Which Dinosaur Lived In The Water?


The Spinosaurus was one of the biggest dinosaurs ever, but it didn’t live on land like a T-Rex. Instead, it was semiaquatic, which means it lived both on land and in water, like rivers, swamps, and lakes, similar to a Hippo. In addition, its front feet were shorter than its back feet, making it easier to walk and swim in the water.

Spinosaurus was a carnivore and an opportunistic predator who ate almost every animal it could catch, both on land and water. These animals were likely scavengers as well. 

With up to 18 meters (or 59 ft) in length and a bodyweight of 20 tons, they were among the biggest dinosaurs that ever roamed the earth.

Other animals that are often depicted as dinosaurs that lived in the water are the plesiosaurs, Ichthyosaurs, and mosasaurs.

But these animals are classified as marine reptiles, not as dinosaurs, making the Spinosaurus the only known dinosaur that could be described as a water dinosaur.

Why Plesiosaur, Mosasaur, and Ichthyosaurus Weren't Dinosaurs.

Animals like the plesiosaurs, mosasaurs, and ichthyosaurs are often depicted as dinosaurs that lived in water. But those animals weren’t classified as dinosaurs. The plesiosaur is easily confused with dinosaurs because they look like a brontosaurus with flippers, but they are, in fact, a group of marine reptiles with long necks. 

What's the Difference Between Dinosaurs and other Reptiles?

When you look at a plesiosaur, you can certainly tell it isn’t a dinosaur because its legs grow out of the sides of its body, not below the hips. Dinosaurs had legs come out directly under their hips, just like birds, while reptiles have legs coming out from the sides


If people think of a dinosaur who lived in water, the Plesiosaurs come to mind. These animals lived 203 to 66 million years ago during the Mesozoic period. They were large sea creatures that lived in the water near coastlines and between 3 and 15 m long (or between 11 and 49 feet) in oceans all over the world.

They had a vast body and small head, big eye sockets so they could see well underwater; they also had sharp snouts where their teeth were attached and four flippers that were more like webbed feet and allowed them to swim as fast as 20 km/h and to hunt fish, mollusks, small crustaceans, and squid.

The plesiosaur would prey on Ichthyosaurs as well, as a discovered plesiosaur fossil with an ichthyosaur embryo in its gut has shown.

Their two nostrils were on top of the head, one on each side, making it easier to breathe when they were completely underwater.

The word “Plesiosaur” comes from the Greek words “plesious” which means “near to” in English, and “sauros” which means lizard, just like the “saur” in dinosaur. So, Plesiosaur could be translated with “near to dinosaurs” or “nearly a dinosaur,” but it isn’t a dinosaur. So who could blame someone for thinking these animals were dinosaurs?


Another prehistoric animal that lived in the water but wasn’t a dinosaur was the mosasaurs. Its name comes from the greek “Mosa” (meaning the Meuse, a river in France) and “sauros” meaning ‘lizard.’
These animals lived 100,5 – 66 million years ago and were 18 meters (60 feet) huge and probably looked like a mix of a fish and a crocodile, but they weren’t dinosaurs either. Having a look at their legs explains why. Like the plesiosaurus, mosasaurs had legs coming out on the side of their body, not directly below them. Their teeth tell us a lot about their nutrition. These animals probably ate any other animal swimming in the Meuse – and they just swallowed them whole. Can you imagine swimming in a river filled with these gigantic creatures that have 2″ long sharp teeth and are ready to eat you just as soon as they notice you?


The Ichthyosaur is another reptile that lived in the water and gets easily confused for a dinosaur. “Ichthyo” comes from Greek and means fish. Of course, you already know that “sauros” means lizard. It’s not a fish, though. The Ichthyosaur was a marine reptile (look at their legs) that lived 250 – 90 million years ago. Like the plesiosaur and the mosasaurs, it was a deadly predator, preying on other animals like fish and squid. 

Was there ever a dinosaur that lived in water?

No! Paleontologists (people who study prehistoric animals) agree no dinosaur lived only in the water. Some dinosaurs lived near water, though, with the Spinosaurus being the best example. This dinosaur lived near water and would hunt fish. So it’s a “true” dinosaur but didn’t live in the water as the plesiosaurus would.


To make a long story short.

If we think about dinosaurs that lived in the water, we typically think about plesiosaurus, ichthyosaurs, or mosasaurs. Even though these animals lived during the same period, they are not classified as dinosaurs but as marine reptiles. The next best thing to a water dinosaur is the Spinosaurus, semiaquatic, like a hippo.

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