Mosasaurs were large marine reptiles that resembled a cross between a crocodile and a giant iguana. The mosasaurs are believed to have completely died out after a huge asteroid impact caused Earth to drastically change.

Mosasaurs were serpentine marine reptiles and ferocious predators. Mosasaurs were not dinosaurs but evolved from semi-aquatic squamates known as the aigialosaurs, close relatives of modern-day monitor lizards and snakes. This happened about 95 MYA, during the Cenomanian age of the Cretaceous.