Scientists have collected fossil dinosaur eggs, sometimes finding more than a dozen in a nest-like area. The fossilized eggs are usually the color of the rock in which they are found. Similar to fossil dinosaur bones, their structures have been fossilized and replaced by minerals over time.

Although the eggs are fossilized, scientists have discovered that dinosaur eggs probably looked similar to those of modern birds, reptiles, and some primitive mammals. Most of the eggs were rounded or elongated, with hard shells. They contained an amnion, a membrane that kept the egg moist, a kind of “private pond” for the young animal growing in the egg. The eggs appeared to be similar in other ways, too. The surface of the shell allowed for the exchange of gases necessary for the young to survive (many of the fossilized eggs exhibit a mottled surface that indicates the shell pores), and the young would crack their way out of an egg when they were ready to enter the world.

No one really knows whether the majority of eggs laid by the dinosaurs were soft-, flexible-, or hard-shelled. The eggshell would have to have been relatively strong to support the weight of a brooding parent, or the overburden of nesting material, while still allowing for the exchange of necessary gases. Hard-shelled eggs had the best chance of fossilizing, so most shells that we find today may not truly represent all the eggs that dinosaurs laid, but just the hardest ones that survived.