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This long period of development may have been what doomed them, Erickson said. After an asteroid crashed into the earth 66 million years ago, triggering a mass extinction, it would have been harder for slowpoke reproducers like dinosaurs to recover.

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“I think it's an important piece for understanding why dinosaurs went extinct,” he said.

To understand why, we have to rewind a bit, to about 10 million years before the asteroid strike. It's the end of the Cretaceous Period. In what is now Mongolia's Gobi Desert, a clutch of 12 eggs the rough size and shape of potatoes has just been laid by a Protoceratops andrewsi, a relative of the more famous Triceratops. Around the same time, near Alberta, Canada, a duck-billed Hypacrosaurus stebingeri laid a huge egg the size of a volleyball. (“Ridiculous,” Erickson said of the almost 10-pound Hypacrosaurus egg.)

While the dinosaurs did whatever it is dinosaurs do while nesting (scientists think some species sat on their nests like birds, while others probably buried their eggs like reptiles), the embryos inside slowly develop. Just before the halfway point of their incubation period, they start to grow teeth.

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In humans and reptiles, teeth are formed from dentin, a liquid that gets laid down every day and then mineralizes, forming a hard layer. Over time, the layers build up like tree rings, one for each day the embryo developed.

“You can basically just count those up and figure out how long it took the dentition to form,” Erickson said. He wondered if dinosaur embryos might exhibit the same phenomenon. So he talked with scientists at the American Museum of Natural History (which houses the Protoceratops eggs) and the University of Calgary (which has the Hypacrosaurus egg) into letting him sample a small amount of tooth from each fossil.

As soon as he popped his slide under a microscope and saw the telltale dentin layers, “I knew we were in business,” Erickson said.

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The number of layers let him calculate a conservative estimate of the incubation times for the two species: about three months for the smaller Protoceratops hatchlings, six months for the larger Hypacrosaurus. Most birds' eggs hatch in a fraction of that time (chickens take three weeks, canaries need just 13 days). Even emperor penguin dads, who famously huddle around their eggs for extended periods to protect them from the harsh Antarctic winter, incubate their young for two months at most.

“It's really surprising,” Erickson said. “I don’t think that people would have entertained the idea that they would have incubated over the better part of the year.”

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David Varricchio, a paleontologist at Montana State University not involved in the study, was less surprised by the long incubation times, noting that modern reptiles also take a long time to hatch. But the method of calculating incubation periods was a compelling one, he said, and it provides another line of evidence for understanding dinosaurs' lives.

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Spending a long time caring for a clutch of eggs probably cramped dinosaur parents' styles. It restricted their habitats to regions where the weather was right for incubating an egg, made migration more difficult, and exposed attending parents to predators, natural disasters and hunger — “all the rigors that go with trying to protect the nest for long periods of time,” Erickson said. Basically, this could have been an extreme version of the lengthy parenting depicted in the film “March of the Penguins.”

And that probably had consequences when the asteroid struck and wiped out most dinosaurs, along with 75 percent of all life on Earth. If a species was going to survive, it needed to be holding all the right cards when it came to physical and life history attributes. Successful creatures were small and adaptable animals that lived fast and died young. For the ancestors of birds, quick incubation times may have boosted their ability to repopulate and evolve to fill ecological niches left vacant after the disaster.

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But dinosaurs were large and probably endothermic (warm blooded), which made them “profligate wasters of energy,” Erickson said. They also took a long time to reach sexual maturity. On top of all this, a slow incubation might have been a “black ace” — contributing to long generational times that made it harder for dinosaurs to bounce back.

“Dinosaurs found themselves holding basically a dead man's hand,” Erickson said.