was a creature of superlatives. As big as a bull elephant, T. rex weighed 15 times as much as the largest carnivores living on land today. Now, paleontologists have for the first time charted the colossal growth spurt that carried T. rex beyond its tyrannosaurid relatives. "It would have been the ultimate teenager in terms of food intake," says Thomas Holtz of the University of Maryland, College Park.
Growth rates have been studied in only a half-dozen dinosaurs, none of them large carnivores. That's because the annual growth rings in bone used to determine age tend to get resorbed in the weight-bearing bones of tyrannosaurids and other big meat-eaters. "I was told when I started in this field that it was impossible to age T. rex," recalls Gregory Erickson, a paleontologist at Florida State University in Tallahassee, who led the study.
But leg bones aren't the only place to check age. While visiting the Field Museum of Natural History in Chicago, Erickson discovered growth rings in many bones that don't bear substantial loads. He and his colleagues then sampled more than 60 bones from 20 specimens of four closely related tyrannosaurids. By plotting the age of each animal against its mass--estimated from the circumference of its femur--they constructed growth curves for each species.
Starting at about age 12, the more primitive tyrannosaurids put on about 310 to 480 grams per day, reaching 1100 to 1800 kilograms in 4 years. T. rex, in comparison, was almost off the chart. As the team describes this week in Nature, it packed on 2 kilograms a day and maxed out at more than 5600 kilograms at age 18.5.
It makes sense that T. rex would grow so fast in such a short time, experts say. Several lines of evidence suggest that dinosaurs had a higher metabolism and faster growth rates than living reptiles do (although not as fast as birds). "Tyrannosaurus rex lived fast and died young," Erickson says. "It's the James Dean of dinosaurs."
Being able to age the animals will help shed light on the population structure of tyrannosaurids. The technique could also help researchers interpret the medical history of individuals, perhaps revealing at what age they suffered various kinds of injuries. "We could see if they had a really rotten childhood or lousy old age," Holtz says.