We have no way of studying an on-going life cycle of dinosaurs, which means it’s impossible to see how the creatures grew as time went on.
Thanks to fossils, we have snapshots of growth and development, but that’s like looking at pictures of a baby, and teenager, and an adult, and knowing everything about the person’s life – growth doesn’t work that way. But, experts can at least theorize, which led them to believe the incubation process for dinosaurs was similar to today’s birds.
The first fossilized dino eggs appeared in France and were at first thought to have belonged to an unknown species of birds (that's a little bit of foreshadowing). This led to them being overlooked for a long time, but sixty-four years later researchers took another look and correctly re-categorized them as dinosaur eggs.
This leads us back to the American Museum of Natural History, which provided the researchers that did the work in Mongolia. Thanks to this refocused study, our knowledge about dinosaurs, eggs, and plenty of other things have grown by leaps and bounds.
Once the researchers discovered the French eggs were, in fact, from dinosaurs, more and more started to pop up as other researchers went back to them and started studying them again. It led to lots of questions about the creatures that had laid – and inhabited – the eggs.
A few of the fossils that have popped up are empty – they successfully hatched, and lived out their lives away from their eggs. A few, however, told a different story, one that had life cut short before they could breathe their first breath. In rare cases, the creatures inside are perfectly preserved.
Until, however, David J. Varricchio from Montana State University published a paper in 2013 putting forth a different theory. Thanks to his study he's found that certain dinosaur eggs had lots of pores, which suggests they had been covered up underground – before they were scheduled to hatch.
If you've ever seen videos of baby turtles pulling themselves out of the sand and crawling toward the water, you've seen a similar process. This, in turn, suggests that the incubation process does have more in common with modern-day reptiles than birds.
It's practically impossible to come to any definite conclusions, but a great deal of this area of paleontology remained a mystery to researchers even after Varricchio's contribution.
Despite all the technology, different fossils, and different minds working to uncover the mysteries, nothing definitive came to light. FSU professor Erickson gave a statement in 2017 to the university summarizing: “Did [dinosaur] eggs incubate slowly like their reptilian cousins, crocodilians and lizards? Or rapidly like living dinosaurs, the birds?”