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Your Complete Guide to Mosasaurs

Article written by: Jayson Kowinsky -

Mosasaur - The Great Marine Reptile

A Tylosaurus proriger Mosasaur from the Western Interior Seasway of North America. Displayed in the Carnegie Museum of Natural History

Mosasaur skeleton: Tylosaurus proriger from the Western Interior Seasway of North America. Displayed in the Carnegie Museum of Natural History.

Fast Facts about Mosasaurs

Mosasaur fossil skeleton - Clidastes propython

Complete skeleton of a 15 foot Mosasaur: Clidastes propython. Clidastes would have looked very similar to a snake in life.

Name: Mosasaur (pronunciation: "moh-suh-sawr") - it means "Lizard of the Meuse River"
'Mosa' stands for the Meuse river in Holland - the location where mosasaurs were first described.
'Saur' (Sauros) is greek for lizard.

Taxonomy: Class: Reptilia - Order: Squamata - Superfamily: Mosasauroidea - Family: Mosasauridae
Subfamilies: Halisaurine, Mosasaurinae, Plioplatecarpinae, Tylosaurinae

Mosasaurs ARE NOT DINOSAURS. They are reptiles and are closely related to snakes and monitor lizards.

Age: Cretaceous
Mosasaurs went extinct at the end of the Cretaceous during the end Cretaceous mass extinction event.

Jurassic World vs Real Life: Half the Size
The Tylosaurus mosasaur shown on the Jurassic Park movie was the largest mosasaur to exist. In real life it grew up to 50 feet in length. It's hard to figure out the exact size of the mosasaur in the movie, but it's conservatively somewhere around 100 feet, which is at least twice the real life size.

Also, the Tylosaurus shown in Jurassic World looks more like an aquatic godzilla. In real life, evidence shows they had a caudal fin (tail) like a shark and very smooth snake-like skin.

Remember, the movie is an action packed Hollywood blockbuster, it's not meant to be scientific.

Discovery: Holland, 1764
The first described mosasaur fossils were found in a limestone quarry on the Meuse River in Holland in 1764.

Native Americans in the Midwest U.S. had found mosasaur fossils long before the first described fossils. They drew images of them, thought the fossils had special powers, and may have thought they were the remains of the mythological beings Wakinyan (Thunder beings) and Unktehila (Water monsters).

Distribution: Nearly Global:
Mosasaurs lived in the seas during the Cretaceous. Fossils are found in Cretaceous rock units on almost every continent from North and South America, to Europe, Asia, and Australia. In Hungary, A freshwater Mosasaur has even been found!

Body Size: 3 to 50 feet
Some, such as Dallasaurus were only 3 feet in length. While others, like Tylosaurus, grew up to 50 feet in length.
Most mosasaurs were over 10 feet in length.

Diet: Anything!
Stomach contents of mosasaurs reveal ammonites, bony fish, sea turtles, plesiosaurs, and even sea birds!
It appears they could eat whatever they wanted.

Physical Appearance:
Mosasaurs were sleek, streamlined, and fast! They had flipper like paddles for arms and legs and a tail fin. They propelled themselves by mianly moving the large fin on their tail in a side to side motion. Mosasaurs were also covered in smooth scales which were very dark in color.

Similar to snakes, Mosasaurs had jaws could expand to help swallow large whole prey. Also, like a snake, mosasaurs had two sets of teeth in their upper jaws. This second set was smaller and set further back in the jaw. These teeth would help hold on to struggling prey as the animal swallowed it whole.

They Breathed Air:
Although mosasaurs were aquatic, they were reptiles, which means they had to surface to breathe air, like a sea turtle today.

Live Birth:
A 2015 study found that Mosasaurs gave live birth in the open ocean.

Mosasaur Facts and Information: The Details

Below is a National Geographic video: Mosasaurs 101

Video clip from National Geographic giving an overview of Mosasaurs.

What is a Mosasaur?

Mosasaurs are considered the Great Marine Reptiles that ruled the seas during the Cretaceous period. Although they are often referred to as the T-Rex of the seas, they were not dinosaurs, but reptiles that returned to the sea during the Cretaceous Period. Even though they are aquatic, these greats beasts were still reptiles that breathed air.

