Masasaurus The Largest Marine Predators

If the Liopleurodon was huge, than Mosasaurus was colossal.
Fossil evidence suggests that Mosasaurus could reach as much as 50 feet (15m) in length, making it one of the largest marine predators of the Cretaceous period.
 Mosasaurus’s head was like that of a crocodile, lined with hundreds of razor sharp teeth which could kill even the most well-armored enemies.

Tanystropheus was a reptile that could reach 20 feet (6m) long

While Tanystropheus was not strictly marine, its diet was mainly fish and scientists think it spent most of its time in the water.
 Tanystropheus was a reptile that could reach 20 feet (6m) long, and it is thought to have been alive during the Triassic period nearly 215 million years ago.

Tylosaurus a species of Mosasaur

Tylosaurus was a species of Mosasaur. It was enormous, reaching more than 50 feet (15m) in length.
The tylosaurus was a meat eater with a very diverse diet. Stomach remains show signs of fish, sharks, smaller mosasaurs, plesiosaurs, and even some flightless birds.
 They lived during the late Cretaceous in the seas that covered North America, where they sat firmly atop the marine food chain for several million years.