How to Weigh a Dinosaur

A new study reveals the proper technique to weigh these extinct creatures

The head of a Tyrannosaurus rex seen at The Natural History Museum of Denmark on June 9, 2020 in Copenhagen
How would you calculate the weight of a T. Rex based only on its bones? There's at least two ways to find the answer, experts found. Photo by Ole Jensen/Getty Images

Weighing a dinosaur is no easy task. These extinct creatures were massive, and for the most part, all that remains are their bones, as their organs and skin have long since decomposed. However, new research has found more than one way to measure the mass of these giant creatures that roamed the planet millions of years ago.

In a paper entitled “The Accuracy and Precision of Body Mass Estimation in Non-avian Dinosaurs,” published this month in the scientific journal Biological Reviews, a team of scientists from the University of New England's Palaeoscience Research Centre evaluated the two existing ways scientists approach calculating how much a Tyrannosaurus rex might have weighed. (Interestingly, neither method involves pulling out an actual scale.)

Led by paleontologist Nicolás Campione of the University of New England, the researchers “examined an extensive database of dinosaur body mass estimates” from as far back as 1905, with weight estimates for individual specimens ranging anywhere from three tons to a whopping 18 tons. (For reference, the average sedan weighs a measly 1.5 tons.)

"Body size, in particular body mass, determines almost all aspects of an animal's life, including their diet, reproduction and locomotion," says Campione in a Royal Ontario Museum press release. "If we know that we have a good estimate of a dinosaur's body mass, then we have a firm foundation from which to study and understand their life retrospectively."

In an essay published by The Conversation, Campione explains that for years, paleontologists followed two rival approaches for tallying a dinosaur's poundage. These methods were long thought to be at odds with each other, but Campione's team found that both techniques are actually quite accurate.

Using limb circumference to find out an animal's mass is already widely used across a variety of modern land animals, like primates, marsupials, and turtles, writes Campione. The same scaling method can be applied to dinosaurs. Researchers essentially measure the bones in living animals, such as the femur in an elephant’s leg, and compare that figure to dinosaur's femur.

The second method involves calculating the volume of 3-D reconstructions of dinosaurs, which serve as approximations of what the creature would’ve looked like when it was still alive.

Occassionally, these methods have come to very different conclusions. For The Conversation, Campione presents a recent example of a discrepancy:

A [3-D] reconstruction of the gigantic titanosaur Dreadnoughtus, which lived roughly 80 million years ago in what is now Argentina, suggested a body mass between 27 and 38 tonnes. Yet its colossal legs suggest it could have supported even more weight: between 44 and 74 tonnes.

But after applying both methods repeatedly to an ample number of specimens in the database, it became clear that the case of the titanosaur was an outlier. "In fact, the two approaches are more complementary than antagonistic," Campione says in a statement.

David Evans, a paleontologist at the Royal Ontario Museum in Toronto and senior author of the paper, says their conclusion illustrates the importance of using both methods in tandem—and highlights the importance of uncertainty, because "dinosaurs, like humans, did not come in one neat package," according to the university statement.

“There will always be uncertainty around our understanding of long-extinct animals, and their weight is always going to be a source of it," he says in a statement. "Our new study suggests we are getting better at weighing dinosaurs, and it paves the way for more realistic dinosaur body-mass estimation in the future."

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