But the newfound plesiosaur is thought to have been a pliosaur, and pliosaurs were different from other plesiosaurs.
With short necks and massive heads, pliosaurs became the top marine predators during the Jurassic period, 200 to 145 million years ago.
Hurum said the newly excavated specimen is 20 percent bigger than what was until now the largest known pliosaur, Kronosaurus from Australia.
Calling the latest find "the T. rex of the ocean," Hurum said it "would have eaten other marine reptiles and maybe some of the huge bony fishes that were around at that time."
The newly excavated pliosaur was unveiled today at the Natural History Museum in Oslo.
Patrick Druckenmiller, a plesiosaur expert at the University of Alaska's Museum of the North, was a member of the expedition team that found and excavated the Arctic fossil.
"Not only is this specimen significant in that it is one of the largest and relatively complete plesiosaurs ever found, it also demonstrates that these gigantic animals inhabited the northern seas of our planet during the age of dinosaurs," Druckenmiller commented.
"Although we didn't get the entire skeleton, we found many of the most important parts," Druckenmiller said. "Amazingly, the paddle [of its forelimb] alone is nearly ten feet [three meters] long."
The fossil was found in permafrost among a prehistoric "graveyard" of large marine reptiles approximately 800 miles (1,300 kilometers) from the North Pole.
(See related pictures: "'Sea Monster' Graveyard Found in the Arctic".)
The site represents one of the richest accumulations of marine reptiles in the world, according to Hurum, who led the Artic fossil hunt.
The creatures swam in temperate seas and sank to the ocean floor after they died, where their bodies were preserved in soft mud.
Some 40 skeletons have been located to date. Most belonged to long-necked plesiosaurs and dolphin-shaped ichthyosaurs, Hurum said.
Another pliosaur specimen was also found last year, however.
"Hopefully it will be just as big as the first," Hurum said.
This newest fossil may help provide pieces that are missing from Monster's partial skeleton, he added.
"It seems to have more teeth, and hopefully there's more of a skull inside the hill [where it was found]. But first we have to move the hill," Hurum said.
The researchers plan to excavate the second pliosaur when they return to Spitsbergen this summer.
Plesiosaur expert Richard Forrest, affiliated with the New Walk Museum in Leicester, England, said pliosaur skeletons are extremely rare.
"As is the case with any big predator, the further up the food chain you go, the fewer you find," Forrest said.
Since pliosaurs were reptiles, which have slow metabolisms, they probably had long breaks between meals, he said.
"If [Monster] was eating something like a large plesiosaur, it would probably get enough food to keep it going for a couple of months," Forrest added.
Pliosaurs are thought to have been ambush predators, using their giant flippers to launch ferocious attacks.
"We don't think they were particularly good at cruising but were very good at accelerating, so they'd lurk in the depths and shoot up to catch things," Forrest said.
Pliosaurs likely had the most powerful bite force of any predator, living or extinct, he added.
"It can't think of any animal that would even come close," Forrest said.
"Inside their enormous skulls they had huge areas of muscle available for biting force. One of these animals would have been big and strong enough to pick up a small car and bite it in half."
Forrest added that he recently studied the remains of a 23-foot-long (7-meter) plesiosaur that almost certainly fell victim to a pliosaur.
"There's evidence that it was hit extremely hard and basically just ripped apart," he said.
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