An Ultimate Guide to Mosasaurus: The Meuse Lizard

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Name Meaning“Meuse lizard”HeightN/A
Pronunciationmo-zaw-SAW-rusLength12–18 meters (39–59 feet)
EraMesozoicLate CretaceousWeight5–15 tons (11,000 – 33,000 lbs)
ClassificationSquamata, Mosasauroidea & MosasauriaLocationNorth America, Europe, Africa

Mosasaurus Pictures

Mosasaurus | dottedhippo via Getty Images

The Mosasaurus

Gage Beasley Prehistoric's Mosasaurus Concept
Gage Beasley Prehistoric’s Mosasaurus Concept

Mosasaurus is a genus of aquatic reptiles that lived in North America and Europe during the Late Cretaceous Period, between 82 and 66 million years ago. 

It was an enormous lizard believed to have inhabited an area that is now the present-day North Atlantic Ocean. 

The reptile’s name translates as “Meuse lizard,” referring to the Meuse River in France, where the first fossil of the Mosasaurus was found.

Growing up to 50 feet long, it is the largest known member of the mosasaur family, a group that includes several other squamate marine reptiles that lived during the Cretaceous. 

Given their size, mosasaurs were the dominant marine reptiles during their time.

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

Mosasaurus was first discovered around 1780, and it was one of the first marine reptiles ever found. 

Numerous other fossils of this aquatic predator and other relatives have turned up since then, providing a wealth of information about the mosasaurs. 

Once thought to be a crocodile or a large whale, we now have a lot of information about the mosasaur and how it may have lived. 

This article details some of the interesting facts about the Mosasaurus, including its physical characteristics, habitat, behavior, and interactions with other species that were around during the Cretaceous Period. 

Physical Characteristics

Mosasaurus was a large squamate reptile, meaning it was related to modern snakes and monitor lizards. 

However, unlike its modern relatives, it was fully adapted to an aquatic lifestyle. 

It had a streamlined body similar to that of modern aquatic animals like dolphins and sharks. 

The Mosasaurus’ long and slender body was well-suited to swimming swiftly through the water, but the reptile still had to come up to the surface to breathe air (similar to modern marine mammals like whales). 

The limbs were modified into paddle-like limbs that may have helped them with maneuvering in the water. 

They also had a long two-lobed tail that curved downward slightly; the appearance of their tail is similar to that of ichthyosaurs (another group of marine reptiles that lived earlier in the Mesozoic). 

Mosasaurus was an impressive creature in terms of size. 

On average, its length has been estimated to be about 40 to 56 feet (12 to 17 meters). 

Gage Beasley Prehistoric's Mosasaurus Concept
Gage Beasley Prehistoric’s Mosasaurus Size Comparison Chart

The Mosasaurus’ skull alone reached lengths of up to 1.7 meters (67 inches). 

The jaws were filled with sharp, conical teeth that were quite robust and efficient for capturing large prey. 

The body of the Mosasaurus and the membranes between their toes and fingers were covered in small overlapping scales.

The scales were diamond-shaped and were quite similar to that of modern snakes. The size of the scales also varied across the entire body.

Habitat and Distribution

Fossils of the Mosasaurus have been found in various locations all over the world, including North America, Europe, and Africa. 

They had a transatlantic distribution, meaning they were present on both sides of the Atlantic Ocean. 

During the Late Cretaceous Period, the continents had a different configuration, and this affected the size and location of the world’s oceans too. 

The range of this marine reptile possibly covered the Northern Atlantic Ocean, the Western Interior Seaway, and the Mediterranean Tethys Sea

Mosasaurus | dottedhippo via Getty Images

The climatic conditions also varied across these different seas. 

In some places, the climate ranged between tropical, subtropical, temperate, and even subpolar. 

Although Mosasaurus was fully adapted to an aquatic lifestyle, it could not breathe underwater; this means it would have had to surface periodically to take in air. 

As a result, they spent more time in shallow water in the continental margins, but they did great in open waters too.

Behavior and Diet

Gage Beasley's Mosasaurus Stuffed Plush Toy
Gage Beasley’s Mosasaurus Stuffed Plush Toy

Mosasaurus had a streamlined body and powerful shark-like tails, which allowed it to swim swiftly through the water. 

It likely used undulating movements of its tail and lower body to propel itself forward, similar to the swimming style of modern-day mackerels, whales, and dolphins. 

This swimming style has been described as sub-carangiform swimming.

