Tyrannosaurus rex: An eating machine

When you think your teen chows down during a growth spurt, consider the dietary habits of the adolescent Tyrannosaurus rex, one of the all-time meat-eaters.

Scientists calculated in a new report that before the dinosaurs reached adulthood at about age 20, they went through an accelerated growth spurt that added nearly 5 pounds a day to their weight -- for up to four years -- on their way to an adult weight of more than 11,000 pounds.

The study published Thursday in the journal Nature examined more than 60 bones from 20 different fossils of four closely related members of the tyrannosaur family that lived in North America in the late Cretaceous period, some 90 million to 65 million years ago. Samples were drawn from several leading paleontology collections in the United States and Canada.

Researchers led by Gregory Erickson, an assistant professor of biological sciences at Florida State University, used an innovative technique to calculate age and growth rates for the animals by counting growth rings in bones, specifically in smaller, non-weight- bearing bones that don't remodel as they grow, making them easier to read under a microscope.

The method is somewhat similar to counting growth rings in trees and has been tested on the bones of modern snakes, lizards and crocodiles to confirm the age of the animals.

"We can now stop guessing about how T. rex and some of its closest relatives grew," said Peter Makovicky, a dinosaur curator at the Field Museum in Chicago and a study co-author. "Knowing the lifeline is important, because we now understand the evolution of T. rex's giantism, one of the most fascinating aspects of dinosaurs."

T. rex was at least 15 times larger than today's largest living land carnivore, the polar bear. Until now, paleontologists had argued that T. rex either got so big by growing faster than its near relatives or by growing for a longer period of time.

But with new estimates of the animals' lifespan drawn from the specimens, which ranged from 2 to 28 years, the researchers determined that the growth spurts were responsible for the huge size. These spurts stopped when the beasts were about 18 years old; the animals stayed at this adult size for about one-third of their lives.

The researchers concluded that "Sue," the hallmark T. rex in the Chicago museum, reached her full size nine years before her death, indicating that the carnivores had a life span of about 30 years.

Thus, T. rex grew faster but had about half the lifespan of the African elephant, the only living land animal of comparable size today.

The study also showed that T. rex's smaller cousins matured in about 14 to 16 years and had late growth spurts that ran several years, but added only about a pound a day during adolescence.

On the Net: www.nature.com; www.fieldmuseum.org; www.amnh.org

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