By Paul Rincon
Science reporter, BBC News
Pliosaur (Tor Sponga, BT)
A fossilised "sea monster" unearthed on an Arctic island is the largest marine reptile known to science, Norwegian scientists have announced.
The 150 million-year-old specimen was found on Spitspergen, in the Arctic island chain of Svalbard, in 2006.
The Jurassic-era leviathan is one of 40 sea reptiles from a fossil "treasure trove" uncovered on the island.
Nicknamed "The Monster", the immense creature would have measured 15m (50ft) from nose to tail.
A large pliosaur was big enough to pick up a small car in its jaws and bite it in half
Richard Forrest, plesiosaur palaeontologist
And during the last field expedition, scientists discovered the remains of another pliosaur which is thought to belong to the same species as The Monster - and may have been just as colossal.
The expedition's director Dr Jorn Hurum, from the University of Oslo Natural History Museum, said the Svalbard specimen is 20% larger than the previous biggest marine reptile - a massive pliosaur from Australia called Kronosaurus.
"We have carried out a search of the literature, so we now know that we have the biggest [pliosaur]. It's not just arm-waving anymore," Dr Hurum told the BBC News website.
"The flipper is 3m long with very few parts missing. On Monday, we assembled all the bones in our basement and we amazed ourselves - we had never seen it together before."
Young girl beside pliosaur flipper (J. Hurum)
The Monster's flipper alone measures 3m in length
Pliosaurs were a short-necked form of plesiosaur, a group of extinct reptiles that lived in the world's oceans during the age of the dinosaurs.
A pliosaur's body was tear drop-shaped with two sets of powerful flippers which it used to propel itself through the water.
"These animals were awesomely powerful predators," said plesiosaur palaeontologist Richard Forrest.
A second large pliosaur has now been found on the Arctic island
"If you compare the skull of a large pliosaur to a crocodile, it is very clear it is much better built for biting... by comparison with a crocodile, you have something like three or four times the cross-sectional space for muscles. So you have much bigger, more powerful muscles and huge, robust jaws.
"A large pliosaur was big enough to pick up a small car in its jaws and bite it in half."
The Monster was excavated in August 2007 and taken to the Natural History Museum in Oslo. Team members had to remove hundreds of tonnes of rock by hand in high winds, fog, rain, freezing temperatures and with the constant threat of attack by polar bears.
Lena Kristiansen prepares specimens in the Natural History Museum, University of Oslo.
'Monster' fossil find in Arctic
They recovered the animal's snout, some teeth, much of the neck and back, the shoulder girdle and a nearly complete flipper.
Unfortunately, there was a small river running through where the head lay, so much of the skull had been washed away.
A preliminary analysis of the bones suggests this beast belongs to a previously unknown species.
The researchers plan to return to Svalbard later this year to excavate the new pliosaur.
A few skull pieces, broken teeth and vertebrae from this second large specimen are already exposed and plenty more may be waiting to be excavated.
"It's a large one, and has the same bone structure as the previous one we found," said Espen Knutsen, from Oslo's Natural History Museum, who is studying the fossils.
Dr Hurum and his colleagues have now identified a total of 40 marine reptiles from Svalbard. The haul includes many long-necked plesiosaurs and ichthyosaurs in addition to the two pliosaurs.
Artist's impression of long-necked plesiosaur (Tor Sponga, BT)
Excavations have also yielded long-necked plesiosaurs
Long-necked plesiosaurs are said to fit descriptions of Scotland's mythical Loch Ness monster.
Ichthyosaurs bore a passing resemblance to modern dolphins, but they used an upright tail fin to propel themselves through the water.
Richard Forrest commented: "Here in Svalbard you have 40 specimens just lying around, which is like nothing we know.
Exacavation at the Monster site
The 2007 fieldwork took place in challenging conditions
"Even in classic fossil exposures such as you have in Dorset [in England], there are cliffs eroding over many years and every so often something pops up. But we haven't had 40 such finds from Dorset in 200 years."
The fossils were found in a fine-grained sedimentary rock called black shale. When the animals died, they sank to the bottom of a cold, shallow Jurassic sea and were covered over by mud. The oxygen-free, alkaline chemistry of the mud may explain the fossils' remarkable preservation, said Dr Hurum.
The discovery of another large pliosaur was announced in 2002. Known as the "Monster of Aramberri" after the site in north-eastern Mexico where it was dug up, the creature could be just as big as the Svalbard specimen, according to the team that found it.
But palaeontologists told the BBC a much more detailed analysis of these fossils was required before a true picture of its size could be obtained.