It also had elongated paddle-like limbs, which it used like hydrofoils to maneuver through the water. 

Scientists believe the Mosasaurus was a predominantly solitary creature. 

Individuals typically lived and hunted on their own. 

Mosasaurus hunting | Racksuz via Getty Images

However, they likely aggregated in certain areas where food was abundant or during mating seasons.

Mosasaurus was the largest known marine animal during the Late Cretaceous. 

As such, it was an apex predator that occupied the top of the marine food chain. 

The massive jaws equipped with robust cutting teeth would have been strong enough to handle pretty much any animal in the Late Cretaceous seas. 

The teeth were sharp and conical, which suggests that they were well-suited for gripping prey. 

Mosasaurus was probably an ambush predator.

Mosasaurus chasing an Ichthyosaur | estt via Getty Images

It used its speed and agility to swiftly get close to prey before going in for a kill. 

The reptile had a varied diet: fish, cephalopods, and other marine reptiles like the plesiosaurs. 

Mosasaurus had a poor sense of smell, but its large eyes were adapted to two-dimensional vision. 

This would have made it easier to spot and capture prey near the ocean surface. 

Like modern squamates, Mosasaurus had a double-hinged jaw, which means it was capable of opening its jaws to eat prey bigger than the length of its skull.

Scientists once found remains of a dismembered fish that was more than one meter long in the gut of a Mosasaurus.

It is also likely that it consumed large prey by dismembering and consuming it in small bits at a time instead of swallowing it whole. 

Life Cycle

juvenile Mosasaurus
Fragmentary skull of a juvenile Mosasaurus (NHMM 200793) from Geulhem, Netherlands | MWAK via Wikipedia (CC0)

Mosasaurus reproduced sexually, and evidence suggests that they were viviparous.

Like modern marine mammals, they gave birth to live young in the water. 

So far, there has been no direct evidence of live birth in the Mosasaurus

However, fossils of other related mosasaurs have been discovered that support the theory that they were viviparous. 

Scientists have found the skeleton of a pregnant Carsosaurus as well as a Plioplatecarpus fossil with two embryos close to it. 

Fossils like this point to internal fertilization and viviparity in the mosasaurs. 

fossil egg
A fossil egg of a marine reptile, found in Antarctica, is seen in this picture obtained by Reuters on June 16, 2020. | University of Chile/Handout via Reuters

However, the discovery of a large fossil egg in 2011 throws some doubt on the widely-held theory of Mosasaurus’ viviparity. 

Scientists think the egg belonged to a marine reptile, and the presence of mosasaur fossils nearby suggests it may have belonged to this reptile. 

However, the fossil egg has not been conclusively identified, so the theory that Mosasaurus gave birth to live young remains valid.

Juveniles were born in the water and were capable of a fully functional marine lifestyle right away. 

Although they were probably born in shallow waters, evidence suggests that the young did not require nursery areas and could live in the open waters from a very young age. 

Evolution and History

Mosasaurus jumping out of the water | estt via Getty Images

Mosasaurus was a member of the mosasaur family of marine reptiles. 

It was the first member of the family to be identified and is the largest mosasaur identified so far. 

Mosasaurs belonged to the larger reptilian order called Squamata, which means they’re related to modern lizards and snakes. 

Mosasaurus and other mosasaurs represent a distinct lineage that evolved specialized adaptations for an aquatic lifestyle.

Although they became dominant after the extinction of other giant marine reptiles like the ichthyosaurs and pliosaurs, Mosasaurus did not evolve from them. 

Mosasaurus | dottedhippo via Getty Images

Instead, experts believe they evolved from a group of terrestrial lizards earlier in the Cretaceous Period. 

Their terrestrial ancestors returned to the water about 98 to 93 million years ago and gradually became more adapted to a marine lifestyle. 

High sea levels and the abundance of prey species in the Cretaceous oceans probably drove the evolution of the mosasaurs.

This explains why they went extinct when sea levels dropped, and productivity collapsed in the seas at the end of the Cretaceous Period. 

The widespread distribution of the Mosasaurus in different oceans with varying conditions triggered further diversification within the genus. 

At least five species have been identified within the genus showing varying adaptations. 

Interactions With Other Species

Mosasaurus inhabited offshore ocean habitats of various depths. | Apokryltaros via Wikipedia (CC BY 2.5)

Mosasaurus was one of the apex predators that dominated the oceans during the last 20 million years of the Cretaceous Period. 

