Norwegian scientists have discovered a "treasure trove" of fossils belonging to giant sea reptiles that roamed the seas at the time of the dinosaurs.

The 150-million-year-old fossils were uncovered on the Arctic island chain of Svalbard - about halfway between the Norwegian mainland and the North Pole.

The finds belong to two groups of extinct marine reptiles - the plesiosaurs and the ichthyosaurs.

One skeleton has been nicknamed The Monster because of its enormous size.

These animals were the top predators living in what was then a relatively cool, deep sea.

Palaeontologists from the University of Oslo's Natural History Museum discovered the fossils during fieldwork in a remote part of Spitsbergen, the largest island in the Svalbard archipelago.

Jorn Harald Hurum, co-director of the dig, said he was taken aback by the sheer density of fossil remains in one area.

"You can't walk for more than 100m without finding a skeleton. That's amazing anywhere in the world," he told BBC News.

Dr Dave Martill, a palaeontologist at the University of Portsmouth, UK, commented: "These sites are very unusual. To find that many individuals is a remarkable thing - that's a bonanza."

Ichthyosaurs bore a passing resemblance to modern dolphins, but they used an upright tail fin to propel themselves through the water.

Plesiosaurs are said to fit descriptions of Scotland's mythical Loch Ness monster. They used two sets of powerful flippers for swimming and came in two varieties - one with a small head and very long neck, and another with a large head and short neck.

The short-necked varieties are known as pliosaurs.

The discovery of a gigantic pliosaur, nicknamed The Monster, was one of the most remarkable discoveries of the expedition.

Its skeleton has dinner-plate-sized neck vertebrae, and the lower jaw has teeth as big as bananas.

Tooth in the neck

The skeleton is not yet fully excavated, but its skull is about 3m long, suggesting the body could be more than 8m from the tip of its nose to its tail.

"What's amazing here is that it looks like we have a complete skeleton. No other complete pliosaur skeletons are known anywhere in the world," said Dr Hurum.

The researchers even found evidence of an attack on one of the creatures. An ichthyosaur tooth is embedded in a neck vertebra from one plesiosaur belonging to the genus Kimmerosaurus.

The fossil hoard comprises 21 long-necked plesiosaurs, six ichthyosaurs and one short-necked plesiosaur. The bones were unearthed in fine-grained sedimentary rock called black shale.

"Everything we're finding is articulated. It's not single bones here and there, and bits and pieces - these are complete skeletons," said Dr Hurum.

After death, the carcasses came to rest in mud at the bottom of the deep ocean, where little or no oxygen was present.

Dr Hurum said an unusual chemistry of the mud could have been responsible for the remarkable preservation of the specimens: "Something happened with the chemistry that's really good for bone preservation. Some skeletons are pale white even though they're in black shale - they look like 'roadkill'."

The marine reptiles found in the Norwegian archipelago are very similar to ones known from southern England. Dr Hurum said the animals could have been living in the same ocean and he now plans to compare the Arctic finds with those from Britain.

The Svalbard excavation was led by Dr Hurum and Hans Arne Nakrem, also of Oslo's Natural History Museum. The museum plans to return to the field site in the summer of 2007 to resume excavations.