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Mosasaur: Facts and Information about the Giant Marine Reptile

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Mosasaur fossil skeleton - Clidastes propython
Complete skeleton of a 15 foot Mosasaur: Clidastes propython.

Mosasaur Gallery: Facts and Information about Mosasaurs

A ~40 foot long Mosasaur fossil skeleton - Tylosaurus proriger
40 foot North American Mosasaur: Tylosaurus proriger

Fast Facts about the Mosasaurs

Name: Mosasaur (pronunciation: "moh-suh-sawr") - The name 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 that 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.

Although these are the first described Mosasaur fossils, Native Americans had found mosasaur fossils long before that in the Midwest United States. They drew images of them, thought the fossils had special powers, and may have thought they were the remains of Wakinyan (Thunder beings) and Unktehila (Water monsters) from their mythology.

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. They propelled themselves by moving the back of their bodies and tails in a side to side motion. Mosasaurs were also covered in smooth scales which were probably 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 dolphin or whale.

Introduction to Mosasaurs - Detailed Mosasaur Fossil Facts and Information

Video clip from Discovery Channel: "Dinosaur Revolution" - Mosasaur Rampage showing a mother Mosasaur making a light meal out of Squalicorax sharks. This is a nice representation of what they may have been like in real life.
Representatives of the Great Marine Reptiles
from the Cretaceous Seas:
A Tylosaurus proriger Mosasaur from the Western Interior Seasway of North America. Displayed in the Carnegie Museum of Natural History
40 foot Tylosaurus proriger, a Mosasaur from North America
Ichthyosaur Fossil Skeleton from Holzmaden, Germany. Displayed in the Carnegie Museum of Natural History
Ichthyosaur Fossil Skeleton
Plesiosaur Fossil Skeleton. Displayed in the Academy of Natural Sciences of Philadelphi
Long Necked Plesiosaur Fossil Skeleton from North America
This image is a manual panorama of two images
Pliosaur Fossil Skeleton of Dolichorhynchops from North America. Displayed in the Carnegie Museum of Natural History
Dolichorhynchops Pliosaur from North America
What is a Mosasaur?

Mosasaurs are not Dinosaurs. Although they are often referred to as the T-Rex of the seas, they were reptiles that returned to the sea during the Cretaceous Period. Even though they are aquatic, these greats beasts were still reptiles, and therefore breathed air, kind of like whales today.

Mosasaurs are considered one of the Great Marine Reptiles that ruled the seas during the Cretaceous period. Other great marine reptiles at that time include: the dolphin like ichthyosaurs, the long-necked plesiosaurs, and the short-necked pliosaurs. Luckily, for us, all Great Marine Reptiles became extinct at the end of the Cretaceous period 65 million years ago. Representatives of these Great marine Reptiles are shown to the left.

Once mosasaurs returned to the seas in the Cretaceous, around 100 million years ago, they rapidly diversified. Numerous subfamilies, genera, and species appear near-globally throughout the Cretaceous. They 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. Fossil skin impressions have been found on occasion, indicating mosasaurs had a scaly skin, similar to a snake.

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

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 teeth, like snakes, were thought to help hold on to 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 this 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.
This is an image of a scale impression
of the mosasaur specimen: SMU 76532 where
the fossil melanosomes were found. They
indicate the mosasaur had a dark coloration.
Image by Johan Lindgren

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 a Mosasaur 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: Specimen: FHSM VP-401). This specimen has three-dimensionally 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.

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 possible 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.

Mosasaur Excavation Video:

This video gives an overview of a mosasaur excavation from part of the preserved Western Interior Seaway in North Texas

The following are brief descriptions of each of the common Mosasaurs in the Western Interior Seaway:
Platecarpus mosasaur skull

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


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

Image of the Tylosaurus proriger mosasaur 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. A complete skeleton of Clidastes propython is also shown at the very top of this page.

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

Image of the Clidastes mosasaur skeleton at the Academy of Natural Sciences of Philadelphia.

New Jersey Mosasaurs

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 at Big Brook, New Jersey Mosasaur fossil tooth found in North Carolina

The left image is 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.

Purchase Your Own Authentic Mosasaur Fossil 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.

Moroccan Mosasaurs

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.
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 the 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? They are fake... Albeit expertly faked!

A whole industry in Morocco has been booming for some time now that centers around faking 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. As time goes on, these fakes are looking better and better.

I would strongly caution anyone buying ANY fossil from morocco unless you absolutely know what you are doing.

Specific types of fake mosasaur fossil are 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. The expert fakers take ground up matrix and add some glue to form a base. They then place real mosasaur teeth in the matrix. They then add fragments of modern animal bone (usually ground up goats) 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.

Fake mosasaur fossil jaw section Fake mosasaur fossil
Mosasaur teeth in fake matrix mosasaur composite - fake matrix and bone

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.

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Recommended Books for learning about the Mosasaurs and other Prehsitoric Sea Monsters:

The following book: Sea Dragons: Predators of the Prehistoric Oceans is an great book that explores the lives of giant seagoing reptiles, including Mosasaurs, Ichthyosaurs, and Plesiosaurs. While Dinosaurs ruled the land, see what ruled the seas!

Oceans of Kansas: A Natural History of the Western Interior Sea (Life of the Past)
By Michael J. Everhart: 2005
This book is all about life in the Western Interior Seaway during the Cretaceous. It descripes everything from the prehistoric sharks, to mosasaurs, to icthyosaurs on an intermediate reading level. The Author, Michael Everhart, is an Adjunct Curator of Paleontology at the Sernberg Museum of Natural History, and an expert on Late Cretaceous fossils of the Western Interior Seaway. This well researched book also contains many photos, illustrations, and drawings.
If you have ever come across his wonderful Oceans of Kansas website, this book is a must!


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