Their large size and massive jaws suggest that they could hunt and prey on other marine animals, including fish, cephalopods, sea turtles, and even other marine reptiles. 

They were efficient killers capable of holding on to large prey with their spike-like teeth, cracking the shell of giant turtles, and swallowing prey whole. 

Apart from the Mosasaurus, the Late Cretaceous seas teemed with other marine reptiles, including relatives in the mosasaur family and non-related marine reptiles. 

Some of these reptiles, such as Prognathodon and Tylosaurus, were large enough to compete for food and other resources. 

Tylosaurus, a genus of mosasaur, in the stormy sea | MR1805 via Getty Images

Although they probably lived in different niches, there’s evidence of violent encounters between Mosasaurus and Tylosaurus

There’s also evidence to suggest that Mosasaurus individuals competed aggressively with other members of their own species. 

Scientists once found a partial skeleton of Mosasaurus conodon with multiple cuts and punctures. 

Experts think these cuts came from another M. conodon and were probably fatal.

These and other similar accounts of fatal Mosasaurus encounters suggest that Mosasaurus competed aggressively among themselves or even engaged in cannibalism.

Cultural Significance

Fossil of Mosasaurus, an extinct mosasaur—Took the photo at Natural History Museum of Maastricht | Ghedoghedo via Wikipedia (CC BY-SA 3.0)

Discovered in the late 18th century, Mosasaurus was the first Mesozoic marine reptile ever found. 

Therefore, fossils of this marine reptile hold significant scientific value. 

It has played an important role in our understanding of how the marine reptiles of the Late Cretaceous evolved and the natural ecosystems where they lived. 

Before the discovery of the Mosasaurus, the theory of extinction was not widely accepted. 

Discovered fossils were often attributed to being some form of living species rather than an extinct one. 

TM 7424, the first known specimen of M. hoffmannii | Ghedoghedo via Wikipedia (CC BY-SA 4.0)

Adriaan Gilles Camper and Georges Cuvier, who studied the first fossil of the Mosasaurus, concluded that it was that of an extinct marine lizard, unlike any other living species. 

This conclusion gave rise to the theory of catastrophism, considered a precursor to the modern theory of evolution. 

Mosasaurus also holds a bit of cultural significance in France.

The first fossils of this reptile were discovered during the French Revolutionary Wars and were seized by French forces from the Fortress of Maastricht in the Netherlands, from where they were then moved to France. 

The fossil was famously called the “great animal of Maastricht” before it was officially named and studied. 

Mosasaurus in Jurassic World | Photo via Jurassic Park Wiki

In pop culture today, Mosasaurus is still commonly referenced as an iconic representation of ancient sea monsters. 

It has been featured in various scientific books, movies, and documentaries about prehistoric creatures

The most famous of these was the cameo in two Jurassic Park movies, “Jurassic World” and “Jurassic World: Fallen Kingdom,” released in 2015 and 2018, respectively. 

It also appeared on-screen briefly in the third installment of the movie. 


Mosasaurus is an extinct genus of aquatic reptiles that were the apex predators of the marine environment during the Late Cretaceous Period. 

They evolved from terrestrial ancestors around 99 million years ago and became the most dominant aquatic carnivores during the late 20 million years of the Cretaceous

Mosasaurus fossils have been found in various marine environments all over the world. 

They lived in prehistoric seas such as the Western Inland seaway and the Tethys Sea.

As the apex predator, Mosasaurus fed on fish, cephalopods, turtles, and other marine reptiles. 

Since it was one of the first marine reptiles ever found, the discovery of the Mosasaurus holds both paleontological and cultural significance. 

It remains one of the most recognizable marine reptiles and an iconic representation of the monsters that once dominated Earth’s marine ecosystem. 


Was the Mosasaurus bigger than the Megalodon?

The Megalodon is the largest shark species and one of the fiercest marine carnivores ever found. 

Based on the size estimates for these two animals, the largest Mosasaurus would have been about the same size as an average Megalodon

Mosasaurus hofffmanni reached lengths of up to 59 feet, which matches the Megalodon’s length. 

Was Mosasaurus a dinosaur or a shark?

Mosaurus was not a dinosaur.

It was a type of marine reptile more related to modern-day snakes and monitor lizards but was adapted to a fully-marine lifestyle. 

Was Mosasaurus bigger than the T-Rex? 

Yes, it was.

Based on their lengths, Mosasaurus was bigger than the famous Tyrannosaurus rex.


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