How much does a stegosaurus weigh?

Paleontologists wish they could be so rude as to walk up and ask a dinosaur her weight. Traditionally, they’ve guessed the mass of the long-dead dinos by making estimates based on the size of the bones in their thighs and upper arms. That has proved to be as accurate as they can get — until now.

Because the Natural History Museum of London’s stegosaurus stenops (nicknamed Sophie) is the most complete in the world, scientists were able to use 3-D scanning to make a new estimate of her weight.

Before she was put on display in 2014, researchers scanned all 360 bones in Sophie’s skeleton, which is about 80 percent of the skeleton she had in life. Once digitized, the bones were measured by volume, which could be translated into mass using comparisons to living animals.

The result is an estimate of 3,527 pounds, the same as the number calculated with the traditional thigh and upper-arm method. Because Sophie represented a unique opportunity to cross-check two methods of weight estimation on the same skeleton, the sum and methods can now be considered more accurate. Their findings were published recently in Biology Letters.

But the findings also highlight just how fickle dinosaur weight estimation can be. Originally, the traditional method produced an estimate of weight nearly twice what the new method found. But when researchers realized that Sophie had still been growing when she died (and that the rest of her body hadn’t caught up to her adolescent limbs), they adjusted their calculations.

Why try to weigh a dinosaur? Without an accurate estimation of weight, it’s hard to make informed guesses about the way an animal lived.

“If we want to estimate how fast an animal runs, you need body mass; if you want to say something about their metabolism, you need to know their body mass,” lead author and Natural History Museum paleontologist Charlotte Brassey said.

One of Brassey’s next projects is to add muscles to the digital scan of Sophie’s skeleton, which could help create simulations of how the 150 million-year-old dinosaur walked.