Old Earth Ministries Online Dinosaur Curriculum

Free online curriculum for homeschools and private schools

From Old Earth Ministries (We Believe in an Old Earth...and God!)

NOTE:  If you found this page through a search engine, please visit the intro page first. 


Lesson 74 - Mosasaurs

Mosasaurs (from Latin Mosa meaning the 'Meuse river', and Greek sauros meaning 'lizard') are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs are now considered to be the closest relatives of snakes, due to cladistic analyses that have taken into account similarities in jaw and skull anatomies. Mosasaurs were varanoids closely related to terrestrial monitor lizards. They probably evolved from semi-aquatic squamates known as aigialosaurs, which were more similar in appearance to modern-day monitor lizards, in the Early Cretaceous. During the last 20 million years of the Cretaceous Period (Turonian-Maastrichtian), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators.


Quick Facts


Length:  Smallest species was about 10 feet, largest species was 57 feet

Date Range:   85 - 65 Ma, Cretaceous Period



Mosasaurus hoffmannii skeleton, Natural History Museum of Maastricht, The Netherlands  (Picture Source)


Mosasaurs breathed air, were powerful swimmers, and were well-adapted to living in the warm, shallow epicontinental seas prevalent during the Late Cretaceous Period. Mosasaurs
Mosasaur Platecarpus tympaniticus
Modern reconstruction of Platecarpus tympaniticus showing crescent-shaped tail fluke   (Picture Source
 were so well adapted to this environment that they gave birth to live young, rather than return to the shore to lay eggs, as sea turtles do.

The smallest-known mosasaur was Carinodens belgicus, which was about 3.0 metres (9.8 ft) to 3.5 metres (11 ft) long and probably lived in shallow waters near shore, cracking mollusks and sea urchins with its bulbous teeth. Larger mosasaurs were more typical: mosasaurs ranged in size up to 17 metres (56 ft). Tylosaurus holds the record for longest mosasaur, at 17.5 metres (57 ft).

Mosasaurs had a body shape similar to that of modern-day monitor lizards (varanids), but
Mosasaur Tylosaurus
Tylosaurus pembinensis specimen nicknamed "Bruce"  (Picture Source
 were more elongated and streamlined for swimming. Their limb bones were reduced in length and their paddles were formed by webbing between their elongated digit-bones. Their tails were broad, and supplied the locomotive power. This method of locomotion may have been similar to that used by the conger eel or sea snakes today. However, more recent evidence suggests that many advanced mosasaurs had large crescent-shaped flukes on the ends of their tails similar to those of sharks and ichthyosaurs. Rather than snake-like undulatory movement, the body probably remained stiff in these mosasaurs to improve hydrodynamic efficiency through the water while the end of the tail provided strong propulsion. The animal may have lurked and pounced rapidly and powerfully on passing prey, rather than hunting for it.

Mosasaurs had a double-hinged jaw and flexible skull (much like that of a snake), which enabled them to gulp down their prey almost whole, a snakelike habit which helped identify the unmasticated gut contents fossilized within mosasaur skeletons. A skeleton of Tylosaurus proriger from South Dakota included remains of the diving seabird Hesperornis, a marine bony fish, a possible shark and another, smaller mosasaur (Clidastes). Mosasaur bones have also been found with shark teeth embedded in them.
Based on features such as the double row of pterygoid ("flanged") teeth on the palate, the double-hinged jaw, modified/reduced limbs and probable methods of locomotion, many researchers believe that snakes and mosasaurs may have had a common ancestor. This theory was first suggested in 1869, by Edward Drinker Cope, who coined the term "Pythonomorpha" to include them. The idea lay dormant for more than a century, before being revived in the 1990s.

