These were the serpentine marine murderers of the Late Cretaceous (90-65 million years ago) Oceans, and enjoyed a brief yet violent reign of around 20-25 million years. They then became extinct; along with over 65% of all life on Earth, during the cataclysmic disaster that also much more famously, killed off ( or simply finished off) the Dinosaurs. They could be over 17 metres long – roughly the same size as the Basilosaurid Predatory Whales that would thrive and also then perish, some 30 million years or so after the Mosasaurs. They ate anything and everything, not even sparing their own kind. It is likely that Snakes and Monitor Lizards, are the cousins of these extinct monsters.