For a long time, scientists believed that dinosaur eggs took about the same time to hatch as birds because of biological connections between birds and dinosaurs. Most birds’ eggs hatch in one week to three months. But dinosaur eggs stayed in the shell much longer, according to a study published in the journal Proceedings of the National Academy of Sciences.

fossilized dinosaur brain
Image source: Wikimedia Commons

A hidden factor in dinosaur extinction

Scientists at the Florida State University found that dinosaurs took between three and six months to hatch out of their eggs. For the entire incubation period, the parents had to stay where the eggs were laid. They couldn’t easily flee predators or go out in search of food. The long incubation time made the eggs vulnerable to floods and predators.

It also meant that adults were reproducing at much slower rates than birds or mammals. It could have played a role in the extinction of dinosaurs following the asteroid impact. Greg Erickson, the lead author of the study, said animals that reproduce quickly are more likely to survive mass extinction events, and are better equipped to adapt to challenges.

Growth lines on teeth helped determine incubation time

Erickson and his colleagues studied the teeth of embryonic dinosaurs to determine how long it took them to hatch from the eggs. The teeth have incremental growth lines that can be used to estimate an animal’s age. Every day, a layer of dentine fills in the inner part of the tooth and mineralizes. It leaves growth lines on teeth that can be measured.

Dinosaur fossils and fossilized eggs are not rare, but recovering fossilized embryos is incredibly difficult. Erickson sought help of the American Museum of Natural History, which has a large number of fossilized embryos in its collection. Researchers studied the embryos of Protoceratops andrewsi and Hypacrosaurus stebingeri. The Protoceratops eggs were the size of potatoes, but Hypacrosaurus eggs were as big has a volleyball.

Scientists plan to study the incubation time of more specimens

They determined that the Protoceratops had an incubation time of three months. It was six months for the larger Hypacrosaurus. In contrast, canaries take less than two weeks, and chickens need just three weeks. The discovery indicates that the fast incubation of modern birds probably evolved much later, said Erickson. The study also helps explain how birds with their fast incubation times and reproduction rates thrived after dinosaurs disappeared.

However, the study has certain limitations. The researchers analyzed specimens of only two species. Erickson plans to study more dinosaurs, including carnivorous ones to see if they too have similar incubation times.