Mosasaurs (from Latin Mosa meaning the ‘Meuse river', and Greek meaning 'lizard') are large, extinct, marine reptiles. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1770s.

During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian), with the extinction of the Ichthyosaurs and decline of Plesiosaurs, mosasaurs became the dominant marine predators.

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 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 Dallasaurus turneri, which was less than a 1 metre (3.3 ft) long. Larger mosasaurs were more typical: Hainosaurs holds the record for longest mosasaur, at 17.5 metres (57 ft).

Mosasaurs has a body shape similar to that of modern-day monitor lizards (verandas), but were more elongated and streamlined for swimming. Their limb bones are reduced in length and their paddles were formed by webbing between their elongated digit-bones. Their tails were broad, and supplied the locomotive power.