For a long time, scientists have thought that non-avian dinosaurs became extinct because of a massive asteroid that struck Earth some 66 million years ago – but a new study suggests that something less dramatic may have also played a role in their demise.

Researchers have examined rare fossils of dinosaur embryos and discovered that the incubation period of non-avian dinosaurs was way longer than we previously realised – at least as long as six months for one species, in fact – meaning dinosaurs were at a distinct evolutionary disadvantage to other animals that developed much more quickly.

"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," says biologist Gregory Erickson from Florida State University.

Scientists previously thought that the incubation period of dinosaurs would have been similar to their avian ancestors – with their eggs hatching in between 11 to 85 days.

While dinosaur eggs could be quite large – weighing up to 4 kilograms, and being about the size of a volleyball in some instances – researchers hypothesised that rapid incubation may have allowed the eggs to develop quickly, rather than the slower incubation of comparable reptile eggs, which can take many months before they hatch.

To test if this were true, Erickson and fellow researchers examined embryo fossils from two different species: Protoceratops, a small, sheep-sized dinosaur; and Hypacrosaurus, an enormous duck-billed dinosaur.

After running the embryonic jaws through a CT scanner, the team extracted several of the forming teeth and analysed them under powerful high-resolution microscopes.

The technique enabled the scientists to identify what are called "von Ebner lines". These growth lines are present in all animal's teeth, but it's the first time scientists have identified them in dinosaur embryos.

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

"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."

When they did so, they found the Protoceratops embryo was almost three months old, and the Hypacrosaurus specimen was nearly six months.

"I was stunned," Erickson told Maddie Stone at Gizmodo. "As a biologist, understanding incubation periods of an egg-laying animal has myriad implications for the group."

And in the case of these dinosaurs, the implications of such drawn-out incubation periods could have been particularly grave.

"Having a slow incubation period – three to six months – would have exposed eggs to predation, droughts, and flooding for long periods of time," Erickson told Gizmodo.

"If there were attending parents, you can imagine the parents would have been exposed for long periods of time, too."

87324923478-dinosaur-1Hypacrosaurus. Credit: Darla Zelenitsky/University of Calgary

Those risk factors would have become immeasurably more pressing in harsh environmental circumstances – such as after the Chicxulub asteroid impact 66 million years ago – when competition between different species for limited resources in the aftermath would have been especially fraught.

In other words, it's possible the combination of long incubation periods and a world-changing cataclysmic event created hostile conditions that were just too tough for non-avian dinosaurs.

The researchers acknowledge that they've only examined two fossils so far, both of which belong to a group called Ornithischian dinosaurs, which are characterised by their pelvic structure.

But we don't yet know about the incubation periods of Theropods – which are more closely related to today's birds, and include the likes of Tyrannosaurus rex and Velociraptor.

The researchers want to take a look at their embryonic teeth next, provided they're lucky enough to find rare samples.

"What would be really interesting now is to see if small theropod dinosaurs like Velociraptor also incubated slowly," palaeontologist Stephen Brusatte from the University of Edinburgh in the UK, who was not involved with the study, told Carolyn Gramling at Science.

"If more modern-style birds are the only ones that incubate very quickly, it could be that this feat of biology gave them a better lotto ticket for surviving the asteroid impact that killed off all of the other dinosaurs."

We'll have to wait and see if the team is able to source more dinosaur embryos to continue their research.

But in the meantime it's fascinating to think that these ancient creatures – which we always think of as being so fearsome and fierce – may have been brought down by something as seemingly innocuous as a little extra time in the egg.

And yet, it's just the latest evidence that dinosaurs – despite their awesome power and size – had a lot of things holding them back, evolutionary-wise.

"These animals were profligate wasters of energy… even the smallest dinosaurs took over a year to mature," Erickson told James Gorman at The New York Times.

"The dinosaurs found themselves holding some bad cards. They had a dead man's hand."

The findings are reported in Proceedings of the National Academy of Science (link down at time of writing)