A file photo taken on July 10, 2007 shows the skeleton of a Brachiosaurus branchai dinosaur at the Museum of Natural History in Berlin. The giant, long-necked Brachiosaurus may have been as much as six schoolbuses lighter than previously thought, say scientists whose new computing method could place all dinosaurs in a lower weight class. (AFP PHOTO/DDP/MARCUS BRANDT)

Despite their large skeletons, dinosaurs were lighter than previously thought, a new study has found.

University of Manchester biologists used lasers to measure the minimum amount of skin required to wrap around the skeletons of modern-day mammals, including polar bears, giraffes and elephants.

They found the animals had almost exactly 21% more body mass than when they were just "skin and bones."

They applied this 21% body mass to a giant Brachiosaurus skeleton in Berlin's Museum fur Naturkunde. It had previously been estimated the Brachiosaurus weighed as much as 80 tonnes. The new calculations reduced that figure to just 23 tonnes.

"One of the most important things palaeobiologists need to know about fossilized animals is how much they weighed. This is surprisingly difficult," lead author of the study Dr. Bill Sellers said in a release Wednesday, noting their research supports the view "these animals were much lighter than traditionally thought."

The study is found in the journal Biology Letters.