15 Prehistoric Marine Reptiles and Water-Dwelling Dinosaurs

Leave a comment / / Updated on: 21st January 2024

Ancient waters were not friendly..

Between 256 and 66 million years ago, prehistoric marine reptiles were the most dominant animal group on the planet.

Although they evolved slightly earlier, the Mesozoic Era is often regarded as the age of the reptiles because they increased in diversity and complexity during this period—and they were everywhere! 

The famous dinosaurs that were the dominant terrestrial overlords were reptiles.

Some species also evolved to fly.

These winged reptiles, known as pterosaurs, were the first animals to evolve powered flight.

The aquatic ecosystem also had reptiles that ruled as apex predators. These marine reptiles weren’t like fishes at all.

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Gage Beasley’s Prehistoric Shirt Collection
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Gage Beasley’s Prehistoric Plush Collection

They’re more similar to modern whales and other marine mammals because they still had to surface to breathe air. 

The prehistoric marine reptiles came in different shapes and forms, from semi-aquatic dinosaurs that could also live on land to crocodile-like creatures and long-necked pliosaurs and plesiosaurs.

In this post, we’ll explore some of the most remarkable marine reptiles and water-dwelling dinosaurs ever discovered. 

How Did Marine Reptiles Qualify for This List

The prehistoric marine reptiles came in different shapes and forms, from semi-aquatic dinosaurs that could also live on land to crocodile-like creatures and long-necked pliosaurs and plesiosaurs.

Our list includes individuals from these four categories: 

Semi-aquatic dinosaurs

These include dinosaurs that spent a significant amount of their time in the water.

While these were not entirely water-dwelling animals, there are records of these semi-aquatic reptiles hunting prey in these marine ecosystems.

Dinosaur 3d rendering, Tyrannosaurus Rex swimming
Tyrannosaurus wasn’t afraid of the water, but did it make our list? / FOTOKITA via Istock


These are marine reptiles in the suborder Plesiosauroidea.

They’re known for their long, snake-like neck, small head, broad body, and short tail. 


These are marine reptiles that belong to the Pliosauroidea suborder.

They’re similar to plesiosaurs but had shorter necks and large crocodile-like heads with massive toothed jaws.


These are extinct members of the suborder Crocodylomorpha.

They’re crocodile-like reptiles that lived an exclusively marine or semi-aquatic lifestyle. 


In addition to these main groups, our list also includes more reptilian groups like the nothosaurs, ichthyosaurs, and mosasaurs that lived in Earth’s oceans at various times during the Mesozoic Era. 

The 15 Prehistoric Water Dwellers


Liopleurodon 3D Render
Liopleurodon 3D Render / CoreyFord via Istock
Name MeaningSmooth-sided tooth
EraMesozoic – Early Jurassic
ClassificationSauropterygia, Plesiosauria & Pliosauroidea
Length7–8 meters (23–26.2 feet)
WeightUp to 10 meters (33 feet)
LocationEurope (England and France)

Liopleurodon was a 10-meter-long (33 feet) marine reptile that ruled the prehistoric seas of Europe during the Middle to Late Jurassic Period.

It was a pliosaur, a family of short-necked marine reptiles that were the apex predators of the aquatic ecosystem during the Mesozoic. 

Liopleurodon had an elongated head, a relatively short neck, and four long flippers attached to a thick torso.

On average, adults may have weighed up to 1.7 tons (3,700 pounds).

It was an apex predator that preyed on fish, squids, and other marine animals. 

The Liopleurodon is renowned as a fast-swimming predator.

The reptile’s broad, flat flippers helped with propulsion through the water.

Scientists also think it had a well-developed sense of smell because it had forward-facing nostrils.

This is an adaptation seen in apex predators to help them detect prey easily.

It also had massive, smooth-sided teeth, which measured up to three inches in length.

The reptile’s name, which translates as “’smooth-sided teeth,” is a reference to its massive teeth. 


A profile view of Deinosuchus riograndensis
Deinosuchus riograndensis / Sphenaphinae – License
Name MeaningTerrible crocodile
EraMesozoic – Late Cretaceous
ClassificationCrocodylomorpha, Crocodylia & Alligatoroidea
LengthUp to 10.6 meters (35 feet)
Weight4 to 10 metric tons (4.4 to 11 short tons)
LocationUnited States

A relative of modern crocodiles and alligators, Deinosuchus was a large prehistoric crocodilian that lived between 82 and 73 million years ago.

