We now know how long dinosaurs took to emerge from their eggs.

Scientists from Florida State University (FSU) suggest, in a new study in the journal Proceedings of the National Academy of Sciences, that the answer is from 3 to 6 months, depending on the type of dinosaur.

Beyond the wow factor of the information on its own, the finding has implications for our understanding of how dinosaurs lived and why they went extinct, according to the FSU researchers.

Study lead Gregory Erickson explained in a press release that we know "virtually nothing" about dinosaurs' embryonic lives.

"Did their eggs incubate slowly like their reptilian cousins – crocodilians and lizards? Or rapidly like living dinosaurs – the birds?" he said.

Thanks to some rare embryo fossils and some high-tech equipment, Erickson and his team appear to have answered those questions.

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First the scientists gathered embryo fossils from two dinosaurs: Protoceratops, whose tiny eggs weighted just 194 grams (7 ounces), and the enormous, duck-billed Hypacrosaurus, with 4 kilo (9 pound) eggs.

Then the team put the embryonic jaw of each through a CT scanner, to visualize the forming teeth, and extracted a number of individual teeth for study beneath a high-powered microscope.

Under microscopic view, Erickson and his team found growth lines on the teeth that helped the researchers establish a timeline for embryonic development.

"These are the lines that are laid down when any animal's teeth develop," Erickson said. "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."

In the end, the team determined that the tiny eggs from the sheep-sized dinosaur Protoceratops took about three months to hatch, while Hypacrosaurus took about 6 months.

Researchers examined a fossilized embryo of the dinosaur Hypacrosaurus. Credit: Darla Zelinitsky

The FSU team noted some of the key points their find implies.

First, answering Erickson's earlier questions, the discovery suggests dinosaur egg development more closely resembled that of primitive reptiles than birds. That contradicts previous theories that dinosaur incubation was bird-like in its speed, with eggs hatching in 11 days to just under two months.

Second, the team says its findings throw into doubt theories about some dinosaur migration patterns. Animals once theorized to summer in the Arctic and winter in lower Canada, for example, might not have had the time for such journeys, given the time required for long periods of egg development, maturation, and then migration.

Third, and most important of all, say the FSU researchers, is what their find has to say about the extinction of dinosaurs. With slow egg-hatching times coupled with more than a year of maturation outside the egg, the creatures would have been at a disadvantage compared to other animals that survived the great dinosaur extinction event.

"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.

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