The remains of a giant meat-eating sea monster that patrolled the oceans during the reign of the dinosaurs have been unearthed on an island in the remote Arctic archipelago of Svalbard.

Norwegian fossil hunters recovered the rear half of the formidable reptile's skull in south-west Spitsbergen in what has been described as one of the most significant Jurassic discoveries ever made.

The predator has been identified as a new species of pliosaur, a group of extinct aquatic reptiles that had huge skulls, short necks and four flippers to power them through the water.

Measurements of the partial skull and 20,000 other bone fragments uncovered at the site showed that the creature was at the top of the food chain, preying on squid, fish and other marine reptiles.

The pliosaur's head was twice as big as that of a Tyrannosaurus rex and was filled with an impressive set of 12-inch teeth. Palaeontologists estimate the beast was 15 metres long, weighed 45 tonnes and hunted the oceans 147 million years ago.

"This is really big. We have parts of the lower jaw that are huge compared with anything we've ever seen," said Espen Madsen Knutsen, a palaeontologist on the team at the University of Oslo that studied the creature. "It could have eaten anything it came across."

Researchers got their first glimpse of the beast's remains on the last day of an expedition in 2007, during which they uncovered bones from a smaller pliosaur. Jørn Hurum at the University of Oslo's Natural History Museum, who led the expedition, noticed some large bones sticking out of the ground. The team marked the site, took a GPS reading of the location, and returned last August to excavate the remains.

Hurum's team was stunned to find the remains were from a larger pliosaur than any uncovered to date. They have spent recent months cleaning and measuring the bone fragments to build up a picture of how the creature would have looked when it was alive.

Among the remains, the team noticed a spherical bone, called a basioccipital condyle, found in all mammals and reptiles that joins the base of the skull to the spinal cord. The bone measured 15cm across, making it the largest of any pliosaur known, and twice the size of the same bone in T. rex.

To find out how the beast moved in the water, they called in Frank Fish, an expert on the biomechanics of flippers, at West Chester University in Pennsylvania. Using a wind tunnel, they reconstructed the forces that the creature's huge flippers generated and found that it probably cruised the oceans using its front two flippers only, deploying all four to lunge forwards and take its prey by surprise.

Using a CT (computed tomography) scanner on loan from the Natural History Museum in London, another team member, Patrick Druckenmiller from the University of Alaska, created a three dimensional image of the beast's brain, which showed it was small and elongated, similar to that of a great white shark.

The team then travelled to Florida's St Augustine Alligator Farm and Zoological Park to join evolutionary biologist Greg Erickson from Florida State University to work out how powerful the creature's bite was. Calculations based on the animal's jaw bones suggest it could have bitten into its prey with a force of 150 kilonewtons, or four times the force thought to be exerted by the jaws of a T. rex.

The discovery was announced as Canadian experts unveiled their own remarkable finding, the fossilised remains of what may have been the least fearsome predator ever to stalk the continent.

The carnivorous dinosaur, the smallest ever found in what is now North America, was the size of a small, skinny chicken, ran about on two legs wielding razor-sharp claws, and had an enlarged sickle-shaped claw on its second toe.

"It was half the size of a domestic cat and probably hunted and ate whatever it could for its size," said Nick Longrich, a palaeontologist at the University of Calgary, who led the expedition. Among its prey would have been insects, small mammals, amphibians and possibly small dinosaurs that lived in the swamps and forests of the late Cretaceous.

The remains of the diminutive dinosaur, called Hesperonychus, were excavated at the 75m-year-old Dinosaur Park Formation site in Alberta in 1982, but had lain unstudied for 25 years. When Longrich began studying the bones, he suspected they were from juveniles because they were so small, but closer inspection revealed them to be from adults, according to a report in Proceedings of the National Academy of Sciences.

"Its discovery just emphasises how little we actually know, and it raises the possibility that there are even smaller ones out there," said Longrich.

"Small carnivorous dinosaurs seemed to be completely absent from the environment, which seemed bizarre because today the small carnivores outnumber the big ones. It turns out that they were here and they played a more important role in the ecosystem than we realised. For the past 100 years, we've overlooked a major part of North America's dinosaur community," Longrich said.



