Estimating the weight of prehistoric animals by examining their fossilized skeletons is notoriously difficult. Estimates for the weight of larger species, such as dinosaurs, can vary by a factor of three or four. Now British researchers have developed a new way to estimate weight and conclude that our previous estimates are much too high: Dinosaurs were, in fact, much lighter than we thought.

A team headed by biologist William I. Sellers of the University of Manchester used a laser scanner to compile a three-dimensional image of the skeletons of various animals, then calculated the minimum amount of skin that would be necessary to cover the skeleton. Using well-known estimates for the average density of tissue, they could then calculate the weight of the animal. When they applied this technique to animals whose weight was known, including reindeer, polar bears, giraffes and elephants, they consistently underestimated body mass by 21%, they reported in the journal Biology Letters.

The team then went to Berlin’s Museum fur Naturkunde and scanned the nearly complete skeleton of the Berlin brachiosaurus, Giraffatitan brancai, one of the largest complete sauropod dinosaurs. Using their algorithm, they then concluded that the living animal weighed 23,200 kilograms (51,150 pounds). Previous estimates for the brachiosaurus mass had gone as high as 80 metric tons, or 176,000 pounds.

“Our method provides a much more accurate measure and shows dinosaurs, while still huge, are not as big as previously thought,” Sellers said.

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