Baby Dinos Lived in Eggs for 3-6 Months

Dinosaur babies stayed in their eggs for 3 to 6 months, much longer than scientists previously thought. The new findings, published in the journal Proceedings of the National Academy of Sciences, might help explain the extinction of the powerful predators.

Dinosaurs are long gone, leaving behind only remnants of themselves in mysteriously-massive fossilised body parts that have captured the wandering mind of man. Ever since their discovery, researchers have persevered in collecting more and more information about these scary creatures. However, their embryology remains mostly unknown.

“Some of the greatest riddles about dinosaurs pertain to their embryology—virtually nothing is known,” says Gregory Erickson, one of the authors. “Did their eggs incubate slowly like their reptilian cousins—crocodilians and lizards? Or rapidly like living dinosaurs—the birds?”

Co-author Darla Zelenitsky explains that time spent inside the egg represents an essential stage of development: we could decipher whether dinosaurs were more like birds or reptiles in their growth. However, the great rarity of dinosaur embryos accounts for a poor understanding of this developmental phase. Because of the lack of knowledge, scientists could only assume that the embryonic lives of dinosaurs probably lasted from 45 to 80 days—a short incubation period being a trait displayed by birds which are thought to be the descendants of dinosaurs. Erickson says that a reptile egg will normally hatch after twice the period of time a bird egg comparable in size takes.

Thankfully, this has been partially solved: recent analysis of rare fossilised dinosaur embryo teeth shows the opposite of common scientific opinion on the subject.

75-million-year-old hatchling fossil of dinosaur Protoceratops andrewsi. Its fossilised embryos reveal the extremely long incubation of the creature: the little ones lived in their eggs for 3 months. Photo credits: Mick Ellison/AMNH.

The fossils came from two species of dinosaurs, Protoceratops andrewsi, which had small eggs (194 grams), and Hypacrosaurus stebingeri, whose eggs were bigger (over 4 kilograms). Putting their embryonic jaws through CT-scan, dentition was examined. Then, the teeth were viewed under a microscope, and counts of growth lines carved on the teeth, were analysed; this feature, whose usefulness is like that of tree rings, indicated that dinosaur hatching was more similar to reptiles.

“These are the lines that are laid down when any animal’s teeth develops,” Erickson said. “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.”

From these results, the team deduced that the smaller Protoceratops embryos were in their eggs for approximately 3 months while the Hypacrosaurus ones spent 6 months on the inside.

The longer incubation might even have curbed the survival chances of dinosaurs. Having to guard over eggs for more months is rather challenging: the parents would be more likely to succumb from starvation or attacks from their predators because of this long-lasting responsibility. Furthermore, natural calamities would have exacerbated their vulnerability as they would be less resilient in the aftermath of extinction events: if their incubation was slower, and they took over a year to reach maturation, while they also needed more resources, they would be at a considerable disadvantage as opposed to other animals following a disaster. This could help elucidate the mystery of their disappearance from Earth.

“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,” Erickson said.

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