A hatchling Protoceratops andrewsi from the Gobi Desert. Image: AMNH/M. Ellison

Why did the dinosaurs go extinct? We may never be completely sure, although a giant asteroid and a bunch of enormous volcanic eruptions probably had a lot to do with it. But here’s another factor you may not have considered: too much time in the egg.

Dinosaur eggs took a surprisingly long time to hatch, according to a study published today in the Proceedings of the National Academies of Sciences, which determined that two well-preserved dinosaur embryos—a Protoceratops and a Hypacrosaurus—were three and six months old respectively when they died inside their shells. If long incubation times were the norm among dinosaurs, the risk to babies and adults alike would have made it hard to compete in the post-apocalyptic wasteland following the Chicxulub asteroid impact 66 million years ago.

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“I was stunned,” Florida State University biologist and lead study author Gregory Erickson told Gizmodo, when asked how he felt after discovering that baby dinosaurs spent up to half a year inside their eggs. “As a biologist, understanding incubation periods of an egg-laying animal has myriad implications for the group.”

Many of our assumptions about the lives of dinosaurs are based on their living descendants—birds—and embryonic development is no exception. Birds stand out among egg-laying animals, in that they produce a small number of large eggs with very short incubation periods, anywhere between 11 and 85 days. This strategy promotes high survivorship: the less time you spend chilling in a gooey amniotic sac inside a shell, the less likely you are to become somebody else’s brunch.

A fossil embryo of the dinosaur Hypacrosaurus was examined in the study. Image: Darla Zelinitsky

Since birds are “living dinosaurs” and since other attributes of avian reproduction seem to trace back to Cretaceous ancestors, most paleontologists figured that dinosaurs also hatched quickly. “It was assumed that you could take the size of a dinosaur egg and put it on a regression line for how fast birds incubate their eggs,” Erickson said. “This is a good speculative approach, but it’s not empirical.”

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To get actual data on how quickly dinosaurs hatched, Erickson and his colleagues needed a way of aging unborn embryos. An idea for how to do this came to Erickson in the early 1990s, when he was doing his masters’ thesis at Montana State University. There, he discovered that daily growth lines—so-called “von Ebner” lines—could be used to age adult dinosaur teeth.

Over 20 years later, Erickson and his colleagues obtained access to two extremely well-preserved dinosaur embryos: one of a sheep-sized herbivore called a Protoceratops, another of the enormous duck-bill Hypacrosaurus. Using CT-scanning, they examined the tiny monsters’ tiny teeth up close, counted the von Ebner lines, and learned that each of the embryos was several months old at the time of death. These results place the first lower limits on dinosaur incubation times. (Because neither specimen had hatched yet, actual incubation times for the two specie could be longer.)

Daily growth lines in the dentine in an embryonic tooth of Hypacrosaurus. Image: G. M. Erickson

Not only does Erickson’s finding fly in the face of conventional wisdom, it implies that the badass reptilians of the Cretaceous were at a decided disadvantage when it comes to coping with environmental disturbance. “Having a slow incubation period—three to six months—would have exposed eggs to predation, droughts and flooding for long periods of time,” Erickson said. “If there were attending parents, you can imagine the parents would have been exposed for long periods of time, too.”

In the years following the asteroid impact that punched out the 10,000 square-mile Chicxulub crater, the world was in a state of extreme upheaval: devastating volcanic eruptions, rapid climate change, sudden ecological collapse. Taken together with other aspects of dinosaur biology, the risk of prolonged incubation periods would have made it difficult for dinosaurs to compete with smaller, faster-reproducing reptiles, birds and mammals.

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“With regard to their life history and physiological attributes, dinosaurs were basically holding a dead man’s hand,” Erickson said. “They were profligate wasters of energy, which is bad in a resource-depleted environment. Some of [our] work shows that dinosaurs took over a year to reach maturity. You throw in very slow incubation times on top of that, and these attributes are collectively a bunch of black eights and aces.”

The study only looked at two specimens, and it remains to be seen whether long incubation times were a universal feature of dinosaur biology. Still, it’s sobering to think that one of the most infamous extinction events in Earth’s history may have been tipped off, at least in part, by excessive parenting.

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[PNAS]