Mosasaurus marine reptile skull fossil. Fossilised skull of a juvenile Mosasaurus beaugei marine reptile. Mosasaurus marine reptiles were predatory carnivores that reached around 9 metres in length. Their anatomy was similar to present day monitor lizards. They inhabited the seas of the Late Cretaceous Period, from 100 million to 65 million years ago. They fed on fish, but probably also hunted smaller marine reptiles. This specimen is 70 million years old, and was found in Mauritania.