Once mosasaurs returned to the seas in the Cretaceous, around 100 million years ago, they rapidly diversified. Numerous subfamilies, genera, and species quickly appeared on a near-global level, even Antarctica. Some developed teeth for crushing shells, and some even expanded into fresh water environments. In 2012 Laszlo Makadi published a paper on the discovery of a fresh water mosasaur from Hungary that lived in the rivers, similar to the freshwater river dolphins today.

Although each genera has slightly different morphological features, they all share similar traits. Mosasaurs are all long and sleek. They have arms and legs that evolved into flippers. Their jaws contain numerous conical teeth. These teeth are not designed for cutting, but instead for grasping. Their jaws are also double hinged, meaning they can greatly expand in order to swallow prey whole, like a snake. Also, like a snake, fossil skin impressions show mosasaurs had a scaly skin.

A Flipper from the mosasaur Clidastes propython. Notice it's a modified arm

A Flipper from the mosasaur Clidastes propython. Notice it's just a arm, like a dolphin flipper.

Other Marine Reptiles of the Cretaceous

Mosasaurs are considered the Great Marine Reptiles of the Cretaceous. However, there were many other marine reptiles that lived before the mosasaurs. Other great marine reptiles that predate mosasaurs include: the dolphin like ichthyosaurs, the long-necked plesiosaurs, and the short-necked pliosaurs. Examples of these Great marine Reptiles are shown below.

Other Great Marine Reptiles:
Ichthyosaur Fossil Skeleton from Holzmaden, Germany. Displayed in the Carnegie Museum of Natural History

Ichthyosaur Fossil Skeleton from Holzmaden, Germany. Displayed in the Carnegie Museum of Natural History. Notice Ichthyosaurs would look like todays dolphins.

Plesiosaur Fossil Skeleton. Displayed in the Academy of Natural Sciences of Philadelphia

Long Necked Plesiosaur Fossil Skeleton from North America. Displayed in the Academy of Natural Sciences of Philadelphia.

Pliosaur Fossil Skeleton. Displayed in the British Musuem of Natural History, UK.

Pliosaur Fossil Skeleton: Rhomaleosaurus cramptoni. Cast displayed in the British Musuem of Natural History, UK. Mary Anning's photo is next to it, I think, because she discovered similar plesiosaur and pliosaur fossils on the Dorset coast.

Mosasaur Teeth - Like a Snake

Mosasaurs, like snakes had two rows of teeth in their upper jaw, the main set, and a smaller set toward the rear and center of their mouths. These extra teeth called pterygoid teeth are thought to help hold and swallow lager prey whole. Also, like snakes, their jaws could expand, again to help them swallow whole prey. The second row of teeth can be seen in the image below of a mosasaur from the Mace Brown Museum in Charleston, SC.

View of the second set of teeth in the upper jaw of a mosasaur

A view of the second set of teeth in the upper jaw of a Tylosaurus mosasaur skull cast. From the Mace Brown Museum in Charleston, SC.

Mosasaur Color and Swimming Style: New Research

Image by Johan Lindgren showing the fossilized scales where the tiny melanosomes were found, indicating the color of the mosasaur.

Fossil scales of the mosasaur specimen: SMU 76532 where the melanosomes were found. They indicate the mosasaur had a dark coloration, like a whale. Image by Johan Lindgren (article reference at bottom of page).

What color were Mosasaurs?

New research shows they were most likely a dark color, similar to a Sperm Whale coloration.

In the past, the color of prehistoric animals was strictly confined to the realm of artists. Over the past few years, paleontologists have been able to study melanosomes in fossil feathers in dinosaurs and birds to determine their colors and color patterns. Now, paleontologists have found fossilized melanosomes in skin impressions from an ichthyosaurs , ancient turtle, and a mosasaur.

The research done by Johan Lindgren, et al (2014), shows these prehistoric aquatic reptiles were "at least partially dark colored in life." "The mosasaur contained so much of this pigment, it would have been very dark in color." Lindgren says this dark color scheme, similar to a sperm whale, would help with thermoregulation, protect against UV radiation when at the surface, and provide camouflage when deep diving.

How did Mosasaurs Swim?

