Cross-sections of its bones show that their structure is remarkably robust, resembling that of a different T. rex known to have died around the age of 28. And its main leg bone, or femur, in particular provided a vital clue to Scotty's size.

By studying many living animals, scientists have found that the wider an animal's femur, the more weight that the bone tends to hold up. Scotty's femur was a whopping eight inches across—which means that Scotty's two legs could hold up more than 19,500 pounds, give or take a couple tons. When the same methods are applied to Sue, the famously complete T. rex at the Field Museum, that fossil comes out about 900 pounds lighter.

Lean and mean

However, this bone-measuring method isn't foolproof. For one, animals don't use their skeletons to passively hold up their weight; bones also endure the forces of motion. There's some evidence that tyrannosaurs may have been faster and more agile than other groups of large predatory dinosaurs, such as the earlier allosaurs. Perhaps tyrannosaur leg bones were slightly over-engineered to take the stress of running, which would lead researchers to overshoot Scotty's actual weight.

In addition, body mass is just one way of parsing bigness, and not all predatory dinosaurs had the same dimensions. Tyrannosaurs such as T. rex appear to have had stockier builds, while other species had longer, more slender bodies. This variety, some researchers argue, may even hold within the T. rex species, which includes some more “slender” specimens.

No species demonstrates this conundrum better than Spinosaurus, a semiaquatic dinosaur that lived in what's now northern Africa about a hundred million years ago. The animal was about 50 feet long from its snout to the tip of its tail, which would make it longer than T. rex. But when estimating Spinosaurus's weight based only on femur size, it comes out at just 3,600 pounds.