Tyrannosaurus rex Could Have Weighed 9 Tons, Grew Fast

Oct. 16, 2011 — -- Johnny Cash might have said life wasn't easy for a dinosaur named Sue. She grew up quick and she grew up mean, packing on nearly 4,000 pounds a year as a teenager.

"We estimate they [Tyrannosaurs] grew as fast as 3,950 pounds per year (1790 kg) during the teenage period of growth, which is more than twice the previous estimate," said John R. Hutchinson of The Royal Veterinary College, London in a press release.

Modeling the teenage growth spurts of an ancient mega-predator was made possible by comparing computer models of smaller, younger Tyrannosaurs with those of full grown adults, including the tyrant lizard queen, Sue, the largest T. rex skeleton yet discovered.

The new models also found that the 42 foot-long Sue may have been much heavier than earlier estimates. She tipped the scales at approximately 9 tons, according to the computer model.

"We knew she was big but the 30 percent increase in her weight was unexpected," said co-author Peter Makovicky, curator of dinosaurs at the Field Museum of Natural History in Chicago.

Hutchinson, Makovicky and a team of researchers used 3-D scans of five mounted T. rex skeletons to create computer models of the long dead predators. The computer models allowed them to estimate the weights of the dinosaurs in a range of health conditions, from dinos on diets to tubby Tyrannosaurs.

"These models range from the severely undernourished through the overly obese, but they are purposely chosen extremes that bound biologically realistic values" says study co-author Vivian Allen of the Royal Veterinary College.

"The real advantage to our method is that the models can be adjusted to accommodate the variation that is inherent in nature, so we don't have to pick an arbitrary result, but rather deal with more meaningful ranges of results," added co-author Karl T. Bates of the University of Liverpool.

Reconstructing the Cretaceous era killers using scans of mounted skeletons improved on earlier methods of estimating dino weights.

"Previous methods for calculating mass relied on scale models, which can magnify even minor errors, or on extrapolations from living animals with very different body plans from dinosaurs. We overcame such problems by using the actual skeletons as a starting point for our study," Makovicky said.

The reconstruction of Sue's skeleton may have been a bit different than she was in life, but Makovicky doesn't think this threw off their calculations.

"Sue's vertebrae were compressed by 65 million years of fossilization, which forced a more barrel-chested reconstruction" says Makovicky.

"Nine tons is the minimum estimate we arrived at using a very skinny body form, so even if we made the chest smaller, adding a more realistic amount of flesh would make up for the weight," Makovicky said.

T. rex packed on the pounds as a teenager, but that was about on par with other dinosaur growth rates.

"Our new growth rate value actually erases a deficit between the previous growth rate estimate and what is expected for a dinosaur of this size," said Makovicky.

Putting on all that weight slowed a T. rex down.

"The total limb musculature of an adult T. rex probably was relatively larger than that of a living elephant, rhinoceros, or giraffe, partly because of its giant tail and hip muscles. Yet the muscles of the lower leg were not as proportionately large as those of living birds, and those muscles seem to limit the speed at which living animals can run. Our study supports the relative consensus among scientists that peak speeds around 10-25 miles per hour (17-40 kph) were possible for big tyrannosaurs," said Hutchinson.

The research was published in the journal PLoS ONE.