As far as we know, all dinosaurs reproduced by laying eggs, as do most other sauropsids (reptiles). It is very difficult to determine what species of dinosaur laid the eggs that have been discovered, because only a few dinosaur embryos have been found inside the fossil eggs.
Did dinosaurs lay big eggs? Some probably did, but there is a limit to how big an egg can be. Bigger eggs need thicker shells for support, but because the developing embryo must breathe through the shell, it cannot be too thick.
Mark Norell, Macaulay Curator and chair in the Division of Paleontology, discusses dinosaur reproduction in the video above. Find answers to more dinosaur questions in the Museum’s original video series, “Dinosaurs Explained.”
All dinosaurs hatch from eggs, including extinct dinosaurs and modern birds; as do crocodiles, the living group most closely related to dinosaurs. Until the 1980s, discoveries of fossilized eggs and bones of young dinosaurs were extremely rare, but dinosaur eggs have now been discovered on several continents, and fossils of hatchlings, juveniles, and adults have been found for most major groups.
One remarkable find was in Montana, where fossils of duckbill dinosaurs, including eggs, nests, hatchlings, juveniles, and adults were found together in one death assemblage, or mass grave. The eggshells in the nests were badly broken, arousing speculation that the hatchlings might have crushed the eggs while moving around the nests. Some paleontologists think this site was a nesting colony, where adult dinosaurs cared for their young during the first several months after hatching.
One of the greatest highlights of the American Museum of Natural History’s expedition to Central Asia occurred in 1923 at the Flaming Cliffs of Mongolia. It involved the discovery of eggs that, after first analysis, were thought to belong to the dinosaur Protoceratops. Roy Chapman Andrews, the leader of the expedition and future director of the Museum, described the scene in his field notes:
“On July 13, George Olsen reported at tiffin [lunch] that he had found some fossil eggs... We felt quite certain that his so-called eggs would prove to be... geological phenomena. Nevertheless, we were all curious enough to go with him to inspect his find. We saw a small sandstone ledge, beside which were lying three eggs partly broken… The brown striated shell was so egglike that there could be no mistake. Granger finally said, ‘No dinosaur eggs have ever been found, but the reptile probably did lay eggs. These must be dinosaur eggs. They can’t be anything else.’ The prospect was thrilling, but we would not let ourselves think of it too seriously, and continued to criticize the supposition from every possible standpoint. But finally we had to admit that ‘eggs are eggs,’ and that we could make them out to be nothing else. It was evident that dinosaurs did lay eggs and that we had discovered the first specimens known to science." (The New Conquest of Central Asia, 1932.)
Paleontologists presumed that the fossil eggs at Flaming Cliffs were laid by Protoceratops because it was the most common dinosaur at the locality where the eggs were found. However in the 1990s Museum expeditions discovered identical eggs, one of which contained the embryo of an Oviraptor, like dinosaur—which changed scientists' view of which dinosaur laid these eggs. Turns out Oviraptor was a parent, not an egg thief.
Genuine fossil eggs usually have an easily identifiable shell which differs significantly from the enclosed sediments either by having a fine surface ornamentation (the smoother the "shell," the less likely it is to be a non-bird dinosaur egg) or a specific type of crystalline structure in cross-section. Also, because eggshell tends to be brittle, the shell is almost always heavily cracked with clear shifting of the eggshell bits.
Some ancient dinosaurs made nests, laid eggs, and tended to their babies—just like today’s birds and crocodiles.
All dinosaurs reproduced by laying eggs, just as living birds and many modern reptiles do. But surprisingly, the babies that hatched out of sauropod eggs were generally no bigger than a modern adult goose. Sauropods didn't start out extremely big--they just grew very, very fast.
They are some of the rarest of rare artifacts: fossil dinosaur eggs with the embryo still inside. And they are prized for what they can tell paleontologists about the adults that laid them.
In 1993, Museum scientists found an egg in the Gobi Desert that contained the remains of a tiny dinosaur embryo from 80 million years ago. Like most scientific discoveries, this one raises as many questions as it answers.