Aptly named “terrible crocodile,” the Deinosuchus was one of the largest predators in North America when it was alive.

It measured up to 10.6 meters (35 feet) in total length and had massive jaws capable of crushing large dinosaurs.

Scientists have found fossils of dinosaurs like Albertosaurus and Appalachiosaurus with teeth marks from this crocodilian.

When it wasn’t munching dinosaurs for dinner, the terrible crocodile fed on turtles, fish, and other small aquatic and terrestrial prey. 

Deinosuchus was an apex predator that lived on the edges of the Western Interior Seaway, a shallow sea that covered most of North America during the Cretaceous Period.

It was capable of living in the water and on land but probably spent more time in the brackish water, only coming on land to hunt prey. 


3D rendering of Spinosaurus
3D rendering of Spinosaurus / Vac1 via Istock
Name MeaningSpine Lizard
EraMesozoic – Late Cretaceous
ClassificationDinosauria, Saurischia & Theropoda
DietCarnivorous, primarily piscivorous (fish-eating)
Height5–7 meters (16.5–23 feet) tall
LengthUp to 15 meters (49.2 feet)
Weight7.4 metric tons (16,314 pounds)
LocationNorth Africa (Tunisia, Algeria)

While most of the marine reptiles discovered so far were either vicious crocodiles or other reptilian forms, only a few of them were actual dinosaurs.

The Spinosaurus is one of the few dinosaurs that have been confirmed to be water-dwelling.

It lived in the waters of North Africa back in the Late Cretaceous. 

Spinosaurus is one of the longest carnivore dinosaurs ever discovered.

With an estimated length of about 14 meters (46 feet), this dinosaur is comparable in size to large theropods like the Tyrannosaurus and Giganotosaurus.

It had a body mass of up to 7.4 metric tons (16,314 pounds).

The most prominent feature of this dinosaur, however, was its prominent back sail formed by tall vertebral spines.

This was probably used for display or thermoregulation.

Most experts agree that the Spinosaurus was a piscivore (fish-eating) dinosaur.

Still, it may have also switched to terrestrial habitats occasionally to compete with the large carnivore theropods for prey. 


Nothosaurus 3D Illustration
Nothosaurus 3D Illustration / Warpaintcobra via Istock
Name MeaningFalse Lizard
EraMesozoic – Triassic
ClassificationSauropterygia, Nothosauroidea & Nothosauria
Length7 meters (23 feet)
Weight200 to 300 kg (440 to 660 lbs)
LocationEurope (Germany)

Nothosaurus is one of the oldest marine reptiles ever discovered.

It belongs to a group of reptiles that evolved from the first terrestrial reptiles to return to an aquatic lifestyle after the Permian Period ended.

The name “Nothosaurus” translates as “false reptile” and was a dominant aquatic predator during the Triassic Period, about 240 to 210 million years ago. 

Up to a dozen Nothosaurus species have been discovered so far.

The largest species measured between five and seven meters (16–23 feet) on average. 

Nothosaurus lived a semi-aquatic lifestyle similar to that of present-day seals.

It had a streamlined body, a long tail, and webbed feet, which it used to propel and steer itself through the water.

The Nothosaurus’ limbs were less specialized for swimming compared to the pliosaurs and plesiosaurs. 


Pliosaurus irgisensis
Pliosaurus / DiBgd – License
Name MeaningMore Lizard
EraMesozoic – Jurassic
ClassificationSauropterygia, Plesiosauria & Pliosauroidea
Length12 meters (40 feet)
Weight25 to 30 metric tons (50,000—60,000 lbs)
LocationEurope (England), South America

Pliosaurus was a genus of marine reptiles that lived on the shores of Western Europe during the Late Jurassic Period (150–145 million years ago).

It was a close relative of the Plesiosaurus and was once classified into the same group.

Despite their similarities, Pliosaurus is distinguished from the plesiosaurs by its massive head, robust body, and a relatively shorter neck, which is a sharp contrast to the small head and extremely long neck of the plesiosaurs. 

Pliosaurus was an apex predator with a voracious appetite.

It likely hunted a variety of prey, including other marine reptiles such as plesiosaurs, ‬ichthyosaurs, smaller pliosaurs, and large fish like the Leedsichthys.

They reached an impressive size of up to 40 feet long and weighed between 25 and 30 tons.