Soft tissue

Despite the relatively high number of mosasaur remains collected worldwide, knowledge of the nature of their skin coverings remains in its early stages. An incredibly small amount of mosasaurid specimens collected from around the world retain fossilized scale imprints; this
Mosasaur skin
Scales of Tylosaurus proriger (KUVP-1075)
 lack of knowledge is possibly due to the delicate nature of the scales, which nearly eliminates possibility of preservation, in addition to the preservation sediments types and the marine conditions under which the preservation occurred. Until the discovery of several mosasaur specimens along with their remarkably well preserved scale imprints from late Maastrichtian deposits of the Muwaqqar Chalk Marl Formation of Harrana in Jordan, knowledge of the nature of mosasaur integument was mainly based on very few accounts describing early mosasaur fossils dating back to the upper Santonian-lower Campanian such as the famous Tylosaurus specimen (KUVP-1075) from Cove County, Kansas. Material from Jordan has shown that the body of mosasaurs, as well as the membrane between the fingers and toes, was covered with small overlapping diamond-shaped scales resembling those of snakes. Much like modern reptiles, there existed regional variations in the type and size of the scales that covered the mosasaurs. In Harrana specimens, two types of scales were observed on a single specimen, keeled scales covering the upper regions of the body as well as smooth scales covering the lower regions. As ambush predators, lurking and quickly capturing prey using stealth tactics, it is suggested mosasaurs benefited greatly from the non-reflective keeled scales.

More recently, a well preserved fossil of Platecarpus tympaniticus has been found that preserves not only skin impressions, but also internal organs. There are several reddish areas in the fossil that may represent the heart, lungs, and kidneys. The trachea is also preserved along with part of what may be the retina in the eye. The placement of the kidneys is farther forward in the abdomen than it is in monitor lizards, and is more similar to that of cetaceans. As in cetaceans, the bronchi leading to the lungs run parallel to each other instead of splitting apart from one another as in monitors and other terrestrial reptiles. In mosasaurs, these features may be internal adaptations to a fully marine lifestyle.

In 2011, collagen protein was recovered from a Prognathodon humerus dated to the Cretaceous.


Sea levels were high during the Cretaceous Period, causing marine transgressions in many parts of the world and a great inland seaway in what is now North America. Mosasaur fossils have been found in the Netherlands, Belgium, Denmark, Portugal, Sweden, the United Kingdom, Angola, Morocco, Australia, New Zealand, and on Vega Island off the coast of Antarctica. Mosasaurs have been found in Canada in Manitoba and Saskatchewan and in much of the contiguous United States. Complete or partial specimens have been found in Alabama, Mississippi, Tennessee, and Georgia -- as well as in states covered by the Cretaceous seaway: Texas, southwest Arkansas, New Mexico, Kansas, Colorado, Nebrask, South Dakota, Montana, and the Pierre Shale/Fox Hills formations of North Dakota. Lastly, mosasaur bones and teeth are also known from California, Mexico, and Peru.

Many of the so-called 'dinosaur' remains found on New Zealand are actually mosasaurs and plesiosaurs, both being Mesozoic predatory marine reptiles.

Evolutionary antecedents

Based on features such as the loosely-hinged jaw, modified/reduced limbs and probable locomotion, many researchers believe that snakes share a common marine ancestry with mosasaurs, a suggestion advanced in 1869, by Edward Drinker Cope, who coined the term "Pythonomorpha" to unite them. The idea lay dormant for more than a century, to be revived in the 1990s. Recently, the discovery of Najash rionegrina, a fossorial snake from South America cast doubt on the marine origin theory.

In 2005, research reported in Netherlands Journal of Geosciences, confirmed that the recently uncovered Dallasaurus turneri is an early link between land-based monitor lizards (such as the Komodo dragon) and the aquatic mosasaurs.

In popular culture

Mosasaurs appear in the BBC television series Sea Monsters. Mosasaurs also feature heavily in the ITV television series Primeval. In this show, the mosasaurs are depicted incorrectly as having skin more like a crocodile's. A highly evolved mosasaur appeared in an episode of Godzilla: The Series. It was inhabiting Loch Ness as the legendary Loch Ness Monster.

The IMAX 3D film "Sea Monsters" features several mosasaurs (including Tylosaurus and Platecarpus) with detailed animated recreations of their movements and activities.
In Disney's Fantasia, there is a scene where a pod of mosasaurs are swimming by and one of them grabs a pteranodon.



Return to the Old Earth Ministries Online Dinosaur Curriculum homepage.

horizontal rule


Bay State Replicas - T-rex teeth, claws, skulls, jaws, foot, rib, femur, complete skeleton (1/20 scale), brain cavity

Black Hills Institute - Full size skulls, skeletons, 1/6th scale disartictulated skull, teeth, arm, leg, femur, foot, claws.  Wall-mounted half-skull.