"Beyond the question of a reasonable doubt, these are the parents of those eggs," Curry Rogers said. (Related: "Dinosaur Eggs Discovered Inside Mother -- A First" [April 14, 2005].)
The researchers examined thin cross-sections of bone from these dinosaur parents to determine whether they were fully grown.
Similar to growth rings on a tree, fast-growing bones have lots of blood vessels. They also have a line marking where growth paused, then another pulse of growth, then another line, Curry Rogers explained. The outer layer of fully grown bones is a stack of lines.
Two of the brooding dinosaurs examined in this study lacked the stacked lines, indicating the dinosaur parents mated before they reached full maturity.
Peter Makovicky, the assistant curator of paleontology at The Field Museum in Chicago, Illinois, was not involved in the research.
He says the study, published last month in the online edition of the journal Biology Letters, shows fast growth and delayed sexual maturity came late in the history of bird evolution.
"It's quite likely that the earliest birds, things like archaeopteryx, had the same pattern we see in these dinosaurs," he said.
Lead author Erickson said the finding supports a study he and colleagues published last year in the journal Science that found tyrannosaur mortality went up when they hit their teens.
The researchers hypothesized the spiking death rates are related to the hardships of sexual maturity and parenthood.
"This [new study] is consistent with that this is the strongest evidence [of] the timing of their sexual maturity," Erickson said.
With most scientists recognizing the link between dinosaurs and birds, scientists are beginning to study what traits make birds stand apart from dinosaurs, study co-author Curry Rogers said.
"We are as a group pretty comfortable with the idea that birds are meat-eating dinosaurs with fancy feathers and the ability to fly," she said. "And so it really does wow us to tease apart the more intricate parts of this evolutionary story."
Given the similarities between dinosaurs and birds, why and when did birds evolve such fast growth rates?
The answer may lie in escape from predators, study author Erickson said.
Though early sexual maturity offers the advantage of contributing to the gene pool, the energy required for reproduction can hinder growth.
But birds can't fly until they are almost fully grown, explained Erickson, and flight is a primary defense against capture by a predator.
"So one of the ways to cheat the system and grow faster and escape predation is to forgo sexual reproduction," he said.
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