Dino-Size Spurt: T. Rex Teens Gained 5 Pounds a Day

Brian Handwerk
for National Geographic News
August 11, 2004
Teenagers are known for rapid growth spurts, but some 65 million
years ago juvenile Tyrannosaurus rex dinosaurs may have
outstripped them all.

Studies indicate that between 14 and 18 years of age, T. rex grew from a 1-ton dinosaur into a 6-ton colossus—adding as much as 5 pounds (2.3 kilograms) a day. The animals reached full adult size by about age 20.

The rapid growth spurt explains how the legendary carnivores became gigantic—by growing at a much faster rate than many of their dinosaur relatives.

"The most intriguing thing about dinosaurs is their great size," said Gregory M. Erickson, professor of biological science at Florida State University. "How dinosaurs attained gargantuan proportions has remained one of the great mysteries in paleontology. Here we cracked the code for one family of dinosaurs, the Tyrannosauridae," said Erickson, who co-authored the new study.

T. rex dwarfed today's largest living land carnivore, the polar bear. Only one other carnivore is believed to have been bigger: Giganotosaurus, a behemoth that grew to as much as eight tons during the Cretaceous period 110 million years ago, more than 40 million years before the time of T. rex..

T. rex's rapid growth must have been fueled by eating on a truly gargantuan scale—whether by aggressive predation, scavenging, or some combination of the two.

"I think this helps us to focus in on how much energy [T. rex] would have had to assimilate in a short time," Erickson told National Geographic News. "If it had grown more like [modern] reptiles, it would have taken hundreds of years to reach its size. When you realize that it only lived perhaps 30 years, that's a lot of meat [to eat] in a short time."

The new growth and age findings are described in the August 12 issue of Nature by Erickson and his collaborators: Mark A. Norell of New York's American Museum of Natural History, Peter J. Makovicky of Chicago's Field Museum, Philip J. Currie of Alberta, Canada's Royal Tyrrell Museum of Paleontology, Scott A. Yerby of California's Stanford University, and Christopher A. Brochu of the University of Iowa.

The report prompted Erickson to contend that T. rex "lived fast and died young."

Size Matters

It's not certain why dinosaurs grew to such large sizes, but there are clues to the process of gigantism and its possible advantages.

"In carnivorous reptiles, such as crocodiles, for example—through their history, gigantism evolved numerous times," Erickson said. "Every time we see 30-to-40-foot [9-to-12-meter] crocs, it is related to the prey available. When there's giant prey available, giant crocs appear" and fill that niche.

Gigantism offers some advantages. As animals grow larger, they become less vulnerable to certain predators. Size can also provide access to new food sources, such as high leaves.

Erickson notes that giant animals also enjoy efficiency advantages in locomotion and energetics. "For instance, ectothermic [cold-blooded] animals like reptiles, if very large, probably wouldn't cool down overnight, so they wouldn't be as dependent on the sun to warm them back up."

Bones Yield "Growth Rings"

The key to T. rex's growth history was found in bones kept by some of the United States' major museums. Many dinosaur bones, like trees, feature seasonal growth rings.

"If you were to cut a bone in half and put a slice under a microscope, you can actually see these growth lines," Erickson said, noting that such research has become more popular in the last decade. "Counting the growth lines is exactly like counting the growth lines in a tree—it's the entire record of the animal's growth."

Clustered growth lines near the end of T. rex's development show that the animals reached an adult size and stopped growing, rather than gaining size throughout their lifespan.

The team examined the growth rings of adults, juveniles, and subjuveniles for four different species of Tyrannosauridae. Dinosaurs related to T. rex the researchers studied for comparison were Albertosaurus sarcophagus, Gorgosaurus libratus, and Daspletosaurus torosus.

The scientists estimated specimens' ages using the growth-line method, and approximated body mass by leg-bone size. Body-mass estimates ranged from 29.9 to 5,654 kilograms (66 to 12,465 pounds). That age-mass data allowed the researchers to establish a growth curve.

The researchers verified their results by comparing the dinosaurs' growth rings to those of modern alligators and lizards. Those animals' growth rings are similar—but their growth patterns are not.

T. rex's growth rate is comparable to that of the modern elephant. But while elephants have a lifespan of more than 70 years, T. rex lived an average of 30 years.

Just as in the biological study of living species, establishing an extinct animal's growth curve is a key stepping-stone to broader understanding.

With the life-history parameters, we can better understand T. rex evolution, biology, locomotion, and population dynamics, said Peter Makovicky, dinosaur curator at the Field Museum and a co-author of the study.

Erickson said that future research will focus on the growth of other dinosaur species.

"We'll follow up by looking at other dinosaur groups and see how they became gigantic, [to see] if this is the pattern that dinosaurs followed. If that's the case, it would be a very interesting evolutionary pattern."

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