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Mosasaur Facts and Information: The Details

Mosasaurs were the great marine reptiles that ruled the seas during the Cretaceous period. Although they are sometimes called the "T. rex of the seas," they were not dinosaurs. They were squamate reptiles that returned to the ocean and rapidly evolved into powerful marine predators.

Below is a National Geographic video: Mosasaurs 101

What Is a Mosasaur?

Once mosasaurs returned to the sea around 100 million years ago, they diversified quickly. Numerous subfamilies, genera, and species appeared across the world’s oceans. Some developed teeth for crushing shells, some specialized in fish or marine reptiles, and some even moved into freshwater environments.

Although each genus has its own details, most mosasaurs share a similar body plan. They had long streamlined bodies, paddle-like arms and legs, jaws filled with grasping teeth, and flexible snake-like skulls that helped them swallow large prey. Fossil skin impressions show they had smooth scales rather than the rough, crocodile-like skin often shown in older reconstructions.

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Other Marine Reptiles of the Cretaceous

Mosasaurs were the dominant marine reptiles of the Late Cretaceous, but they were not the only great marine reptiles of the Mesozoic. Earlier marine reptile groups included dolphin-like ichthyosaurs, long-necked plesiosaurs, and short-necked pliosaurs. These groups were already famous ocean predators long before mosasaurs became dominant.

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Mosasaur Teeth: Snake-Like Jaws and Pterygoid Teeth

Mosasaurs had teeth built mainly for gripping, not chewing. Most mosasaur teeth are conical and often show striations, cutting edges, or faceted surfaces. Like snakes, mosasaurs also had a second set of smaller teeth on the roof of the mouth called pterygoid teeth. These helped hold struggling prey and move it backward as the animal swallowed.

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Mosasaur Color and Swimming Style: New Research

What Color Were Mosasaurs?

New research suggests at least some mosasaurs were dark colored in life, similar to the coloration of a sperm whale. For many years, prehistoric animal colors were left entirely to artists, but paleontologists can now study melanosomes in exceptional fossils. Lindgren et al. (2014) found evidence of pigment in fossil skin from an ichthyosaur, an ancient turtle, and a mosasaur. The mosasaur contained enough pigment to suggest a very dark appearance, which may have helped with thermoregulation, protection from UV radiation, and camouflage while diving.

How Did Mosasaurs Swim?

Exceptionally preserved fossils show mosasaurs were efficient swimmers. Lindgren et al. (2011) described a well-preserved mosasaur specimen with three-dimensional muscle fibers, suggesting the front of the body was held relatively rigid while the rear of the body and tail provided most of the propulsion. Other specimens preserve outlines of a shark-like tail fluke. Taken together, mosasaurs appear to have been fast, maneuverable marine reptiles that used both the tail and forelimbs during powerful swimming.

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North American Mosasaurs: Mosasaurs of the Western Interior Seaway

During the Cretaceous, North America was split by a long, shallow sea called the Western Interior Seaway. This seaway stretched from the Gulf region northward through the central United States into Canada. Its marine deposits, including the Smoky Hill Chalk and Pierre Shale, preserve an incredible fossil record of mosasaurs, plesiosaurs, giant sea turtles, pterosaurs, fish, and sharks.

Three of the best-known mosasaur genera from the Western Interior Seaway are Platecarpus, Tylosaurus, and Clidastes. Each had a different body shape and ecological role, ranging from smaller agile hunters to enormous apex predators.

Mosasaur Excavation Video

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Platecarpus

Platecarpus is a common mosasaur from the Western Interior Seaway. It was a medium-sized mosasaur that reached about 15 feet in length. A remarkable specimen of Platecarpus tympaniticus (LACM 128319) preserves soft tissues, including skin impressions, cartilage, organ tissues, and a partial body outline. The tail is bent downward, indicating a tail fluke that helped it swim like a fast marine predator rather than an eel-like animal.

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Tylosaurus

Tylosaurus is one of the best-known mosasaurs because it is a centerpiece in many museums and has appeared in popular media. Reaching lengths of over 45 feet, it was one of the largest mosasaurs. Tylosaurus was an apex predator in the Western Interior Seaway and could eat a wide variety of prey, including fish, turtles, plesiosaurs, and smaller mosasaurs.

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Clidastes

Clidastes was one of the smaller and more agile mosasaurs. Although some species grew larger, the average length was only a few meters, perhaps around 10 feet. Its narrow rib cage and sleek body suggest it was a fast, maneuverable hunter that could chase smaller prey.

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New Jersey Mosasaurs

Besides the Western Interior Seaway, mosasaurs are also found in Cretaceous outcrops along the East and West coasts of the United States. The first documented North American mosasaur fossils came from New Jersey. One popular fossil-hunting location is the Big Brook area in central New Jersey, where streams cut through Cretaceous sediments and occasionally expose mosasaur teeth and vertebrae. According to Gallagher (2005), at least eight genera and numerous species of mosasaurs have been reported from the Cretaceous of New Jersey.

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Moroccan Mosasaurs and Fake / Composite Fossils

The phosphate deposits of northern Morocco, including the Khouribga area, the Ouled Abdoun Basin, and the Ganntour Phosphate Basin, are rich in mosasaur fossils. These deposits represent parts of the ancient Tethys Sea, which covered North Africa and connected with the early North Atlantic. Mosasaur teeth are abundant, while jaws, vertebrae, and skull material are less common.

Moroccan mosasaur fossils are popular with collectors, but the fossil trade has also produced many composite and fake specimens. Individual mosasaur teeth from Morocco are often real, but many mounted jaws or skulls are assembled from real teeth placed into artificial matrix. Some fake jaws use ground matrix, glue, carved material, or fragments of modern animal bone to create fake roots and jaw sections. Be cautious with any "complete" mosasaur jaw or skull unless it comes from a reputable source with clear provenance.

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Recommended Mosasaur Books and Educational Items:

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References / Scientific Sources

Lindgren, Johan, et al. (2014). Skin Pigmentation Provides Evidence of Convergent Melanism in Extinct Marine Reptiles. Nature. DOI: 10.1038/nature12899.

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

Lindgren, J., Caldwell, M. W., Konishi, T., Chiappe, L. M. (2010). Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur. PLOS ONE, 5(8): e11998. DOI: 10.1371/journal.pone.0011998.

Konishi, Takuya, Lindgren, Johan, Caldwell, Michael W., and Chiappe, Luis. (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.

Gallagher, W. B. (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, M. W., and Ősi, 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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