Mosasaurs were a kind of marine reptile, which florished in the late cretaceous. They lived quite shortly, first Mosasaurs envolved in the middle cretaceous, and died out again and the end of the age of dinosaurs. But in that short period of time they became incredibily succesfull, Mosasaur fossils have been found all over the world and the largest species (Hainosaurus) was up to 17m in length. Mosasaurs were the oceans toppredators, they ate everything that was unluckily enough to cross their path, like fish, squid, turtles, ammonites, other marine reptiles, sharks, birds, and their own kind as well.
Mosasaurs were not related to other marine reptiles, they where actually moniter lizards who returned back to the sea. In the middle cretaceous the Ichtyosaurs died out, so there was plenty of room for a new kind of marine reptile. The lizards adapted to a life in water to fill in that niche.
Although they where the toppredators, there infants were often eaten by sharks. One Mosasaur fossil bears the marks of a shark bite in its spine. Recently, scientists discovered that large Mosasaurs gave birth to there young alive. The smaller species, like Halisaurus, might have been able to get out of the water and lay their eggs on the beach.