Rarity of fossils of young tyrannosaurs explained.

Paleontologists have unearthed only a few juvenile tyrannosaurs, and a new analysis suggests why: Unlike many other creatures both modern and ancient, a large percentage of those meat-eating dinosaurs survived into adulthood.

Many dinosaur bones, including those of Tyrannosaurus rex and its relatives, show growth rings, just as trees do, says Gregory M. Erickson, a paleobiologist at Florida State University in Tallahassee. Now, he and his colleagues have counted growth rings in leg bones to determine the age distributions of four tyrannosaur species.

The 22 specimens of Albertosaurus unearthed at a quarry in Alberta ranged in age from 2 to 28 years, the researchers estimate. The creatures' ages at death indicate that between the ages of 2 and 13, the dinosaurs died at an average rate of about 4 percent each year. Thereafter, the animals began to die at a rate of 10 percent or more per year. Previous research suggests that Albertosaurus reached sexual maturity at age 14, says Erickson. The new data indicate that about 70 percent of the dinosaurs made that milestone.

"These creatures started dying more quickly just as they were reaching their prime," says Erickson. Similar trends apply to T. rex, Gorgosaurus, and Daspletosaurus, the researchers note in the July 14 Science.

Tyrannosaur juveniles probably survived so well because they grew quickly, says Erickson. For example, by the time that an Albertosaurus was age 2, it rivaled in size the adults of other predators living then, he notes. The demands of adulthood, including conflicts for territory and mates, probably brought about the accelerated death rates among older tyrannosaurs. --S.P.