It's hard to imagine the king of the dinosaurs as an uncoordinated teenager, but recent analysis of the fossil record shows that Tyrannosaurus rex, one of the largest meat-eaters to ever roam the land, grew nearly five pounds a day between ages 14 and 18. This growth spurt accounted for more than 70 percent of its adult mass.

A full-grown T. rex weighed in excess of five tons--15 times more than a polar bear, the largest living terrestrial carnivore. Paleontologists have long wondered how, exactly, T. rex attained its great size. "If T. rex grew as a reptile, it would have taken over a 100 years for it to reach its maximum size," says Gregory M. Erickson of Florida State University.

But the new research, published today in Nature, shows that T. rex came to tower over its nearest cousins through an accelerated growth stage. Erickson and his colleagues determined the age-at-death of seven T. rex individuals, as well as 13 specimens of three smaller tyrannosaurid species that lived during an earlier period, by counting bone rings in their fossilized ribs, fibulas and hip bones. Similar rings in modern day reptiles reflect an annual cycle in activity and therefore reveal the age of the organism, just as rings in trees do. From this method, the scientists found that the most senior tyrannosaurid in their sample was just 28 years old.

The team also estimated the body mass of the dinosaurs at the time of death by measuring the circumference of the weight carrying femur. The growth curves of all four tyrannosaurid species showed a spell of exponential weight gain, beginning around the 15th year of life and tailing off four years later. But with growth rates of around a pound a day, the relatives of T. rex only ended up reaching a fourth of its size.

What kind of appetite would one of these T. rex teens have had? Erickson doesn't know exactly, but, he says, "If Purina made T. rex chow, it would take a whole lot of bags." --Michael Schirber