Paleontologists have discovered that dinosaur young took so long to hatch and grow into adulthood that populations failed to recover quickly enough after the devastating impact 65 million years ago.
In contrast, birds and small mammals only took a few weeks for their offspring to emerge giving them a distinct advantage.
The discovery was made by scientists at Florida State University and the University of Calgary, who realised it was possible to calculate how long it took for dinosaurs to hatch based on marks on the teeth of embryos and babies.
Just like tree rings growing a new layer each year, teeth grow a new layer each day, which can be seen in microscopic lines in the dentine.
By simply counting the lines, scientists found that it took dinosaurs between three and six months to hatch.
The lengthy incubation period – in comparison to small mammals – made the hatchlings, and their parents, vulnerable to predators and left them struggling to re-establish their species.
“Some of the greatest riddles about dinosaurs pertain to their embryology, virtually nothing is known,” Gregory Erickson, professor of biological science at Florida said.
“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.”
Because birds are living dinosaurs, scientists have long assumed that the duration of dinosaur incubation was similar to birds, whose eggs hatch within 11 to 85 days.
However, similar sized reptilian eggs typically take twice as long, ranging from several weeks to many months. To find out where dinosaurs fitted in, the team studied the fossils of dinosaur embryos.
“Time within the egg is a crucial part of developmentt, but this earliest growth stage is poorly known because dinosaur embryos are rare,” said Darla Zelenitsky, assistant professor of geoscience at the University of Calgary.
“Embryos can potentially tell us how dinosaurs developed and grew very early on in life and if they are more similar to birds or reptiles in these respects.”
The two types of dinosaur embryos researchers examined were those from a protoceratops, a sheep-sized dinosaur found in the Gobi desert whose eggs were quite small, and hypacrosaurus, an enormous duck-billed dinosaur found in Alberta, Canada with eggs weighing nine pounds.
The team ran the embryonic jaws through a CT scanner to visualise the forming dentition. Then, they extracted several of the teeth to examine them under sophisticated microscopes.
Growth lines on the teeth showed researchers precisely how long the dinosaurs had been growing in the eggs. Prof Erickson said the teeth are “kind of like tree rings, but they’re put down daily. So we could literally count them to see how long each dinosaur had been developing”.
Their results showed nearly three months for the tiny protoceratops embryos and six months for those from the giant hypacrosaurus.