Mosasurs were large marine reptiles and although a typical mosasaur tooth may look overly alarming they would have been fearsome predators. During the last 20 million years of the Cretaceous period, with the extinction of the ichthyosaurs and decline of plesiosaurs, mosasaurs became the dominant marine predators. There were many different species of Mosasaurs that ranged in size from less than a metre to possibly as much 18 metres. Mosasaurs had double-hinged jaws and flexible skulls (much like those of snakes), which enabled them to gulp down their prey almost whole. Their appearance was not dissimilar to a modern day monitor lizard.