This reptile also had a powerful bite force thanks to its massive teeth that measured up to 30 centimeters long.


Ceratosaurus 3D Render
Ceratosaurus 3D Render / Naz-3D via Istock
Name MeaningHorned Lizard
EraMesozoic – Late Jurassic
ClassificationDinosauria, Saurischia & Theropoda
Height3.7 meters (12 feet)
Length6 meters (19.7 feet)
Weight2 tons (4,000 lbs)
LocationNorth America (United States), Europe (Portugal)

Ceratosaurus was a medium-to-large-sized theropod dinosaur that lived in North America during the Late Jurassic.

It was a rare animal, and only a handful of fossils have been discovered so far.  

Ceratosaurus was a bipedal dinosaur that measured up to six meters in length. It had long slender teeth in its lower jaw.

The dinosaur’s teeth were up to nine centimeters (3.5 inches) long.

These teeth would have stuck out of the dinosaur’s closed mouth, giving it a menacing appearance.

Although Ceratosaurus is renowned as one of the biggest hunters of the Jurassic Period, it most likely didn’t hunt terrestrial prey like most of the other carnivorous dinosaurs of that period.

Ceratosaurus lived most of its life in floodplains, swamps, and lake margins.

It probably hunted actively in aquatic habitats, preying on fish, turtles, and crocodiles.


Elasmosaurus 3D Illustration
Elasmosaurus 3D Illustration / Warpaintcobra via Istock
Name MeaningThin plate lizard
EraMesozoic – Late Cretaceous
ClassificationSauropterygia, Plesiosauria & Elasmosauroidea
Length10.3 meters (34 feet)
Weight5 to 7 metric tons (5.5 to 7.7 short tons)
LocationUnited States

Fossils of the long-necked Elasmosaurus once confused paleontologists who were unsure of how to reconstruct it.

In fact, the first reconstruction of this reptile placed its skull at the end of its tail.

This was an understandable error, considering the fact that the reptile’s actual neck was considerably longer than its tail. 

The marine reptile had a streamlined body with paddle-like limbs that helped with propulsion through the water.

The entire body length of this reptile is about 10.3 meters (34 feet), with the neck alone taking up about 7.1 meters (23 feet). 

To further add to the bizarre appearance of this reptile, Elasmosausus had a very small head.

Despite being so long, the Elasmosaurus’ long neck was too stiff for the marine reptile to hold it above water.

This was also not good for propulsion, making the Elasmosaurus a relatively sluggish predator that could only hunt small fish and invertebrates. 


Shastasaurus pacificus restoration.
Shastasaurus / DiBgd – License
Name MeaningMount Shasta Lizard
Pronunciation sha-STAH-saw-rus
EraMesozoic – Triassic
ClassificationSauropterygia, Ichthyosauria & Longipinnati
DietCarnivorous, primarily piscivorous
Length21 meters (68.9 feet)
Weight20 to 25 metric tons
LocationNorth America (Unites States, Canada), Asia (China)

Shastasaurus was a gigantic marine reptile often ranked as the largest marine reptile ever found.

Some estimates suggest that this gigantic reptile reached lengths of up to 69 feet and may have weighed up to two tons.

Shastasaurus had a streamlined body, paddle-like limbs, and a long snout.

It is believed to have been a swift swimmer and a top marine predator of the Late Triassic Period. 

Shastasaurus looked considerably different from other ichthyosaurs.

It had a slender profile and an unusually short, toothless snout instead of the dolphin-like snouts of its relatives.

Experts think Shastasaurus was a suction feeder, feeding primarily on cephalopods and other soft-bodied invertebrates. 


3D rendering of a Shonisaurus
3D rendering of a Shonisaurus / Vac1 via Istock
Name MeaningShoshone lizard
EraMesozoic – Triassic
ClassificationIchthyosauria, Shastasauria & Pachycormiformes
DietCarnivorous, primarily piscivorous
Length21 meters (68.9 feet)
Weight20 to 30 metric tons
LocationUnited States

Shonisaurus was another giant marine reptile from the Triassic Period of North America.

A type of ichthyosaur, this reptile is known for its elongated body and its exceptionally long snout.

Shonisaurus had a massive skull that measured up to 2.75 meters (9 feet) long.

It likely inhabited open oceans and used its streamlined body and powerful tail to swim rapidly. 

