Water Dinosaurs

February 01, 2022 8 min read

water dinosaurs

Before we start to dive into this article and find out what were the different species of Water Dinosaurs, let’s find out about the fascinating history of sea reptiles!



If today the biggest resident of the ocean that know of among the 226 000 know species is the blue whale which is 30 meters long, it only has inhabited our oceans for only 9 million years. This can seems like a long time, but in reality, it is quite a short period of time considering the first water dinosaurs appeared around 250 million years ago.

The predators having dominated the deep seas from 250 to 25 million years ago before our era and commonly called water dinosaurs because of their unattractive aspect and their huge size, are actually giant water reptiles. The noticeable difference between these inhabitants of the prehistorical era is in the dinosaur’s rigid posture since their body is supported by their paws in contrary to water reptiles which are extended on both sides of their bodies. These water monsters were the active members of a violent past, during which the security of the inhabitants of the sea was ephemeral and where death is everywhere.


The triassic period


water dinosaurs trias


It’s during the Triassic era, 227 million years ago that, the first water dinosaurs appeared. During this time, the Pangaea included all the land on Earth into one super continent divided in two: Laurasia in the south and Gondwana in the north.

Water dinosaurs are also called water reptiles, those which appeared at this period are nothosaurs and placodontia.

Nothosaur was one of the first reptiles to have adapted to the marine world. This semi-aquatic predator is piscivorous and lived around 240 million years ago. Having a thin body and a long tail it would move waving. Its palm feet, (certain fossils show clawed fingers which is evidence of a life on land beforehand) allowed it to move quickly to catch its victims. This prehistorical sea monster with its long muscular neck and long flat skull had a jaw with cage shaped teeth.


nothosaur water dinosaurs


Placodontia originated from Europe and is the oldest specimen having been found in the Netherlands. As it names suggest it has teeth which are very flat. It fed essentially on shells and crustaceans. It lived for 50 million years on the ocean’s coasts. In Thetys, vast paleo ocean which will become later the Mediterranean Sea.


placodontia water dinosaurs


The ocean’s colonization by eptiles during the Mesozoic era

Oceans can seem to us nowadays, a hostile and terrifying environment, they are inhabited with giant calamars, sharks, whales, and other venomous fishes. But these living beings might seem like puppies next to marine reptiles, which we call water dinosaurs, with their dreadful appearance but these dinosaurs inhabited the same oceans hundreds of million years ago.

At first land inhabitants, paleontologists think that the increase in number of land dinosaurs lead reptiles to colonize oceans to survive.

Let’s recall that during the Mesozoic era, almost all land was merged as one block, a supercontinent called the Pangaea. This supercontinent was surrounded by a paleo ocean called Tethys which had very rich and nutritious waters full of plankton and an abundant and diverse aquatic fauna. The promise of easy food to catch certainly motivated reptiles to explore the Tethys.


The big family of water dinosaurs

We can divide water dinosaurs into 3 big groups:

  • Ichthyosaurs, from the Greek ichthyosauria, which means giant lizard.
  • Plesiosaurs, from the Greek plesios and sauria which means reptile.
  • Mosasaurus, from the Latin mosa and Greek sauria, which means lizard of the Meuse River.




ichtyosaurs water dinosaurs


Ichthyosaurs are giant water dinosaurs; their size could be from 1 to 20 meters long. The first Ichthyosaur fossil ever found was in England in 1811, 10 years before the first discovered dinosaur fossil!

They had the capacity to swim extremely fast, up to 70km/h and in deep seas thanks to a long and thick tail ending with a fin and 4 limbs. Their eyes were globular, their big and long snout had pointy teeth.

Their pace was like dolphins. This specie from the Mesozoic period suddenly disappeared at the beginning of the superior Cretaceous period. Their size varied from a salmon’s size to a whale’s size. Ichthyosaurs are piscivorous and viviparous, so they gave birth to their offspring under water.

In the big family of ichytosaurs, there are cymbospondylus, Macgowania, Temnosontosaurus, utatsusaurus and stenopterygius.




plesiosaurus water dinosaurs


Plesiosaurus lived during the Mesozoic period and disappeared at the end of the Cretaceous period. The first plesiosaurus fossil ever found was by Mary Anning, famous fossil researcher, at the start of the 19th century.

It is of habit to describe them as a « turtle with a snake across its body » because of its immense neck. Its jaw has sharp and pointy teeth, and it has four fins to swim quickly. These marine monsters could weigh up to 5 tons.

Plesiosaurs also were viviparous, which was proven by a discovery in 1987, with a plesiosaurus fossil of a female carrying an embryo. The legend wants that if Loch Ness exists, it’s a descendant from plesiosaurus. It lived in warm and shallow waters, around 160 million years ago.

In the big family of plesiosaurus, we find plesiosaurus, kronosaurus, plesiopleurodons, microcleidus, hyddrorion, and elasmosaurus.




mosasaurus water dinosaurs 


Mosasaurus dominated the oceans until the end of the superior Cretaceous period, the Maastrichtian period. It lived in the North Atlantic Ocean close to the Netherlands. The first fossils were discovered in 1770.

These marine reptiles had the appearance of a crocodile, had oval jaws, and measured between 13 to 18 meters for up to 600kg. They had a long and flexible tail, and their limbs were shaped like a paddle. The evolution made of them real killing machines.

These carnivorous titans of the sea had sharp jaws that allowed them to cut through preys in only one bite. They attacked ichtyosaurus as well as plesiosaurs. The seabed’s floors were not a restful place for their preys as these dinosaurs were true submarine dinosaurs.

In the big Mosasaurus family, we found mosasaurus, tylosaurus, clidastes, halisaurus, platecarpus and tethysaurus.


Are water dinosaurs dangerous?

