University of Bristol logo Fossil group banner, with title and coposite picture of fossils



Mosasaurs breathed air, but were powerful swimmers, so well adapted to living in shallow seas that they gave birth to live young, rather than return to the shoreline to lay eggs, as sea turtles do. Mosasaurs likely descended from varanid lizards. These predators evolved from terrestrial ancestors in the Early Cretaceous and dominated the oceanic food chain during the Late Cretaceous Period.

The smallest known mosasaur is Carinodens belgicus, which was about 3 to 3.5 m long and probably lived on the sea floor cracking mollusks and sea urchins with its bulbous teeth. Larger mosasaurs were more typical: mosasaurs ranged in size up to 17 m: Hainosaurus holds the record for longest mosasaur, at 17.5 m. Other genera include Mosasaurus, Tylosaurus, Plotosaurus, and Platecarpus.

Mosasaurs had a body form similar to that of a crocodile, although streamlined for fast swimming in marine waters. Their front leg bones were reduced in length, their paddles strengthened by long finger-bones. Their rear legs were atrophied. Their powerful tails, lashed side to side crocodile-fashion, provided locomotion.
Mosasaurs had a loosely-hinged jaw which enabled them to gulp down their prey almost whole, a snakelike habit that has helped identify the stomach contents fossilized within a Mosasaur skeleton, which included the diving seabird Hesperornis, a marine bony fish, a shark, and part of a smaller mosasaur. Mosasaur bones have been found with embedded shark teeth.
Based on features such as the loosely-hinged jaw, modified/reduced limbs and probable locomation, many researchers believe that snakes may be descended from mosasaurs, a suggestion advanced in 1869, by Edward Drinker Cope, who coined the term "Pythonomorpha" to include them. The idea lay dormant for more than a century, to be revived in the 1990s.

Sea-levels were high during the Cretaceous, causing marine ingressions in many parts of the world, and a great inland seaway in North America. Mosasaur fossils have been found in the Netherlands and Sweden, in Africa, in Australia, New Zealand, and Vega Island off the coast of Antarctica. In Canada and the United States, complete or partial specimens have been found in Alabama and Georgia and in almost all the states covered by the seaway: Texas, southwest Arkansas, New Mexico, Kansas, Colorado, the Dakotas and Montana.
The "dinosaurs" of New Zealand, a volcanic island arc that has never been part of a continent, are a unique series of mosasaurs and plesiosaurs, another group of predatory marine reptiles of the Mesozoic era.

On November 16 2005, research in Netherlands Journal of Geosciences confirmed that the recently uncovered Dallasaurus turneri, the first Mosasaur discovered in North America, is an early link between land-based, komodo dragon-like varanid lizards and the aquatic mosasaurs.

  • Hainosaurus
  • Up to 17 m long
  • Late Cretaceous
  • Europe (France, Belgium)








back to top

  • Plotosaurus
  • Up to 10 m long
  • Late Cretaceous
  • North America (Kansas)
















back to top

  • Platecarpus
  • Up to 4.5 m long
  • Late Cretaceous
  • Europe (Belgium), North America (Alabama, Colorado, Kansas and Mississippi)










back to top



Author: Koen Stein
Last updated: 21/11/2005
Return to Fossil groups home page

Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2005-6