The genus name translates as “lizard from the Shoshone Mountains,” a reference to the location in Nevada where up to 37 fossils of this reptile have been found.

Once thought to be a toothless suction feeder, recent research shows that the Shonisaurus was an enormous predator with crocodilian attributes.

Shonisaurus’ jaw was strong enough to crush the shell of ammonites, but it also preyed on marine reptiles, fish, and squid.


Dakosaurus 3D Illustration
Dakosaurus 3D Illustration / Warpaintcobra via Istock
Name MeaningBiter lizard
EraMesozoic – Late Jurassic
ClassificationCrocodylomorpha, Crocodilia & Thalattosuchia
Height1 meter (3.3 feet)
Length4 to 5 meters  (13 to 16 feet)
Weight200 to 300 kg (440 to 660 lbs)
LocationEurope (England, Germany, Poland, Russia),  South America (Argentina)

Dakosaurus was a prehistoric crocodile that lived during the Late Jurassic Period about 152 to 157 million years ago.

This crocodylomorph evolved to live an exclusive marine lifestyle.

It had a robust body, strong flippers, and a large head similar to that of terrestrial theropod dinosaurs.

But the rest of its body was similar to that of mosasaurs.

Dakosaurus had clumsy, leg-like hind flippers, which means it wasn’t a particularly fast swimmer but was also not well-adapted to walking on land. 

Unlike other marine crocodilians adapted to a piscivore diet, Dakosaurus had large laterally compressed teeth similar to that of modern killer whales.

This suggests that the Dakosaurus was an apex predator adapted to feeding on larger prey such as other marine reptiles. 


Cryptoclidus oxonensis
Cryptoclidus oxonensis / Dmitry Bogdanov – License
Name MeaningHidden clavicle
EraMesozoic – Jurassic
ClassificationSauropterygia, Plesiosauria & Cryptoclidoidea
HeightAbout 2 meters (6.6 feet)
Length4 meters (13 feet)
Weight500 kg (1,100 lbs)
LocationEurope (England & France ), North America (Cuba)

Cryptoclidus was a long-necked marine reptile that inhabited the seas of Europe (particularly England) during the Jurassic Period about 150 million years ago.

It had a relatively small head, a long neck, and a streamlined body with paddle-like limbs.

Like other plesiosaurs, the incredibly long neck of the Cryptoclidus was one of its most fascinating features.

The neck contained approximately 60 vertebrae, and it probably used it to reach out and snatch prey from a considerable distance. 

Due to the fragile build of this reptile’s head, it is typically considered a filter feeder that ate small fish and soft-bodied invertebrates like squids.

However, studies suggest that the Cryptoclidus had a well-developed sense of smell, a trait commonly seen in marine predators. 

Cryptoclidus had a pointed snout with numerous sharp teeth adapted for catching and holding onto slippery prey.

Its eyes were large, indicating good vision in the water, and its nostrils were located at the tip of its snout.

This would have made it possible to breathe while staying mostly submerged.


3D rendering of Mosasaurus
3D rendering of Mosasaurus / Vac1 via Istock
Name MeaningMeuse lizard
EraMesozoic – Late Cretaceous
ClassificationSquamata, Mosasauria & Mosasauroidea
HeightCarnivorous, primarily piscivorous
Length18 meters (59.1 feet)
Weight10 to 20 metric tons (22,046.23 to 44,092.45 lbs)
LocationNorth America, South America, Europe, Asia, Antarctica

Mosasaurus was an enormous marine lizard that lived in the North Atlantic Ocean during the Late Cretaceous.

It is named after the Meuse River, one of Europe’s major rivers where the first fossil of this reptile was found. 

Mosasuarus was a member of a diverse reptilian family related to modern snakes and lizards known as mosasaurs.

Members of this family are often referred to as the “Tyrannosaurus rex of the seas” because of their massive size, predatory nature, and the fact that they lived around the same time as the notorious tyrannosaurids.

Mosasaurus was one of the largest members of this family, reaching lengths of up to 17.1 meters (56 ft). 

Unlike modern relatives (snakes and lizards), Mosasaurus was warm-blooded.

It was also fully adapted to an aquatic lifestyle.

But it did have a double-hinged jaw like other squamates, a unique adaptation that allowed it to swallow prey whole.

Mosasaurus likely fed on a wide range of marine life, including fish, turtles, smaller marine reptiles, and even other mosasaurs.


