They belong to the order Squamata, which includes lizards and snakes. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Earliest Late Cretaceous. ... They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.

Is there a real Mosasaurus?

That said, the real Mosasaurus was indeed a large animal, with the biggest specimen known estimated to be around 17 meters or 56 feet long (Grigoriev, 2014). ... Mosasaurus was roughly similar in length to a modern sperm whale (though probably much lighter in mass), which is pretty darn big for a lizard.

What killed the Mosasaurus?

Mosasaurs went extinct during the Cretaceous-Paleogene extinction event which killed all of the dinosaurs. Mosasaurs are related to the living Komodo dragon, but they have flipper-limbs which suit them for an aquatic life.