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In the novel "Jurassic Park," Dr. Henry Wu is explaining how dinosaurs are born. "The incubation period varies with each animal, but in general it runs about two months," he tells the book's hero, Dr. Alan Grant.

Close, but no cigar, according to research published this week. Baby dinosaurs stayed as long as six months in their eggs before they hatched — and that could have contributed to their extinction.

Umanoide

Researchers said they think they've cracked the mystery of how long it took for dinosaurs to hatch in a paper published Monday in the Proceedings of the National Academy of Science. And like modern-day forensic specialists, they found that answer in dental records.

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They looked at "rings" in the teeth of fossilized embryos from two dinosaur genera: Protoceratops andrewsi, a pig-size plant eater, and Hypacrosaurus stebingeri, a duck-billed 30-foot-long giant that roamed Canada and Montana tens of millions of years ago.

"These are the lines that are laid down when any animal's teeth develops," said Gregory Erickson, a professor of biological science at Florida State University, who led the research team. "They're kind of like tree rings, but they're put down daily. We could literally count them to see how long each dinosaur had been developing."

Protoceratops embryos took about three months to hatch, the data show, while Hypacrosaurus took half a year.

"We suspect our findings have implications for understanding why dinosaurs went extinct at the end of the Cretaceous period, whereas amphibians, birds, mammals and other reptiles made it through and prospered," Erickson said.

That's because the longer dinosaurs needed to develop in their eggs, the longer they were defenseless against weather and natural disasters.

Not to mention predators — like the Velociraptors depicted hatching in the movie version of "Jurassic Park."

For them, the eggs of their cousins were especially favored snacks.