For Tyrannosaurs, Teen Years Were Murder

Sean Markey
for National Geographic News
July 13, 2006
If they survived the deadly toddler years, tyrannosaurs apparently had it pretty cushy, at least until they hit dinosaur puberty.

But after these dinosaurs reached sexual maturity, life's harsher realities kicked in again. Beginning at about age 14, tyrannosaurs suffered death rates of nearly 23 percent a year, according to a new study.

Threatened by disease, combat, and the stress of mating or raising offspring, most adults were lucky to hit their early 20s, the study says.

And just 2 percent "lived long enough to attain their maximal size and age" in their late 20s, the study authors write.

The study is the first to chart dinosaur survival rates—using a technique that until now had only been applied to living animals.

Gregory M. Erickson, an evolutionary biologist and paleontologist at Florida State University in Tallahassee, led the research, which will be published in tomorrow's issue of the journal Science. Erickson is also a host of the National Geographic Channel TV series Hunter and Hunted, and the National Geographic Society has partially funded some of his research. (National Geographic News is part of the National Geographic Society.)

Dinosaur Obituaries

The researchers analyzed specimens from four North American tyrannosaur species: Albertosaurus sarcophagus, Tyrannosaurus rex, Gorgosaurus libratus, and Daspletosaurus torosus.

First discovered in 1910, a mass grave of 22 Albertosaurus fossils in Alberta, Canada, supplied the team with the most dinosaur obits (Canada map).

Part of a herd or simply close neighbors, the predators likely died over a period of weeks or months about 70 million years ago, perhaps due to a drought or similar catastrophe.

Using a technique pioneered by Erickson and others, the team counted growth rings—like those in trees—in fossil leg and foot bones to determine how old each dinosaur had been when it died.

The researchers then plotted an age curve, or survivorship pattern, for the entire population.

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].)

Fossil Record

"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 dinosaurs—the ones [where] we have multiple representatives of a species—in 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."

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