The birds with the very longest incubation periods, emperor penguins, take about two months to develop in the eggs. And they’re certainly outliers: most bird breeds take between ten and thirty days to bust out of their shells.
Compared to modern-day reptiles, and especially ancient dinosaurs, that might as well be a blink of the eye in incubation terms.
Speaking again to The Washington Post, Erickson has said: “It's really surprising. I don't think that people would have entertained the idea that they would have incubated over the better part of a year.”
However, for David J. Varricchio from Montana State University – who always maintained that dinosaur reproduction was closer to modern reptiles than that of modern birds – the news didn't come as much of a shock. According to Varricchio, modern-day reptiles will often spend many months developing in eggs before hatching.
Modern science currently holds that birds are the closest living relatives to ancient dinosaurs, which led plenty of researchers to conclude that dinosaurs and birds had similar incubation and development cycles.
However, thanks to Erickson and his team, it's now being theorized that dinosaurs – of the Mesozoic era, at least – had more in common with reptiles than birds.
Today's reptiles, on the other hand, tend to have an incubation period that lands between one and two months, which on average places them a lot closer to dinosaurs.
However, there are more details that lead researchers to believe dinosaurs aren't as closely related to reptiles than previously thought. In fact, the very way that modern reptiles interact with their developing eggs is different from how researchers believe dinosaurs acted.
Today's reptiles take the tactic of burying their eggs underground – it protects them from predators, and helps preserve warmth. Some reptiles will also hang around their clutch to add additional protection.
However, most reptiles abandon their eggs, and leave them to develop, hatch, and live without interacting with them again. With the long incubation periods of dinosaurs now clearer, the question arose of whether dinosaurs hung around for the lengthy incubation period, or left them alone.