Until recently, the swimming style of a mosasaur could only be determined by the skeletal anatomy. Fortunately, in 2011, Johan Lindgren, et al. studied a well preserved fossil mosasaur (Ectenosaurus clidastoides: FHSM VP-401). This specimen has 3D preserved muscle fiber bundles. By studying this exceptionally well preserved specimen, Lindgren suggests the mosasaur held the front of its body rigid and used the rear of its body and tail for propulsion. This makes for a very efficient swimming machine. This is also similar to how crocodiles swim.

Also, one specimen (LACM 128319) had a partial body outline preserved. Based on the outline, the mosasaur had a tail fluke like that of a shark. Other mosasaurs have also been found with this fluke like tail. This means they were probably very fast swimmers.

At the 2019 GSA meeting, Formosa presented research on the unusually large pectoral girdle of mosasaurs. These would have supported large muscle attachments, indicating very powerful forelimbs allowing for powerful bursts of propulsion.

Taking this all together, mosasaurs appear to be unique in that they used their tails and forelimbs for propulsion. They would have been very fast and maneuverable.

Image by Johan Lindgren showing the fossilized scales where the tiny melanosomes were found, indicating the color of the mosasaur.

Skeletal reconstruction and inferred body outline of the plioplatecarpine mosasaur Platecarpus. Notice the tail fluke in diagram B
By Johan Lindgren, Michael W. Caldwell, Takuya Konishi, Luis M. Chiappe [CCBY2.5], via Wikimedia Commons

North American Mosasaurs:
Mosasaurs of The Western Interior Seaway

During the Cretaceous time period, North America was split in half by a long shallow sea called the Western Interior Seaway. The sea was roughly 300 miles wide and ran from Texas northward through the central United States into Alberta and out the Northwest Territories.

Diagram by Williston 1898 showing the three most common mosasaurs from the Western Interior Seaway

Diagram by Williston showing the three common mosasaurs of the Western Interior Seaway of North America. By Williston 1898 ( [Public domain], via Wikimedia Commons

The Western Interior Seaway was the habitat of numerous types of mosasaurs. Part of this seaway is preserved throughout Texas, Kansas, and into the Dakotas as the Smoky Hill Chalk and the Pierre Shale. These formations contain a diverse fauna, including large vertebrates, such as the Great Marine Reptiles, the Pterosaurs (Flying Reptiles), Giant Sea Turtles, Giant Fish, and Sharks.

There are numerous mosasaurs found in the Western Interior Seaway. However, the three most common mosasaur genera found are Tylosaurus, Platecarpus, and Clidastes. Reconstructions from Williston's 1898 publication are shown to the right.
The anterior vertebra of Platecarpus is actually bent downward, as it had a large tail fluke.

Mosasaurs Excavation Video

This video, while a little old, gives an overview of a mosasaur excavations and preparing the fossils for display.

The following are brief descriptions of the 3 common Mosasaurs in the Western Interior Seaway:


Platecarpus mosasaur skull

Platecarpus mosasaur skull, By Daderot (Daderot) [CC0] from the Senckenberg Museum of Frankfurt.

Platecarpus is a common genus of mosasaur found in the Western Interior Seaway. It was a medium sized mosasaur that obtained lengths of around 15 feet. A remarkable specimen of Platecarpus tympaniticus (LACM 128319) was found and described in a few papers, including Lindgren et al. 2010, and Takuya et al. 2012. This specimen has soft tissue preservation, including skin impressions, cartilage, organ tissues including retina tissue, a partial body outline, and an articulated skeleton. The tail is turned down, indicating it probably had a fluke, (like a whale but vertical instead of horizontal). This means they swam fast, like a shark, not like an eel. This unique specimen, LACM 128319 is imaged below:

LACM 128319 fossil mosasaur specimen. This specimen of Platecarpus tympaniticus is the best preserved mosasaur ever found.

Platecarpus tympaniticus, LACM 128319: upper Santonian-lowermost Campanian, Kansas, USA. Specimen photographed under normal light. Scale bar equals 0.5 m."
Image under CCBY2.5 license. From: Lindgren J, Caldwell MW, Konishi T, Chiappe LM (2010) Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur. PLoS ONE 5(8): e11998. doi:10.1371/journal.pone.0011998.g001.


