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Ferocious Growth Spurts

Teenage tyrannosaurs packed on the pounds.

Big dinosaurs were big eaters, especially in their teenage years. During adolescence, Tyrannosaurus rex and its relatives sometimes doubled their weight in just 4 years, according to new studies.

Growth rings in a rib (inset) from the large tyrannosaur named Sue suggest that the creature stopped growing after 4 years of rapid growth and then lived another 9 years.

Growth rings in a rib (inset) from the large tyrannosaur named Sue suggest that the creature stopped growing after 4 years of rapid growth and then lived another 9 years.

Field Museum

T. rex was one of the biggest meat eaters ever to terrorize the planet. Adults could weigh more than 5,000 kilograms. Of course, the dinos weren't born that large, and scientists have long debated how quickly and when they grew to their adult size.

To find out, paleobiologist Gregory M. Erickson of Florida State University in Tallahassee and his colleagues analyzed fossil bones from seven T. rex specimens. The species lived in North America between 68 million and 65 million years ago. The researchers were able to figure out how old the dinosaurs were when they died by counting yearly growth rings in the bones of the creatures.

The smallest dino the team looked at weighed just 30 kilograms and was only 2 years old when it died. The largest, named Sue, weighed in at 5,650 kilograms. At 28, she was also the oldest. Other data showed that a T. rex's growth spurt usually began at about age 14. On average, the animal then gained 2 kilograms per day until around age 18.

Analyses of three related species suggest that those dinosaurs also went through growth spurts at about the same time in their lives.

Relying on other evidence, scientists suspect that an adult T. rex may have been too big to run and had to scavenge for its meals. It's possible the youngsters had a different feeding strategy and were able to scurry around, even to catch prey.

Nonetheless, no matter how they did it, teenage tyrannosaurs certainly knew how to put on the pounds.—E. Sohn

Going Deeper:

Perkins, Sid. 2004. Growth spurt: Teenage tyrannosaurs packed on the pounds. Science News 166(Aug. 14):99. Available at http://www.sciencenews.org/articles/20040814/fob1.asp .

Sohn, Emily. 2003. Dinosaurs grow up. Science News for Kids (Nov. 26). Available at http://www.sciencenewsforkids.org/articles/20031126/Feature1.asp .

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