Mosasaurs first appeared in the mid to late cretaceous period, proliferated and were extinct at the end of the period. They existed between 96 to 65 million years ago, and their extinction was primarily due to the KT mass extinction. Similar to the extinction of dinosaurs, there were several events that led up to their extinction.

 

From the asteroids and volcanic activity, to the collapse of food chain and deprivation of oxygen in the waters, it was only a matter of time before these species went extinct. Finally, the species seemed to have been outcompeted by sharks, as they had better motility, body plans, smarter, and were better hunters. Sharks survived several extinction events, evolved and were able to dominate, whereas mosasaurs died off.