The bones, which belonged to a creature called a champsosaur, were unearthed at Axel Heiberg Island in northern Canada.

The fossils of fish and turtles were also discovered with the reptile.

The bones included this vertebra

What we know of champsosaurs and modern crocodiles, says the team that found the animal, in particular their size, lifestyle and breeding behaviour, means temperatures in the Arctic during the late Cretaceous period must have been much higher than today to allow them to live so far north.

The researchers from the University of Rochester, New York, estimate that the annual mean temperature in that part of the world was greater than 14C ( 57F).

This implies it was rarely, if ever, freezing during the winter, and summer temperatures would have consistently reached into the late 20s and early 30s Celsius (80s and 90s Fahrenheit). In other words, the climate in this part of the Arctic would have been similar to modern-day Florida.

Global warming

The evidence from fossilised plants has encouraged scientists to think that the late Cretaceous was a warm time - but not this warm. The Rochester team says global warming would be one explanation.

The Canadian Arctic: Once like Florida

The fossils were found in a layer of sediment right on top of 300 metres (1,000 ft) of hardened lava, known as basalt. Geologists know that right across the globe this basalt was pumped onto the surface of the Earth in vast quantities and over thousands of years.

These floods of boiling rock would have released many millions of tonnes of gas into the atmosphere including huge amounts of carbon dioxide - the main gas though to drive the greenhouse effect.

"We can't avoid the fact that these fossils are sitting right on top of this extremely large volcanic eruption," says Professor John Tarduno who led the expedition that found the champsosaur bones.

"And if you look around the world, it was an unusually active time, with many eruptions occurring at the same time. It's very reasonable to suggest that so much CO2 was dumped into the atmosphere that it overwhelmed the system, causing global warming."

Climate puzzle

The scenario painted by Rochester will need further explanation because it is not easy to have such warm polar temperatures without also having unrealistically high equatorial temperatures.

The tibia allowed the team to work out the size of the creature

"This will be a puzzle for people who model climate," says Tarduno, "but the fossils, together with the radiometric dating, provide very hard evidence of extremely warm temperatures in the Arctic."

The team found a variety of champsosaur bones from jaw bones and abdominal ribs to hip bones and backbones. The creature would have lived a semi-aquatic existence and would have been about 2.4 metres long (8 ft).

The champsosaur find is reported in the journal Science.