The team conducted similar analyses on Tyrannosaurus, Gorgosaurus, and Daspletosaurus specimens from various museums and collections.
What the scientists found was that tyrannosaur survivorship patterns resemble those of wild populations of long-lived birds and mammals.
The team found no Albertosaurus hatchling fossils at the mass grave. As a result, they concluded that Albertosaurus and other tyrannosaur species had high infant-mortality rates.
Rates between 50 and 80 percent would mirror those of today's wild birds, crocodiles, and mammals.
By contrast, the juvenile stage for tyrannosaurs was a stable and relatively safe one, much as it is for humans.
The study found that 70 percent of tyrannosaurs that lived to age 2 made it to 13.
The reason, researchers suggest, is that two-year-old tyrannosaurs had already grown to six feet (two meters) in length and were probably big enough to protect themselves. (See "Dino-Size Spurt: T. Rex Teens Gained 5 Pounds a Day" [August 11, 2004].)
"We don't tend to find many tyrannosaur specimens until they get close to their adult size, and then we find lots of them. This [new finding] explains that mystery," Erickson said.
"It used to be thought that juvenile tyrannosaurs didn't preserve [as fossils] for some reason," he said. "That doesn't make a whole lot of sense."
As to why the record is so rich in young adult tyrannosaurs, Erickson said, "These animals probably had a real hard time once they sexually matured."
His study notes that only 1 in 50 tyrannosaurs, on average, lived into their dotage.
"There's a reason we don't find these giants-among-giants like Sue," a 42-foot-long (13-meter-long) T. rex. Sue lived to about 28 years old and is on display at the Field Museum in Chicago.
"It's just a small percentage of these animals that ever survived long enough to even be that big."
(Related: "Dinosaurs Come Alive" in National Geographic magazine.)
Peter Makovicky is a curator of dinosaurs at the Field Museum and has collaborated with Erickson in the past. He described the new study as "very exciting."
"It's got some pretty far-reaching implications for the way we understand dinosaur biology and various aspects of the dinosaurian fossil record," Makovicky said.
"We're now able to approach some of these more abundant dinosaursthe ones [where] we have multiple representatives of a speciesin biological ways, in the ways we study living animals and living populations."
"I think that's going to be a really exciting frontier in the years to come."
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES