Research has now shown that it may have taken far longer for baby dinosaurs to hatch than previously thought. Thankfully, some of them - our fine feathered friends - appear to have developed more speedily.
Now-extinct dinosaurs took far longer to develop in the egg than previously thought, according to a new study by US researchers.
In a paper published in the Proceedings of the National Academy of Sciences, scientists found evidence that a long incubation period might have led to the extinction of most dinosaur species.
Contrary to popular understanding, dinosaurs didn't become completely extinct. The fossil record shows that birds were the only dinosaur lineage to survive mass extinction. As such they're still considered "avian dinosaurs." The key to their survival might well have been having eggs that hatched quickly.
Until now, researchers tended to think that both avian and non-avian types had roughly similar incubation periods before hatching - between 11 and 85 days. "Because birds are living dinosaurs, rapid incubation has been assumed for all dinosaurs," the paper said.
But scientists led by Dr. Gregory M. Erickson at Florida State University found something quite different.
They took a look at the fossilized teeth of two rare but well-preserved non-avian dinosaur embryos that lived about 76 million years ago.
The two baby dinos appeared to have grown on a time scale far more similar to reptiles than birds.
'Tree rings' in fossil teeth
One of the two embryos, belonging to the small sheep-sized Protoceretops species and found in Mongolia, was almost three months old. The other, a huge duck-billed Hypacrosaurus embryo that was found in Alberta, Canada, was almost six months old.
The researchers used CAT-scanning and high-resolution microscopes to scrutinize dental growth markers - laid down on a daily basis - known as von Ebner lines.
"These are the lines that are laid down when any animal's teeth develops," said 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."
There was huge variation in incubation times, but both took far longer than birds.
When the going gets tough…
Most dinosaurs were wiped out more than 60 million years ago. While the reasons aren't entirely clear, it's something that has been explained by falling global temperatures compounded by an enormous asteroid collision - the Cretaceous–Paleogene (K–Pg) mass extinction event.
What separated both avian and non-avian dinosaurs from reptiles and amphibians was that they appear to have been "warm-blooded."
The ancestors of birds - which developed shorter incubation periods - would have beenable to move and adapt more easily, while more "efficient" cold-blooded reptiles and amphibians could deal with the drop in temperature.
Old-school dinosaurs, meanwhile, were left out in the cold. Back then, those long nesting periods would have posed a huge challenge. Parents would have had to pick sites that were protected from flood, drought and predation over many months.
"Prolonged incubation exposed non-avian dinosaur eggs and attending parents to destructive influences for long periods," the team concluded. "Slow development may have affected their ability to compete with more rapidly generating populations of birds, reptiles, and mammals following the K–Pg cataclysm."