By combining 3D modeling of a near-complete skeleton with a more traditional weight-estimating technique, researchers have now revealed the weight of one of our most beloved dinosaurs. Turns out, a young adult Stegosaurus weighs about the same as a small rhino. The findings were published in Biology Letters this week. 

There are a few ways to calculate how much an animal weighs based on just its weathered bones. One method is to look at the relationship between the mass and leg bone circumference of modern animals to then estimate the mass of extinct ones. An alternative is to create 3D computer models to get at a volume estimate. For the Stegosaurus, however, these two techniques previously came up with very different masses. 

Now, a U.K. trio led by Charlotte Brassey from the Natural History Museum in London applied both techniques to an 80-percent-complete skeleton of a 150-million-year-old Stegosaurus stenops unearthed from the Upper Jurassic Morrison Formation near Shell, Wyoming. Nicknamed Sophie, the specimen is missing just the left forelimb and part of the tail. “Because this incredible specimen is so complete, we have been able to create a 3D digital model of the whole fossil and each of its 360 bones, which we can research in excellent detail without using any of the original bones,” Brassey says in a news release. 

Using the 3D model, the team fit shapes around the bones to calculate the stego’s body volume, and then they converted that figure to mass using estimates based on animals alive today. Finally, they compared these results to the method using leg bone circumference.

Both techniques agree, they found, once the age of the animal was taken into account. Initially, the method using leg bone circumference provided a result that’s nearly twice that of the 3D model estimate—but that’s because Sophie was a juvenile when it died, and its limbs were growing faster than the overall body. 

Their conclusion: A young adult Stegosaurus weighed 1,560 kilograms (over 3,400 pounds) when it was alive. 

With this new weight estimation, researchers can try to answer more questions about how the stego lived. "If we want to estimate how fast an animal runs, you need body mass,” Brassey tells the BBC. “If you want to say something about their metabolism, you need to know their body mass.” And of course, it also helps to clarify body type. “The main muscles that would be used to pull the hind limbs backwards would have been pretty large and would have attached quite far down the tail,” she adds. “Combined with its wide hips, it’s fair to say this Stegosaurus probably would have had quite a large rear end.”

Here are reconstructions of Stegosaurus stenops. Their preferred model is c and d: 

Images: Natural History Museum (top), 2015 C.A. Brassey et al., Royal Society (middle)