Dinosaur eggs had long incubation periods

Fossil of newborn Protoceratops andrewsi (Photo courtesy Gregory Erickson, FSU)
Fossil of newborn Protoceratops andrewsi (Photo courtesy Gregory Erickson, FSU)

An article published in the journal “Proceedings of the National Academy of Sciences” describes a research on the incubation time of dinosaurs eggs. A team of researchers led by Gregory Erickson of Florida State University analyzed rare fossils of embryos of two species of dinosaurs and concluded that the eggs of these ancient animals hatched after a period of three and six months, depending on the species.

The information available on dinosaurs’ embryonic development are really limited. For example, a question that paleontologists asked whether their eggs’ incubation was slow as that of their cousins ​​crocodiles and lizards or quick as that of their descendants, birds. The discoveries of the last decades on the similarities between dinosaurs and birds could suggest a quick incubation for dinosaurs and also the fact that their eggs were big could be interpreted as an indication to that effect.

Gregory Erickson’s team conducted a research examining rare embryos belonging to two species of dinosaurs. A type of embryo was of Protoceratops andrewsi, a small horned sheep-sized dinosaur. This fossil that has an age between 71 and 75 million years was found in the Gobi Desert in Mongolia and its egg was small, weighing 194 grams. The other type of embryo was of Hypacrosaurus stebingeri, a duck-billed dinosaur that could reach an weight around 4 tons. This fossil that has an age of about 76 million years was found in Alberta, Canada and the weight of its eggs is more than 4 kg.

The analysis was conducted on the embryos’ teeth, in particular by examining their growth markers, an examin of the type done to trees by examining their trunks’ growth rings with the difference that the markers in dinosaur teeth are daily. As it happens more and more often in paleontological studies, the researchers used modern technologies to conduct very sophisticated tests on the fossils. In this case, they did a CT scan to the embryos and examined several of their teeth with sophisticated microscopes.

The result is that according to Gregory Erickson’s team Protoceratops andrewsi’s eggs took at least 83 days to hatch, almost 3 months, while those of Hypacrosaurus stebingeri took at least 171 days, almost 6 months. The incubation time increases with the eggs’ size in both birds and reptiles, so it’s plausible that this was also true for dinosaurs.

There were species with eggs much bigger than those of Hypacrosaurus stebingeri so it’s possible that their incubation times was close to a year. This research focused on two species of ornithischian dinosaurs because it was possible to examine embryos of species belonging to that group. It would be interesting to know if theropods, the other large group, had similar egg incubation times.

The researchers made some considerations about the consequences of a long incubation period of the eggs. In particular, they wondered how much it affected their extinction. When an asteroid struck the Earth about 66 million years ago, the long incubation period became a serious disadvantage and may have been one of the causes of dinosaurs’ extinction.

Fossil Hypacrosaurus stebingeri embryo (Photo courtesy Darla Zelenitsky, University of Calgary)
Fossil Hypacrosaurus stebingeri embryo (Photo courtesy Darla Zelenitsky, University of Calgary)

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