How to Weigh a Dinosaur

According to one new study, Stegosaurus youngsters were rather full-figured . 

The most complete skeleton known from this distinctive dino belongs to “Sophie,” a juvenile at London’s Natural History Museum. Despite her adolescence, she was an enormous animal, measuring 5.6 meters (18.3 feet) long from end to end.

So how much did Sophie weigh? No less than a whopping 3530 pounds, PhD researcher Charlotte Brassey said in a recent press release. If spot-on, her measurement would put the Stegosaurus on par with white rhinos and certain BMWs.

By the way, this is no off-the-cuff guesstimate. Brassey and two colleagues meticulously scanned all 360 of Sophie’s beautiful bones to create a digital model of her skeleton. As Brassey explains, “very simple shapes” were then digitally wrapped “around her body outline, and … that whole body volume [was used] to predict body mass.”

We’ve never had a sure-fire means of figuring out just how heavy long-extinct animals were. And unless someone invents a time machine that doubles as an extra-large bathroom scale, we never will. But even taking educated guesses has proven difficult, thanks largely to anatomical x-factors (how beefy, for example, was T. rex’s tail?).

When zoologist Robert McNeill Alexander attacked this problem during the 1980s, he wielded an unlikely tool: plastic dinosaur models. Donning his best Archimedes impression, Alexander submerged reasonably up-to-date miniatures in water and measured the displacement. Afterwards, he more or less scaled up these results based on each species’ real-life dimensions. This simple approach provided rough tonnage estimates for a variety of dinos. But hindsight hasn’t been kind to Alexander’s models, which—among other things—featured several inaccuracies (neck length, limb girth, etc.) that seriously skewed his data.

A far more popular technique involves femurs, or “thighbones.” In the animal kingdom, there’s a general correlation between femur circumference and total body mass—though the fact that, like birds, many dinosaurs had hollow limb bones does complicate matters.

Still, the challenge is worth grappling with. As Brassey points out, only by doing so can paleontologists answer basic questions about dinosaurian lifestyle and behavior. “If [you] want to estimate how fast an animal runs, you need body mass,” she said. “If you want to say something about … metabolism, you need to know the body mass.”

This brings us back to digital Sophie, who won’t be sitting idly by as Brassey pursues her next project. Instead, she’ll have her virtual beastie hitting the treadmill. “[Now] what I’m looking to do is begin to strap muscles on to our computer models so that we can get her walking to say something about locomotion,” she said.