The researchers added this onset of sexual maturity to their growth graphs and found that the dinosaurs were reaching sexual maturity in the midst of a teenage growth spurt.
"They are definitely not juveniles, but they are not fully grown yet, and they are also going through a time of really rapid growth it's very similar to what we call adolescence," Werning said.
The team also confirmed that a Tyrannosaurus rex bone that North Carolina State University paleontologist Mary Schweitzer found in 2005 contained medullary tissue when it died at 18.
(Related: "He Rex or She Rex? Experts Find Way to Tell Dino Gender" [June 2, 2005].)
All three types of dinosaurs had life spans of about 25 to 30 years. But they didn't reach full adult size until age 20 to 25. Waiting until they were fully grown to reproduce would have been risky, according to Werning.
"It makes a lot of sense that [dinosaurs] wouldn't have the same strategy as birds," she said.
Birds Are Unique
The finding complements research published last year that showed that birdlike dinosaurs—discovered sitting on their eggs just like birds do—also had sex as teenagers.
"What Sarah and Andrew have done in their paper is find exactly the same thing but in a different group of dinosaurs and by using a different marker of reproductive maturity," said Kristi Curry Rogers, a paleontologist at Macalester College in St. Paul, Minnesota.
Curry Rogers is a co-author of the 2007 birdlike-dino study with Gregory Erickson, a paleontologist at Florida State University.
She said both studies highlight the uniqueness of sexual maturity in birds.
"[Bird] evolution is a very fascinating mosaic of characteristics that deserves a lot more study," she said. "We're still at the beginning of teasing apart all the fine details."
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