How the Dino Soared

Scientists discover T. rex was once a typical growing teen

WEDNESDAY, Aug. 11, 2004 (HealthDayNews) -- As far as teenage growth spurts go, this one was off the charts.

Tyrannosaurus rex, better known as T. rex, was one of the largest dinosaurs to ever roam the earth. As it turns out, researchers have just discovered it got to be gargantuan through a massive growth spurt between the ages of 14 and 18.

During the spurt, the dinosaur grew about 4.6 pounds a day and took on about 70 percent of its adult mass. This allowed T. rex, which fed on other animals more than 65 million years ago, to end up weighing between 5 and 7 tons. There was little or no growth for the remaining decade of its life.

The new finding appears in the Aug. 12 issue of Nature.

Paleontologists had speculated that T. rex could have attained such proportions in one of two ways. "They can grow for long periods of time or they can accelerate growth rates," explained study co-author Peter Makovicky, the dinosaur curator at the Field Museum in Chicago. "A lot of animals do a combination of both, but with T. rex, it's the second strategy."

And what that strategy entailed in terms of teenage food requirements is hard to imagine. "One of the main aspects of accelerating your growth rate is that your food intake requirements become huge," Makovicky said. "So, obviously, just like any teenager, you've got a three- to six-ton animal raiding the fridge."

To determine T. rex's growth trajectory, the scientists relied on a method normally reserved for trees: They counted rings.

Like trees, dinosaur bones have growth rings, which serve as a record of age. The problem is that many of the weight-bearing bones hollowed out as the animals grew. About four years ago, however, the study's lead author, Gregory Erickson of Florida State University, noticed that smaller, non-weight-bearing bones such as the fibula and tibia were solid.

Still, most museums would have been reluctant to let any scientist take a saw to the precious fossils had it not for another recent development: a spurt of paleontological discoveries resulting in more bones.

This let the researchers analyze 60 bones from 20 dinosaurs including Sue, a T. rex who was discovered in South Dakota in 1990 and now resides at the Field Museum.

They then calculated body size based on the circumference of the thighbone. "The thighbone in a biped has to carry all of the body weight and you can use the relationships between the mass and the circumference along with some modeling combined with extrapolation from living animals to project what it might have weighed," Makovicky said.

Scientists now know that Sue was 28 years old when she died 67 million years ago and weighed, at her peak, a shade over 5.6 metric tons.

"In terms of carnivorous dinosaurs, T. rex was one of the biggest," Makovicky said. It was beaten only by Giganotosaurus carolinii, whose stomping grounds included what is now Argentina.

In terms of modern-day comparisons, only the African elephant, at 5 tons, comes close to T. rex. But elephants take longer to reach adulthood and can live 60 or more years. Sue, at 28, was already showing signs of aging. "This is pretty old for a T. rex," Makovicky said.

While humans don't compare in size, the human growth curve is remarkably similar to that of the T. rex. "We grow slowly until about 12 or 13, and then we take off just like T. rex, and over the next four years we start approaching our full mature adult size," Makovicky said. "Around the time we're 18 or 19, we're full grown and that's what T. rex did."

Why was T. rex's growth rate four to seven times faster than that of its ancestors? As always, it probably had something to do with food, possibly the fact that herbivores were also getting larger.

"There was probably some change in targeted prey during that four-year growth period," Makovicky said. "The ability to hunt larger items was acquired and bigger prey items would give you more pounds of meat."

That's more pounds of meat all around.

More information

Visit the Field Museum for more on the T. rex named Sue.

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