Dinosaurs weren’t just super-sized reptiles—they were bird-like too, with colors and feathers. They may have even been partially warm-blooded. But researchers haven’t been able to figure out one important part of their lifecycle: Did dinosaur eggs incubate rapidly, like modern-day birds? Or did they follow a more reptilian pattern, taking weeks or even months to hatch? New research in Proceedings of the National Academy of Sciences suggests that when it came to hatching babies, dinosaurs were squarely on the lizard-like side of the equation.
Researchers examined the teeth of dinosaur embryo fossils including Protoceratops, a sheep-sized dino collected in Mongolia’s Gobi Desert, and an embryo of a duck-billed Hypacrosaurus, a large dino with eggs the size of volleyballs found in Alberta, Canada, according to a press release. The team ran the jaws of the embryonic thunder lizards through a CT scanner to get fine detail of the developing teeth. They also examined several of the teeth under the microscope.
Growth lines on the teeth revealed that the baby Protoceratops had been in its egg at least three months and the duck-billed dino was cooped up in its shell for six months. In fact, points out Maddie Stone at Gizmodo, the incubation times could be longer since the embryos died in the shell.
“These are the lines that are laid down when any animal's teeth develops,” Gregory Erickson, professor of biological sciences at Florida State University and lead author of the study says in the press release. “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.”
The new information upends some ideas about dinosaurs. For instance, the long incubation period means it’s less likely the creatures migrated. Instead, points out James Gorman at The New York Times, while some dinosaur species buried their eggs and went on their merry way, others likely stuck around to protect their slow-developing eggs and babies, which would be a tempting treat for predators.
The biggest takeaway, however, is the slow incubation period’s impact on dinosaur survival during an extinction event. The slow embryonic development along with the year or so to reach maturity places the lumbering lizards at a disadvantage compared with smaller creatures that could reproduce more quickly after a global catastrophe, like the asteroid or comet that hit the earth, creating the Chicxulub crater in present-day Mexico, 66 million years ago.
Dinos, on the other hand, would have been stuck in one place trying to raise a clutch of eggs under harsh conditions. “Having a slow incubation period—three to six months—would have exposed eggs to predation, droughts and flooding for long periods of time,” Erickson tells Stone. “If there were attending parents, you can imagine the parents would have been exposed for long periods of time, too.”
That long incubation period, combined with climate changes and an asteroid strike or other world-changing events could have pushed the dinos over the edge. "With regard to their life history and physiological attributes, dinosaurs were basically holding a dead man’s hand," Erickson said. "They were profligate wasters of energy, which is bad in a resource-depleted environment. Some of [our] work shows that dinosaurs took over a year to reach maturity," he says. "You throw in very slow incubation times on top of that, and these attributes are collectively a bunch of black eights and aces."