There were monstrous reptiles in the deep, back in the time of dinosaurs.

They swam with mighty flippers, two fore and two hind, all four accelerating on attack. In their elongated heads were bone-crushing jaws more powerful than a Tyrannosaurus rex’s. They were the pliosaurs, heavyweight predators at the top of the food chain in ancient seas.

Much of this was already known. Now, after an analysis of fossils uncovered on a Norwegian island 800 miles from the North Pole, scientists have confirmed that they have found two partial skeletons of a gigantic new species, possibly a new family, of pliosaurs.

This extinct marine reptile was at least 50 feet long and weighed 45 tons, the largest known of its kind. Its massive skull was 10 feet long, and the flippers, more like outsize paddles, were also 10 feet. The creature  not yet given a scientific name but simply called the Monster or Predator X  hunted the seas 150 million years ago, in the Jurassic Period.

“Everything we are finding is new to science,” said Jorn H. Hurum, a paleontologist at the University of Oslo who directed the excavations on the Arctic island of Spitsbergen in the Svalbard archipelago. He described new details of the find in a telephone interview last week.