Tylosaurus proriger mosasaur skull

Tylosaurus proriger mosasaur skull from the Carnegie Museum of Natural History.

Tylosaurus is probably one of the most well known mosasaurs, due to the fact it's a common centerpiece in many museums, and it's been featured in the Jurassic Park games, comics, and the new movie "Jurassic World". Reaching lengths of over 14 meters (45 feet), it was also one of the larger mosasaurs. Tylosaurus was an apex predator in the Western Interior Seaway of North America during the Cretaceous. This beast could eat just about anything. Smaller mosasaurs and Plesiosaus have been found in the stomach contents of these great marine predators.

Tylosaurus mosasaur fossil specimen from the Academy of Natural Sciences of Philadelphia

A giant Tylosaurus proriger mosasaur on display at the Academy of Natural Sciences of Philadelphia.


Fossil Clidastes propython mosasaur skull

Clidastes propython mosasaur skull from the Carnegie Museum of Natural History.

Clidastes was a one of the smaller types of mosasaurs. Although some species were larger, the average length of this genera is only a few meters, perhaps 10 feet in length. This mosasaur was also slimmer than the others, the rib cage was narrower, giving it a more sleek, or eel like look. It was probably very agile, and able to chase down smaller prey.

Clidastes mosasaur fossil specimen from the Academy of Natural Sciences of Philadelphia

A Clidastes mosasaur skeleton on display at the Academy of Natural Sciences of Philadelphia.

New Jersey Mosasaurs

Mosasaur fossil tooth found at Big Brook, New Jersey

A mosasaur tooth as found in a stream bank in New Jersey. In this image, striations and a cutting edge is visible, which is characteristic of a mosasaur tooth.

Besides for the Western Interior Seaway, mosasaurs are also found in Cretaceous outcroppings along the East and West coast of the United States. Although Native Americans had found mosasaur fossils in the mid west, the first documented mosasaur fossils from North America come from New Jersey.

One popular fossil hunting location is the Big Brook area in central New Jersey. Here, brooks cut through cretaceous sediments, occasionally washing out a mosasaur fossil (usually a tooth or sometimes a vertebra). According to Gallagher, 2005 there are at least 8 genera and numerous species of mosasaur found in the Cretaceous of New Jersey. The common genera of mosasaur appears to be Mosasaurus sp. Since mainly only mosasaur teeth are found, and they change shape and size depending on tooth position, no attempt is made here to further identify mosasaur teeth from New Jersey.
To try your hand at finding a mosasaurus tooth, go to the Big Brook Fossils Section for more detailed information, including directions.

Mosasaur fossil tooth found in North Carolina

This image shows a mosasaur tooth found in a stream bank in North Carolina. Although this tooth is very worn, it still has visible striations and a cutting edge. These characteristics differentiate mosasaur teeth from similar looking crocodile teeth.

Moroccan Mosasaurs

A mosasaur skull from Morocco

This is a mosasaur skull from Morocco on display at the Natural History Museum in London.

The Phosphate Deposits throughout Northern Morocco, including the area of Khouribga, the Ouled Abdoun Basin, and the Ganntour Phosphate Basin are chalk full of Mosasaur fossils. These deposits represent the ancient Tethys sea that covered North Africa and Europe. This sea merged with the early North Atlantic Ocean.

Mosasaur teeth are abundant, while jaw and vertebra fragments can sometimes be found. Some beautiful specimens have been excavated, and new genera have been discovered.

There are numerous genera of mosasaur found in the deposits, from common Mosasaurus, Prognathodon, and Cladistes, to an unusual mosasaur called Globidens. Globidens looked like a small mosasaur, but has highly unusual teeth. They look like small rounded buttons and were ideal for crushing shells.

These fossils are very popular among enthusiasts. With that said, the industry of fake mosasaur fossils has rapidly grown to support the demand.

Fake / Composite Fossils

Fossils are very popular among collectors. Ebay is full of them! Because of this, fake fossils are even more popular. fake fossils are often easier and cheaper for locals to create. For example, it is illegal to export fossils from China. So why are there so many Chinese fossils on the market, including 1000's of those little Keichuosaurs and Dinosaur eggs? Some are illegal, but most are fake... Albeit expertly faked!

