Dinosaur hatchlings emerged from their eggs after an incubation period of three to six months, scientists have found in a new groundbreaking study that could help understand why these prehistoric reptiles went extinct. Researchers led by Gregory Erickson from Florida State University in the US studied the biology of these ancient creatures and used embryonic dental records to solve the mystery of how long dinosaurs incubated their eggs.
"Some of the greatest riddles about dinosaurs pertain to their embryology - virtually nothing is known," Erickson said. Scientists had long theorised that dinosaur incubation duration was similar to birds, whose eggs hatch in periods ranging from 11-85 days. Comparable-sized reptilian eggs typically take twice as long - weeks to many months. Since the eggs of dinosaurs were so large - some were about four kilogrammes or the size of a volleyball - scientists believed they must have experienced rapid incubation with birds inheriting that characteristic from their dinosaur ancestors.
Researchers including those from University of Calgary in Canada and the American Museum of Natural History in the US decided to put these theories to the test. "Time within the egg is a crucial part of development, but this earliest growth stage is poorly known because dinosaur embryos are rare," said Darla Zelenitsky, assistant professor at University of Calgary. The two types of dinosaur embryos researchers examined were those from Protoceratops - a sheep-sized dinosaur found in the Mongolian Gobi Desert whose eggs were quite small (194 grammes) - and Hypacrosaurus, an enormous duck-billed dinosaur found in Canada with eggs weighing more than four kilogrammes.
Researchers ran the embryonic jaws through a CT scanner to visualise the forming dentitions. They extracted several teeth to further examine them under microscopes. Growth lines on the teeth showed researchers precisely how long the dinosaurs had been growing in the eggs. "They're kind of like tree rings, but they are put down daily. We could literally count them to see how long each dinosaur had been developing," Erickson said.
The results showed nearly three months for the tiny Protoceratops embryos and six months for those from the giant Hypacrosaurus.
Prolonged incubation put eggs and their parents at risk from predators, starvation and environmental risk factors. Theories that some dinosaurs nested in the more temperate lower latitude of Canada and then travelled to the Arctic during the summer now seem unlikely given the time frame for hatching and migration. The biggest ramification from the study relates to the extinction of dinosaurs.
Given that these warm-blooded creatures required considerable resources to reach adult size, took more than a year to mature and had slow incubation times, they would have been at a distinct disadvantage compared to other animals that survived the extinction event. The study was published in the journal PNAS.