 If in the general audience’s minds, influenced by movies about killer sharks, white sharks are the most dangerous creatures in our oceans. But it would almost look harmless next to a water dinosaur. Among the numerous water dinosaurs there is, let’s focus on the most dangerous ones.




kronosaurus water dinosaurs


Let’s start with kronosaurus which comes from the Greek mythology, Kronos, son of the land and sky. Kronosaurus lived during the inferior Cretaceous period 120 to 190 million years ago. It belongs to the pliosaurus family, which are short-necked dinosaurs.

Known for its vivacity, it is on top of the food chain; ancestor to the crocodile, its head was about 2 meters long and its body 13 meters long. It fed on nautilus, ammonites, and other plesiosaurus dinosaurs.




liopleurodon water dinosaurs


Liopleurodon comes the Greek meaning « smooth side teeth », it is a pliosaurus. It is also called the white shark from the Jurassic period. This carnivorous dinosaur lived 160 million years ago during the middle and superior Jurassic period. It had 4 powerful fins which made it dreadful.

It had a strong sense of smell which allowed it to spot prey from kilometers away. Cousin of Kronosaurus, it is also on the top of the food chain. Measuring from 5 to 7 meters and could weigh up to 17 tons.

Dreadful predator, it could stay underwater and hold its breath for an hour letting little hope for preys. Certain scientists think that this sea monsters which could swallow its preys in one bite and could have descendants today in the deep seas…scary, right?




dakosaurus water dinosaurs 


Dakosaurus, which means « biter lizard » in Greek belongs to the Metriorhynchidae family. This ancestor of the crocodile is a hunter and a scavenger, it lived during the inferior Cretaceous period, 153 to 143 million years ago.

With a 6 meters length, it had a solid and powerful jaw that didn’t let any chance to its preys. This Water Dinosaur lived only in water and had fins instead of paws…




shonisaurus water dinosaurs 


Shonisaurus, meaning « lizard of the Shoschone Mountains » in Latin, where it was found in 1920 in Nevada (USA), belonging to the giant ichtyosaurus family. This giant sea monster lived where North America currently is, 224 to 208 million years ago. It could measure up 20 meters long and weigh up to 20 tons.

Scientists think that Water dinosaurs were rounder, and heavier but recent studies show they had a thinner silhouette. It had a long and thin snout, only young specimens had teeth. The high number of fossils found, let researchers think that this amateur of soft preys lived in a group.



Cryptoclidus, which means «hidden clavicles » lived 170 million of years ago during the superior Jurassic period. This Water dinosaur belongs to the plesiosaurus family. With a length of 3 to 4 meters, its neck would measure up to 2 meters and its fins gave it a lot of power to move underwater easily and quickly.

Amateur of shallow waters, it lived where Occidental Europe currently is. It needed to come back occasionally at the surface to breathe. Its head was long and had a jaw with numerous teeth that looked like needles which fit together to trap small fish and shrimps.




clidastes water dinosaurs 


We do not know many things about Clidastes, it lived during the superior Cretaceous period, 80 million years ago. It belonged to the Mosasaurus family. This water dinosaur measured 3 meters long for 45 kilograms, bigger specimens could have reached up to 6 meters.

Given of a long body, a small head, and a pointy snout with sharp teeth, it moves with agility in the shallow waters. Researchers found Clidastes fossils in North America, Europe, North Africa, and South East Asia.




plotosaurus water dinosaurs


Plotosaurus’s first fossil discovered was in 1937 in California, Plotosaurus is part of the mosasaurus family. It was 9 to 1 » meters long and could reach 5 tons.

Faster than the most mosasaurus, it had short fins, a large tail and an aerodynamic fusiform body which allowed it to move quickly in the ocean. Its big eyes gave it a piercing vision to spot its prey from afar.




ophtalmosaurus water dinosaurs 


Ophtalmosaurus, called big eyes monster, was part of the ichtysaurus family. It had the particularity to have eyes with an average diameter of 23 centimeters. Bigger specimens were 15 meters long and their eyes diameters could reach 30 centimeters.

This other sea giant lived during the Jurassic period. It looked like a dolphin with a slender body and a long snout. The structure of their eyes, protected by a sclerotic ring allowed them to go in 1000meters deep waters. Researchers estimate they had the capacity to see in 500 meters deep waters.

Ophtalmosaurus had salient teeth which allowed it to attack its living preys in the reefs like ammonites. Always thanks to their huge eyes, they were masters of nocturnal hunting, calamars and other fishes passing close to their snout had to be careful. A female ophtalmosaurus gave birth to 2 to 3 litters that lived under water.


Dinosaurs or Sea reptiles, there is still a lot to explore

They disappeared at the same times as dinosaurs, 65 million years ago and continue to fascinate children like adults. Such deep research regions as the oceans are when they are inhabited, water dinosaurs haven’t given paleontologists all their secrets just yet. Heroes of fantastic films that fed our fantasies about a hidden presence or a report from the crazy experience of scientists, they are both familiar and strangers to us.

From small to big, they intrigue us, impress us, and question us on our place in the universe. Humans have never met them, but do we secretly wish that we did?


The Smallest Water Dinosaur 


smallest water dinosaurs 


The smallest of Water dinosaurs was cartorynchus lenticarpus and measured only 40 cm of length and weighed around 2kg. It lived during the inferior Triassic period, around 248 million years ago. It was discovered in China; its appearance is similar to a seal’s. Its morphology allowed it to be on land as well as in water.


The Biggest Water Dinosaur


biggest water dinosaurs 


Recent discoveries in the UK have put a light on an ichtysaurus of 26 meters long and almost as big as a blue whale. We can imagine that paleontologists that will continue this research will allow us to know these sea giants and reveal new species unknown today.




water dinosaurs logo

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