A whole industry in Morocco has been created that centers around the creation of fake fossils. This industry is now perhaps larger than authentic fossil collecting. Fake fossil shops abound throughout the Phosphate regions of Morocco. Anything from Trilobites, Ammonites, to Mosasaurs are all faked.

I would strongly caution anyone buying ANY fossil from morocco unless you absolutely know that the dealer is reputable.

A popular type of fake mosasaur fossil is individual teeth with the roots and/or jaw sections attached. Some people have bought these and taken them apart to see how they are made. They take ground up matrix and add some glue to form a base. Then they place real mosasaur teeth in the matrix. They add fragments of modern animal bone (usually ground up goat) to make the roots and bone, and add a lighter colored matrix to fill in the gaps. The end result is a nice set of "mosasaur" jaws. If you ever see mosasaur jaws like the ones below, they are fake.

I think they are popular because they make nice display pieces... if you don't mind the jaw section came from a modern goat! Almost all mosasaur teeth with attached roots are also made via this method.

A mosasaur skull from Morocco

Any mosasaur fossil that looks remotely like the ones imaged above are fake composites. Real teeth are often used, but the matrix and bone is made from dust, glue, and the bone fragments of modern animals.

Purchase Your Own Authentic Mosasaur Tooth:

The Fossil Era store has a nice selection of real Mosasaur fossils for sale . There are a wide range of sizes and species of mosasaur to choose from. They also have a selection of mosasaur bones and teeth in real jaw sections.

Recommended Mosasaur Books and Educational Items:

Oceans of Kansas: A Natural History of the Western Interior Sea (Life of the Past)
Michael Everhart, 2017

This book delves into life in the Cretaceous Western Interior Seaway, covering prehistoric sharks, mosasaurs, and ichthyosaurs at an intermediate reading level. Authored by Michael Everhart, a Paleontology Adjunct Curator, and expert on Late Cretaceous fossils, it offers well-researched content, complemented by numerous photos, illustrations, and drawings. A must-read for fans of Everhart's Oceans of Kansas website

The Princeton Field Guide to Mesozoic Sea Reptiles
Gregpry S. Paul, 2022

This is a comprehensive exploration of ancient oceangoing creatures, revealing their energetic nature and adaptability to diverse habitats, including polar regions. With detailed accounts of 435 species and stunning illustrations, it challenges preconceptions and offers a vivid journey through 185 million years of Mesozoic history. It describes plesiosaurs, ichthyosaurs, mosasaurs, sea snakes, sea turtles, marine crocs, and more."


Lindgren Johan, et al. (2014) Skin Pigmentation Provides Evidence of Convergent Melanism in Extinct Marine Reptiles. Nature, published online January 08, 2014; doi: 10.1038/nature12899

Lindgren Johan, Everhart Michael J., Caldwel Michael W. (2011) Three-Dimensionally Preserved Integument Reveals Hydrodynamic Adaptations in the Extinct Marine Lizard Ectenosaurus (Reptilia, Mosasauridae). PLoS ONE, 2011; 6 (11): e27343 DOI: 10.1371/journal.pone.0027343

Lindgren J, Caldwell MW, Konishi T, Chiappe LM (2010) Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur. PLoS ONE 5(8): e11998. doi:10.1371/journal.pone.0011998

Takuya Konishi, Johan Lindgren, Michael W. Caldwell, and Luis Chiappe. (2012) Platecarpus Tympaniticus (Squamata, Mosasauridae): Osteology of an Exceptionally Preserved Specimen and Its Insights Into the Acquisition of a Streamlined Body Shape in Mosasaurs. Journal of Vertebrate Paleontology 32(6):1313-1327.

W.B. Gallagher (2005) Recent mosasaur discoveries from New Jersey and Delaware, USA: stratigraphy, taphonomy and implications for mosasaur extinction. Netherlands Journal of Geosciences 84-3: 241-245

Makadi L, Caldwell MW, Osi A (2012) The First Freshwater Mosasauroid (Upper Cretaceous, Hungary) and a New Clade of Basal Mosasauroids. PLoS ONE 7(12): e51781. doi:10.1371/journal.pone.0051781

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