How do you weigh a dinosaur?
In December 2014, London's Natural History Museum unveiled an amazing 150 million year old Stegosaurus fossil, named Sophie. Standing proudly in the museum's Earth Hall, Sophie is a big attraction for visitors. But that's not the only thing that's big about her - as Kat Arney discovered when she went to meet the researcher who's dedicated the past year to studying the skeleton, Charlotte Brassey...
Kat - So, we're here in the Natural History of Museum. Lots of people around us and this beautiful skeleton here, introduce me to this lady.
Charlotte - So, this is Sophie the Stegosaurus, and she is 155 million years old. She's come all the way from Wyoming in the US and she is arguably the most complete Stegosaurus skeleton ever found. What's particularly exciting about her is the skull. The skull is entirely intact. The one in display is plastic but we've got the real one in the basement and we're still studying it.
Kat - Well, let's go and learn a little bit more about what you've been doing to understand more about what she look like maybe in the actual flesh.
Charlotte - So, this is the stores where we keep all the dinosaurs.
Kat - Tell me how you're studying Sophie to try and work out a bit more of what she looked like in real life.
Charlotte - My background is in computer modelling, so what I've done in the last year, I've created a 3D computer model of every single bone in Sophie's body. From that, we can then begin to estimate things like body mass. We reconstruct how the muscles would've been placed across the skeleton, we're looking at reconstructing how she would've walked, and also reconstructing the jaw muscles on her skull to look at what she could've eaten.
Kat - Obviously, when we see dinosaurs in museums, they are all these bones. Paint me a picture of what you think she might have looked like in the glorious flesh.
Charlotte - Well, the interesting thing about dinosaurs in general is the group of muscles that they use to kind of pull their leg backwards as they move are very different to that what we see in modern mammals. So, you can see from Sophie, she's got a very long tail and she would've had very bulky muscles attaching a longer tail onto the back of her legs. Actually, if you go up on a balcony and you look from the top down on Sophie, you'll see that she's got very, very wide hips. So, those two facts combined, she would've had quite an impressive rear end.
Kat - A hefty bulk.
Charlotte - Yes.
Kat - How do you start working out how big a Stegosaurus was and maybe how much she weighed?
Charlotte - In the past, people have based it on simple measurements of the leg bones. So typically, you would go into a museum collection full of modern animals. So, modern mammals and birds and crocodiles. And you would do something like you'd measure the length of the thigh bone or maybe the circumference around the middle of the thigh bone. And then you would also - knowing the body mass of the modern animal, you plot that on a simple scatter plot. So, you plot body mass against this measure, look at bone length say, and you put a straight line through it, and you would then use that line to predict the body mass of fossil animals. So, we could then go and measure say, the length for Sophie's thigh bone or the circumference around the middle of her thigh bone and use that line on our scatter plot and back estimate the body mass of Sophie. That's really useful if all you have in the fossil record are isolated bones. The vast majority of fossil animals that we know of, come from very fragmented remains. We don't have the whole skeleton. What's really exceptional about Sophie is that we've got so much of the skeleton. So, it seems a shame to be reconstructing her body mass from just these simple measurements alone. We wanted to try and use the whole skeleton. So that's where my computer models come in. so, once we've got a computer model of Sophie, I import it into just a simple CAD package like engineers use and then we can wrap very simple shapes around the 3D object of Sophie. Once we've got a volume for Sophie, we can then look at the relationship between body mass and volume in modern animals, and then use that relationship to estimate her body mass based on the whole skeleton.
Kat - How much do you think she might have weighed? What sort of heft was she on the ground?
Charlotte - So, our best guess at the moment is something around 1.6 tons which intuitively seems likely to us. If you actually look at Sophie on display, she is smaller than an elephant. So, you wouldn't be expecting body masses of 3 to 4 tons. She does look something like the size of a rhino and that's about right for a rhino.
Kat - Now that you've started to work this out about Sophie about how much she might have weighed, what she looked like, what next? What do you still want to figure out about her?
Charlotte - So, body mass is really fundamental from where we want to go from here. So, in any kind of equation that I might use for say, estimating how fast she might have moved, maybe reconstructing their metabolism, how much would they have to eat. All those things, all those equations and models require an estimate for body mass. So, that's why we have to get this really nailed down very early on and from that, we can build upon this. So, the thing that I'm working on now is beginning to strap muscles onto my 3D models looking at how efficient her muscles would've been for moving her limbs and then eventually, reconstructing how she would've walked.
Kat - Real life Walking With Dinosaurs.
Charlotte - Exactly, yeah. You've got to get your information from somewhere haven't you?