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Dinosaur eggs took months to hatch — putting parents and babies at risk

Long incubation time may have played a role in their extinction

A hatchling Protoceratops andrewsi fossil from Mongolia
M. Ellison/American Museum of Natural History

Some dinosaur eggs took as long as six months to hatch — an incredibly long time that may have given dinos a disadvantage at the time of their extinction 65 million years ago. New research shows that some dinosaur embryos took twice as long as bird eggs of similar size to hatch. And that has implications for dinosaurs’ survival: long incubation puts both parents and eggs at risk of being attacked by predators or killed by natural disasters.

For a long time, scientists believed that dinosaur eggs took as long as bird eggs to hatch — between 11 and 85 days. That’s because birds are dinosaurs’ living descendants, so scientists thought birds must have inherited the quick incubation periods from their dino ancestors.

In the study, published yesterday in Proceedings of the National Academy of Sciences, researchers tested that theory by analyzing the teeth of two unhatched dinosaur embryos. One embryo belonged to a sheep-sized dinosaur found in Mongolia called a Protoceratops. Another one to a giant duck-billed dinosaur found in Canada called a Hypacrosaurus. The growth markers in the embryos’ teeth showed that the Protoceratops eggs took almost three months to hatch, while the Hypacrosaurus eggs took almost six months. Because some dinosaurs were bigger than the Hypacrosaurus, incubation time could have been even longer for some dinos.

A fossilized embryo of the dinosaur Hypacrosaurus
Darla Zelinitsky

The implications of the long incubation periods are enormous. If dino parents had to spend half a year or more tending their eggs, they couldn’t have migrated easily. They would have had to stay in one place for a long time, putting them and their eggs at risk of being eaten by other predators. The eggs would also have been at high risk from floods and droughts for long periods of time.

That means that when an asteroid hit Earth 65 million years ago, kicking off a time of rapid climate change and mass extinction, dinosaurs weren’t reproducing as fast as other animals. In short, the long incubation time could have contributed to their extinction. Now, scientists will have to see if the same holds true for other types of dinosaur.