While Hypacrosaurus had been a large dinosaur, it was by no means the biggest of them. Modern reptiles and birds tell us that larger eggs – and these larger animals – have longer incubation times and take longer to hatch.
Thus, it’s likely that the Hypacrosaurus eggs had plenty of time to go before they were ready to step out into the world. Erickson, in fact, believes that the incubation periods for these eggs could have stretched up to a full twelve months.
The team was able to calculate the amount of time during the incubation period the dinosaur spends developing teeth: close to forty percent.
The team was also able to surmise the fact that the dinosaurs took a long time to hatch – Erickson's team estimates these dino eggs incubated for around double the time compared to avian eggs of the same size. But Erickson knew there was more information to find, so he looked at the two types of embryos individually.
When analyzing the two types of eggs he had access to, Erickson discovered some interesting facts. The team discovered that the smaller Hypacrosaurus would have taken somewhere around three months to hatch, while the larger Protoceratops probably would have spent twice that — six months — developing before hatching.
This study is the first time that researchers have been able to pinpoint with any degree of specificity the incubation periods for non-avian dinosaurs. But that's not even everything Erickson and his team discovered.
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.