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Weight of the world’s most complete Stegosaurus revealed

The Museum's 150-million-year-old Stegosaurus stenops would have weighed around 1,600kg in life, similar to the size of a small rhino.

The Stegosaurus, revealed at the Museum in December, is exceptionally well-preserved, allowing scientists to learn a lot about the lives of stegosaurs.

Museum scientists Dr Charlotte Brassey and Prof Paul Barrett, along with Dr Susannah Maidment of Imperial College London, combined a 3D model of the skeleton with a more traditional weight-estimating technique to create an accurate picture of the animal’s weight.

‘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,’ said Dr Brassey.

Dr Brassey and the team can now use the weight estimation to find out more about how Stegosaurus lived, such as its walking speed and how much it needed to eat to keep itself going.

Multiple methods

Using the 3D model of the skeleton, the team fit shapes around the bones to calculate the volume of the Stegosaurus, before converting the figure to mass using estimates from living animals. They then compared the results of this method with another common method that uses the circumference of the leg bone.

The second method initially gave a result almost twice that of the 3D model estimate. However, since the Stegosaurus was only a young adult when it died, its limbs were growing faster than its overall body. When this was taken into account, the two estimates agreed on a figure of around 1,600 kg.

‘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.’

Body types

The study provides an explanation for why estimates based on body volume and leg-bone circumference often disagree in fossil studies.

‘These findings identify just how important exceptionally complete specimens like this are for scientific research and collections,’ said Prof Barrett. ‘We can use the same techniques on other complete fossils to find out much more about the wider ecology of dinosaurs.’

The team are confident in their results since the two methods agree, but there is still a lot to learn about the body types of dinosaurs. The anatomy of Stegosaurus would have been particularly unusual, as they evolved from animals that initially walked on two legs.

‘Dinosaurs are related to modern birds and crocodiles, but their anatomy could have been quite different in some cases,’ said Dr Brassey. ‘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. Combined with its wide hips, it’s fair to say this Stegosaurus probably would have had quite a large rear end.’