Inside London’s Natural History Museum sits ‘Sophie,’ one of the world’s most complete Stegosaurus skeletons. Sophie was a young adult when it died. Today, scientists at the museum have announced they have figured out the weight of the Stegosaurus. Sophie tips the scales at around 1.6 tons. A modern day equivalent would be about the same size as a small rhino.

Dr. Charlotte Brassey, Professor Paul Barrett and Dr. Susannah Maidment created a 3D model to help flesh out the entire Stegosaurus. By combining a 3D model of the skeleton with tried and true weight-estimating techniques, they were able to determine how much it weighed.

Using the 3D model, the team of scientists fit shapes around the bone to calculate the Stegosaurus’ volume. Then, they converted the figure to mass using estimates from living animals. After that, they compared the results with a common technique that uses the circumference of the leg bone.

Having a good idea of the Stegosaurus’ weight can be used to determine how this young Stegosaurus lived. What was its walking speed? How much did it need to eat? Those are questions Brassey and her team can now tackle.

“Calculating body mass in animals that have been dead for many millions of years is no easy task,” said Dr Maidment. “Our study is the first to attempt different methods on the same animal, and has highlighted how and why different body mass estimation methods come up with different results. The age of the animal when it died is very important.”

Prof Barrett says these same techniques can be used on other complete fossils to “find out much more about the wider ecology of dinosaurs.”

The team’s confidence in their results is high as the two methods are in agreement. The anatomy of a Stegosaurus presented its own challenges and is described as “particularly unusual.” It evolved from animals that initially walked on two legs, not four.