The four-legged, long-necked animal—think something Brontosaurus-like—stomped around present-day Argentina during the Upper Cretaceous, between 84 million and 66 million years ago. Its name means “fearer of nothing,” and a full-grown Dreadnoughtus probably didn’t worry much about being eaten (fiery death from above is a different matter).
At roughly 85 feet long, Dreadnoughtus would have been able to simultaneously sit in all of first and economy class on a classic Boeing 737. Original estimates of its heft, based on the circumference of its leg bones, suggest Dreadnoughtus probably came in at around 60 tons—or a little bit less than two (empty) Boeing 737s.
That estimate made Dreadnoughtus a contender for the much-debated title of biggest dinosaur, in league with giants like Argentinosaurus and another dinosaur recently found in Argentina, which have enormous bones but no skeletons complete enough to reliably calculate weight.
The new study uses a different method to calculate Dreadnoughtus’ mass and puts it closer to 30 or 40 tons, more comparable to a single jetliner. Rather than scaling up from the size of the limbs, the authors first estimated the volume of the dinosaur. They started with the skeleton, and wrapped a body shape around it, expanding or shrinking the shape based on information from birds, crocodiles, and mammals. Then, they estimated the density based on information from modern-day dinosaur relatives. With the volume and density, plus a few more adjustments, scientists could derive a mass.
No matter what they did, Dreadnoughtus never reached the 60-ton benchmark.
The team also estimated the weights of two other giants, the extinct sauropods Giraffatitan and Apatosaurus.
“In all three cases, the range that comes out from our model is at the lower end of the [original] equation range,” says study author Karl Bates of the University of Liverpool.
Not everyone is convinced the new analysis is accurate. Dreadnoughtus is not the ideal specimen for this type of analysis, says Drexel University paleontologist Ken Lacovara, who published the dinosaur’s initial description. “It’s very complete for a super-giant dinosaur, but there are other sauropods that are more complete,” he says.
He also notes that if Dreadnoughtus was, in fact, less massive than its limb bones indicate, it had some unnecessarily enormous legs. “There’s no physical reason, no biomechanical reason I know of, that would require Dreadnoughtus to have anomalously large limb bones,” he says. “It’s more parsimonious to think that Dreadnoughtus had the limbs it needed to have.”
Determining the size and shape of a dinosaur—or any extinct animal—is inherently tricky. While paleontologists can connect fossilized bones and string together a skeleton, clues that would ordinarily be used to define a creature’s softer anatomy are missing. So, scientists are forced to estimate how big these beasts were based on extrapolations from animals alive today.
“There is no good way to do these reconstructions of extinct animals,” Bates says. “The fairest message is that both these methods are equally wrong. Ours is just wrong at the lower end.”
Follow Nadia Drake on Twitter.