on earth. They weighed more than 75 tons, which is almost as heavy as a blue whale; he reached lengths of up to 69 feet (21 meters), the height of a seven-story building. The Shastasaurus is quite slim overall, but his ribcage measures six feet (2 meters) across. You would think that because of his size, he was a carnivore; the Shastasaurus lived on small fish, cephalopods like squids, and octopuses.

The Great Beast Of The Past That Ruled The Underwater World

What ancient reptiles still exist today?

Thanks to fossils, we have learned more about the ancient beasts that roamed the earth, the sky, and the waters. Because of many remains, scientists got a glimpse of how these creatures lived and have gotten information on how some of these swimming reptiles survived extinction periods. Here are a few names of ancient reptiles that still exist in our modern waters.


Fossils of this prehistoric fish made a lot of think that it was also an extinct species. But, to our great surprise, a citing of this fish was made in 1983. According to fossil records, they started existing about 360 million years ago. It is a member of the Osteichthytes taxonomic group, bony fish; they can live for more than 60 years and grow up to six feet in length. 

They are very different from other fish as they have a hinged joint in their skull, which helps them open their mouth wider, and possess an electrosensory organ called the rostral organ, which helps them detect prey. This mechanism does not exist in modern vertebrates anymore. 

The Great Beast Of The Past That Ruled The Underwater World


The Lamprey has been around for 360 million years and is mostly found in the Atlantic Ocean and some great lakes. They are similar to eels and don’t have any bone structure but a cartilaginous skeleton with a one-tail fin. They feed themselves by sucking the nutrients from a fish’s bloodstream; he doesn’t have a jaw but a lot of tiny and sharp horned shaped teeth that help to catch and latch on to its prey; his razor-sharp tongue will then proceed to scrape away the scales of the fish until it reaches the flesh. 

A thousand years back, they were considered a delicacy only fit for kings, and today, Lamprey’s are key to scientific research on the evolution of back-boned animals.

The Great Beast Of The Past That Ruled The Underwater World

Horseshoe crab

Looking at a horseshoe crab, you might think they are venomous or can sting; quite the opposite actually, they are harmless but fierce survivors. This primitive crab has been on earth for 450 million years, so they were there when dinosaurs existed and have survived five extinctions. You can find them around the eastern US coastline.

They contribute a lot to the biomedical industry as their blue blood help to find bacterial toxins. The absence of bacterial toxins makes sure that vaccinations, surgery, and injection are safe for us. So, thanks are to the Horseshoe crab! 

The Great Beast Of The Past That Ruled The Underwater World


The Nautilus was a prehistoric mollusk that developed when the continents were still taking form. They are not one or two, nor three but five million years old! In the beginning, there were more than 10,000 different species of Nautilus; today, that number has significantly gone down.

The Nautiluses’ diet consists of crabs, lobsters, and fish that they track with 90 chemosensors (retractable tentacles that find food scents). 

They have an internal tube called the hyponomoe, which helps control the amount of water and air inside their shell. Their mastery of buoyancy was the inspiration behind submarines.

The Great Beast Of The Past That Ruled The Underwater World

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Elisha Hosea


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