Halisaurus arambourgi
Halisaurus arambourgi / DiBgd – License
Name MeaningSea lizard
EraMesozoic – Late Cretaceous
ClassificationSquamata, Mosasauria & Mosasauroidea
DietCarnivorous, primarily piscivorous
Length4.6 meters (15 feet)
Weight100 kg (220 lbs)
LocationUnited States

Halisaurus is another member of the mosasaur family. However, it was smaller in size and less popular compared to the Mosasaurus.

It lived during the Late Cretaceous between 85 and 70 million years ago. The name Halisaurus means “ocean lizard.” 

Halisaurus was a relatively small mosasaur, reaching lengths of about 10 to 15 feet.

It had a slender and streamlined body which was well-adapted for a fully-aquatic lifestyle.

Its body was supported by four flippers for swimming and steering in the water.

Halisaurus was one of the earliest mosasaurs to evolve, so it had many primitive features, such as poorly-developed flippers, compared to other mosasaurs that evolved later. 

Despite being small by mosasaur standards, Halisaurus was still a notable predator.

It had an elongated snout filled with sharp teeth.

These teeth were perfect for capturing and consuming prey, which likely consisted of fish, smaller marine reptiles, and other marine organisms.


Ichthyosaurus communis
Ichthyosaurus communis / Nobu Tamura – License
Name MeaningFish Lizard
EraMesozoic – Triassic
ClassificationSauropterygia, Ichthyosauria & Ichthyosauroidea
DietCarnivorous, primarily piscivorous
Length4 meters (13 feet)
Weight250 to 500 kg (550 to 1,100 lbs)
Location Europe (England, Germany), North America (Unites States)

Ichthyosaurus was the first member of the Ichthyosaur family of marine reptiles ever found.

It is also one of the best-known members of the group.

The marine reptile lived in parts of Europe during the Late Triassic Period. 

Ichthyosaurus had a dolphin-like build, with a sleek and streamlined body and flippers for swimming.

The size range for the different species found so far varies between 9.8 and 10.8 feet in length for the largest specimens. 

Despite being a reptilian, Ichthyosaurus was viviparous.

This means it gave birth to live young instead of laying eggs like other reptiles.

Scientists have found Ichthyosaurus fossils with baby specimens inside them.

This adaptation isn’t entirely strange for a marine predator since many shark species today also deliver their young live instead of laying eggs. 

Ichthyosaurus was an apex predator, well adapted to life in the water.

It had a long snout filled with sharp, cone-shaped teeth for catching and gripping slippery prey.

This suggests a diet mainly consisting of fish and other small marine animals. 


Tylosaurus 3D Illustration
Tylosaurus 3D Illustration / Warpaintcobra via Istock
Name MeaningProtuberance lizard
EraMesozoic – Late Cretaceous
ClassificationSquamata, Mosasuria & Mosasauroidea
DietCarnivorous, primarily piscivorous
Length15.24 meters (50 feet)
Weight 5 to 7 metric tons (11023–15432 lbs)
LocationUnited States

Tylosaurus was one of the largest and most powerful marine predators of the ancient seas.

It lived during the Late Cretaceous and is considered one of the deadliest marine hunters known from that period.

Tylosaurus was a mosasaur, which means it is more closely related to snakes and monitor lizards.

With an estimated length of up to 40–50 feet, Tylosaurus was one of the largest members of that family. 

Like other mosasaurs, Tylosaurus had a streamlined body with a long, muscular, and vertically flattened tail that helped propel the reptile through water.

It also had paddle-like limbs for steering in the water.

As an apex predator, Tylosaurus most likely ate anything big enough that swam in the water, including fish, seabirds, primitive sharks, plesiosaurs, and even other mosasaurs. 


Liopleurodon chasing sharks 3D Rendering
As you’ve just seen, prehistoric waters were full of apex predators / Daniel Eskridge via Istock

Great whites and killer whales are the apex predators of the marine world today.

But before them, prehistoric marine reptiles like the ones on this list were the top dogs of the marine ecosystem.

As this article shows, these reptiles came in different shapes and forms, from majestic warm-blooded ichthyosaurs to formidable crocodylomorphs like the Deinosuchus.

These magnificent beings ruled the oceans of the Mesozoic Era while the terrestrial dinosaurs reigned on land.

Which of these marine reptiles is your favorite?

Share with us in the comment section below!


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