Mosasaurus, meaning Lizard of the Meuse River, were aquatic reptiles during the Upper/late cretaceous period, between 100.5-66 Million Years ago. After the extinction of Pliosaurs and Ichthyosaurs they became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the cretaceous period.
Mosasaurs had double-hinged jaws and flexible skulls (much like those of snakes), which enabled them to gulp down their prey almost whole. They also had a second row of teeth in their upper jaw.
Dependant on species, Mosasaurs could grow between 3-50 feet in length. Globidens Mosasaurs evolved rounded stubby teeth as they fed mainly on shelled creatures, these teeth acted like a hammer to crush the shells of their prey.