File photo: The skeleton of a Brachiosaurus branchai dinosaur at the Museum of Natural History in Berlin.

Paris - One of the heftiest dinosaurs that strode the Earth may have weighed as much as six buses less than thought, according to a new formula that will also place other dinos in a lower weight class.

The giraffe-necked, tree-top-grazing Berlin Brachiosaur was previously estimated to weigh as much as 80 tons.

But a new calculation of its mass, published by scientists on Wednesday, suggests it would have tipped the scales at a relatively featherweight 23 tons.

“Our results would suggest that many of the previous estimates (for all dinosaurs) are indeed too heavy,” study author Bill Sellers told AFP of the findings by a team in Britain and the United States.

For most dinosaurs, the discrepancy would not be as big as that for the Brachiosaurus, “but certainly we would suggest that lighter estimates are likely to be correct”.

The team of biologists used 14 large-framed modern mammals including the bison, camel, elephant, giraffe, horse, elk, polar bear and rhinoceros to devise a new method of estimating body mass using only the skeleton.

“It's a mathematical technique that effectively wraps a skin as tightly as it can around the bones,” explained Sellers of the University of Manchester in northwestern England.

“This gives us a 'skin and bones' model (from) which we can measure the volume” - a method he said was “completely objective” and did not require artistic interpretation.

The study revealed that the weight of modern-day animals was 21 percent more than the so-called “wrapping volume” - which equation was then applied to the dinosaur bones.

“Mammals are only distantly related but the way they stand and move is broadly similar to four-legged dinosaurs so we think they are about as good a model as we can get,” said Sellers.

One earlier method involved taking an artist's reconstruction sculpture of the animal and measuring its volume by dipping it in water.

“We have done work estimating body mass in the past and were worried about the fact that we were relying on a certain amount of artistic interpretation and we wanted to find a method that avoided this and was completely objective,” Sellers said.

Body weight is key to determining how an animal lived - its lifespan, agility and food requirements.

“The weight of an animal is the single most important thing that a biologist needs to know - almost everything from anatomy to physiology to behaviour to ecology is dependent on body size.”

The Berlin Brachiosaurus, or Giraffatitan brancai, was chosen for the study as it is one of the most complete dinosaur fossil specimens available.

The animal lived in the late Jurassic period (from about 200 million to 145 million years ago), measuring about 25 metres from nose to tail. - Sapa-AFP