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Mosasaur facts for kids

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Temporal range: Upper Cretaceous
Mosasaurus hoffmanii
Scientific classification

Conybeare 1822
Modern reconstruction of Platecarpus tympaniticus showing crescent-shaped tail fluke

Mosasaurs were large, predatory marine lizards of the Upper Cretaceous. The first fossil Mosasaur, Mosasaurus hoffmanni, was found in the Netherlands in 1776.p7 It was named in 1822 by W.D. Conybeare.


Mosasaurs breathed air, were powerful swimmers, and were well-adapted to living in the warm, shallow epicontinental seas of the Upper Cretaceous. Mosasaurs 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. Thousands of fossils have been found, from every continent. There are about 40 known species. Most skeletons have been found in North America in chalk laid down in the Western Interior Seaway.

An interesting fact is that ichthyosaurs had died out by the middle of the Upper Cretaceous, and also plesiosaurs and sea-going crocodiles were in decline. The reasons for this are not known; perhaps there was increased competition from large predator fish. It is this large marine predator niche that mosasaurs apparently occupied. They gave birth to live young, just as the Ichthyosaurs had done. They flourished in the later Cretaceous, only to become extinct at the K/T extinction event.


Mosasaur skull
Plioplatecarpus primaevus skull at the Oxford University Museum of Natural History.

The type species was estimated to be 33' (10m) long. Mosasaurus had four paddle-like limbs on a long, streamlined body and a long, powerful tail. The large head had huge jaws, up to 4 ft (1.2 m long) with many teeth. The jaws could open about 3 feet (1 m). The lower jaw is loosely hinged to the skull with a moveable joint on each side (behind the teeth). This loose joint let it swallow huge prey. They would have hunted fish, turtles, molluscs, and shellfish. Ammonites have been found bearing mosasaur teeth marks.

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 molluscs and sea urchins with its bulbous teeth. Larger mosasaurs were more typical: mosasaurs ranged in size up to 17 metres (56 ft). Hainosaurus holds the record for longest mosasaur, at 17.5 metres (57 ft).

Other species

Many other species of mosasaur have since been found, for example, Tylosaurus. Mosasaurus maximus was found at Onion Creek, Texas. All have the same general body style and pattern of life, though some, like Prognathodon, had crushing teeth for dealing with shellfish. Present thought is that their closest living relatives are the monitor lizards.

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