The original study used the thinnest circumference of two bones -- the humerus and femur -- to extrapolate total body mass, which is a commonly accepted method. The researchers came up with an estimate of 60 tonnes, though they pointed out their specimen had still been growing, and could very well have been larger at maturity.
The new study involves mathematically reconstructing a "skin" volume around the bones of a dinosaur, then expanding that skin outline to account for muscle, fat and other tissues. Using this method, researchers from the University of Liverpool came up with an estimation of 40 tonnes. To have the mass that was estimated last summer, the authors say, Dreadnoughtus would need to carry more than twice the volume they believe it did.
"Our analysis suggests that only the lower estimates produced by previous methods are plausible," lead author Karl Bates of the University of Liverpool's Institute of Ageing and Chronic Disease said in a statement. "Estimates of 60 tonnes and above do not fit with our current understanding of the mass characteristics of living land animals."
When asked about the new study, the lead of the original Dreadnoughtus team, Kenneth Lacovara, expressed skepticism over the methods used.
"The method of measuring the skeleton and using it to estimate mass is based on the observation in living animals that, from a biomechanical standpoint, animals have the limbs they need -- no more and no less." Lacovara told The Washington Post. The new study seems to suggest that Dreadnoughtus was an exception, with massively sturdy limbs it didn't need to support its frame.
He also pointed out that in terms of traditional mass estimation, Dreadnoughtus is remarkably complete -- it has just 45 percent of its skeleton and some 70 percent once reconstruction is brought into play, but it has the bones you need to estimate mass. But he's not sure you can estimate the volume of an animal's skin based on that number of bones. In fact, his team was quite clear in their paper on one point: They weren't at all certain about the overall size of the skeleton. They didn't have its tail, its head came from another dinosaur, and so on. If any of these uncertain variables were tweaked, Lacovara explained, the volume would change.
"Some artists make dinosaurs look chubby, some make them look like they're shrink-wrapped," Lacovara said. "Volume just isn't preserved in the fossil record, and that's what they're using. Historically, scientists have worked with the data that exists, which is a pretty good policy."
He also isn't sure why Dreadnoughtus is the singular target of the new method.
"If I was making a new method, I'd apply it to as many skeletons -- and more complete ones than ours -- as possible," he said. "Ours just isn't a great candidate for this."
And he worries that the findings would be misleading.
"I imagine if you did this to all dinosaurs who'd had their masses estimated with traditional methods, they'd all change proportionally," he said. "But this just makes Dreadnoughtus shrink. I'm not exactly sure how Dreadnoughtus got involved."
We can't get Dreadnoughtus on the scale, so no one really knows what the dino weighed. But if Dreadnoughtus somehow managed to break the rules and weigh less than the standard method says it should have, all dino weight estimates are being called into question.
But even if that's the case, one fact remains: Dreadnoughtus was a bruiser.
"One thing we can all agree on is that these giant dinosaurs were among the heaviest land animals ever to walk the Earth," said Matthew T. Mossbrucker, director and chief curator of the Morrison Natural History Museum, who wasn't involved in either study. "Surely Dreadnaughtus pushed the limits of terrestrial animal mass."