Mosasaurs were air-breathing marine reptiles that appeared in the Early Cretaceous, descended from aquatic lizards. They radiated into considerable diversity, with a peak of 38 genera.
During the last 20 million years of the period, with the extinction of the ichthyosaurs and pleiosaurs, mosasaurs became the dominant seafaring predator. Powerful swimmers, they coursed through the water using their strong tails, like sharks and ichthyosaurs. Their 4 flipper limbs provided fine movement control.
The smallest mosasaur was less than 1 meter long. Larger mosasaurs were more typical, with many species over 4 meters. The largest mosasaur reached 17 meters.
Mosasaurs had large, sharp teeth in double-hinged jaws, long snouts, and flexible skulls, much like sharks (an instance of convergent evolution), affording a fearsome bite and the ability to swallow large chunks. Sharks were mosasaurs’ main competitors: an evolutionary pressure which may have refined sharks toward their modern forms. Both sharks and mosasaurs had a worldwide presence.
Mosasaurs may have been endothermic, as were ichthyosaurs – an unusual adaptation for reptiles.
Today’s monitor lizards are a close relative to mosasaurs, which went extinct 66 MYA.