Last year, scientists described a dinosaur that was a bit of a whopper, to say the least. Weighing in at an estimated 60 metric tons, the colossal titanosaur Dreadnoughtus threatened to claim the highly sought after title of world’s most massive dinosaur. While unquestionably enormous, a new analysis of the beast’s bones suggests that scientists may actually have significantly over-estimated its weight. In fact, if the dinosaur sat at the lower end of its estimated range, it may only have been half as massive as previously thought. The study has been published on June 10 in the journal Biology Letters.  

As the name suggests, titanosaurs were pretty darn huge dinosaurs. In fact, this group of gargantuan sauropods – the herbivores with the incredibly long necks – includes several species that are considered to be the biggest land animals discovered so far. Argentinosaurus, for example, is the current record holder for both the most massive and the longest terrestrial animal ever to have roamed the planet.

But then the “fears nothing” Dreadnoughtus schrani came stomping onto the scene last year. Discovered in Patagonia, Argentina, this Upper Cretaceous specimen (estimated to have lived between 84 and 66 million years ago) was in remarkably good shape, offering bones from all major skeletal regions. With 70% of the skeleton below the skull available for analysis, Dreadnoughtus represented one of the most, if not the most, complete giant titanosaur discovered yet.

Aside from being an awesome find, such a well-preserved collection of bones gave scientists the opportunity to reliably estimate its length, which came in at a staggering 26 meters (85 feet). But since it wasn’t quite fully mature when it died, it could perhaps have grown to be even bigger. After measuring the beast’s leg bones, the same team was also able to estimate its mass, which turned out to be a rather hefty 60 tonnes. But after scientists revealed that some other sauropods, which were only just overshadowed by Dreadnoughtus in terms of body size, didn’t weigh nearly as much as this, a team of researchers, headed by the University of Liverpool, decided to take another look at its bones.

This time, rather than using a scaling equation based on leg bone dimensions, the researchers adopted a 3D skeletal modeling technique to first reconstruct the animal’s skin volume. They then expanded or shrunk this shape to account for other soft tissue, like muscle and fat, using information gleaned from other modern-day species, such as birds and crocodiles. Finally, once again using data from extant relatives, they estimated the density of the animal’s tissues.

Although this technique produced a range of possible weights, none of these even came close to the highest earlier estimate of 60 tonnes. The smallest figure was just 28 tonnes, and the largest was 38, suggesting that only the lowest estimates from the previous study were reasonable. But Ken Lacovara, lead author of the original study, was quick to defend his team’s work, pointing out to National Geographic that Dreadnoughtus is in fact not the most complete sauropod skeleton and thus there are perhaps better specimens out there for this kind of analysis.

This may be the case, but present study author Karl Bates from the University of Liverpool highlights the fact that reconstructing an extinct animal’s size and shape is exceedingly difficult and relies on the extrapolation of data from living species. No method is therefore right or wrong: all that scientists can do is give an estimate based on the best available data.

[Via University of Liverpool, Biology Letters, Science and National Geographic]