Mosasaurus. Artwork of a Mosasaurus marine lizard swimming underwater. This large carnivorous lizard was around 9 metres in length. Its anatomy was similar to present day monitor lizards, and it is considered a close relative. It inhabited the seas of the late Cretaceous period, from 100-65 million years ago. It would have fed on fish, but probably also hunted smaller marine reptiles. It used its tail for propulsion, steering with its limbs.