As far as paleontologists can determine, all dinosaurs reproduced by laying eggs. The first fossilized dinosaur eggs were found in France in 1869, but not everyone agreed the eggs were from dinosaurs. Although it may seem somewhat obvious to us now, it took time before scientists agreed that dinosaurs nested and laid eggs. The proof was found in the 1920s in the Gobi Desert, where nests and eggs of a group of Protoceratops were found. Since that time, over 200 sites with fossil eggs of various dinosaurs have been found all over the world, including in the United States, France, Mongolia, China, Argentina, and India.

It should also be noted, though, that there are some modern reptiles that do not lay eggs outside their bodies; rather, their eggs remain inside them, where they hatch and then emerge as live young (this is called being ovovivaporous). Some scientists suggest that this is a result of adapting to colder climates. And although the idea is highly debated (and no real physical evidence has been found), some scientists speculate that polar dinosaurs may have reproduced in this way.