The colossal creature would have weighed 60 tonnes and stood 18 metres (60 feet) tall. This ground-shaking monster has been named Sauroposeidon, which means "earthquake god lizard".

The researchers from the University of Oklahoma think the Sauroposeidon had the longest neck in the fossil record.

Sauroposeidon towers over brachiosaurus which itself dwarfs man (Mat Wedel)

"It's truly astonishing. It's arguably the largest creature ever to walk the earth," said Richard Cifelli.

Professor Cifelli led the team that examined bones unearthed in south-eastern Oklahoma in 1994. When they were first catalogued, he said he thought they might be the trunks of prehistoric trees.

But closer examination revealed that they belonged to a larger relative of the better-known Brachiosaurus, which could stretch its head up to 13.5 metres (45 feet). The diplodocus, often regarded as the biggest creature ever, reached 15m (50ft).

Each of the neck bones of the creature is about 120 centimetres (four feet) long.

"The neck on our creature is about a third longer than that of the Brachiosaurus," said team member Mathew Wedel. "It's a lot longer and a lot more specialised."

Like a giraffe - but bigger

Sauroposeidon was giraffe-like in shape, with a short body and long neck, but was 30 times larger than the largest giraffe ever known.

The massive load was made lighter by the bones being filled with tiny air cells, revealed by scanning the fossils at the university's hospital.

"No matter how small the dinosaur's brain was, just lifting it up was a challenge," Professor Cifelli said. "It's remarkable how large the bones are."

It would be very hard to imagine that a neck could get much longer and still function, he added.

"An old design"

"He's an old design. By this time, that body plan is just not working anymore. By the time this guy comes along, they are dying out in North America. He is pretty much the last of his kind."

Sauroposeidon lived about 110 million years ago. At that time, Sauroposeidon inhabited the delta of a massive river system. The Gulf of Mexico had swamped most of Texas, bringing the shoreline to Oklahoma.

The findings are due to be published in the March issue of the Journal of Vertebrate Paleontology.