How Did Dinosaurs Lay Eggs? Nests, Incubation and Surprising Soft Shells

Imagine a 30-metre-long sauropod, heavier than ten elephants, carefully laying eggs no bigger than a football. It sounds strange, but that is exactly what fossils show. Dinosaur eggs and nests, found on every continent, tell us a remarkable amount about how these animals reproduced, how they raised their young and how closely they were related to modern birds.

All dinosaurs hatched from eggs

As far as we know, every dinosaur laid eggs. That is not surprising: their closest living relatives, crocodiles and birds, both lay eggs, and birds are themselves living dinosaurs. Unlike many reptiles, no dinosaur is known to have given birth to live young.

Dinosaur eggs have been found by the thousands, sometimes in huge nesting grounds. At Auca Mahuevo in Argentina, for example, palaeontologists discovered vast fields of titanosaur eggs, some with tiny embryos still inside, preserved with traces of skin.

Surprisingly small eggs

Even the biggest dinosaurs laid relatively small eggs. Titanosaur eggs were roughly round, about 12 to 15 centimetres across, and the longest known dinosaur eggs, from large oviraptorosaurs, were about 45 centimetres long but narrow.

There is a physical reason. The bigger an egg, the thicker its shell must be to avoid breaking, but a thick shell makes it harder for oxygen to get in and for the chick to break out. So instead of a few huge eggs, many large dinosaurs laid many small eggs, sometimes 20 to 40 per clutch, and their babies grew very fast after hatching.

Different dinosaurs, different nests

Fossil nests reveal a range of nesting strategies:

Burying eggs, as crocodiles and turtles do, uses the warmth of the soil or rotting vegetation. Some titanosaur eggs in Argentina were laid in soils heated by geothermal activity, like natural incubators.

Brooding, sitting on the eggs, is the bird strategy. Several skeletons of the oviraptorosaur Citipati from Mongolia’s Gobi Desert were found crouched on top of their nests, arms spread over the eggs, exactly like a bird keeping its eggs warm. They seem to have been buried by a sandstorm or a collapsing dune.

The famous “egg thief” that wasn’t

In 1923, an American Museum of Natural History expedition to the Gobi Desert found nests that were the first to be widely recognised as dinosaur eggs. A skeleton found on one nest was named Oviraptor, “egg thief”, because scientists assumed it had been caught raiding the eggs of Protoceratops.

In the 1990s, a fossil embryo found inside a similar egg turned out to be an oviraptorosaur. The animal had not been stealing eggs, but guarding its own. Its unfortunate name, under the rules of scientific naming, cannot be changed.

How long did eggs take to hatch?

Bird eggs hatch quickly: about three weeks for a chicken, and up to about 80 days for the largest birds. Were dinosaur eggs similar?

In 2017, Gregory Erickson and colleagues found a clever way to find out. Teeth grow in daily layers, like tree rings but much finer. By counting these lines in the teeth of fossil embryos, they estimated incubation times of about 3 months for the small horned dinosaur Protoceratops and about 6 months for the duck-billed Hypacrosaurus. Dinosaur eggs took much longer to hatch than bird eggs of similar size, closer to reptile eggs. Long incubation would have exposed nests to predators and floods for months.

Soft eggs: a recent surprise

Until recently, scientists assumed all dinosaur eggs had hard, mineral shells like bird eggs. But that left a puzzle: why were eggs from some groups, such as early sauropods and horned dinosaurs, so rarely found?

In 2020, a team led by Mark Norell published a study in Nature reporting that eggs of Protoceratops from Mongolia and of the early sauropodomorph Mussaurus from Argentina had soft, leathery shells, more like turtle eggs. Soft shells rarely fossilise, which explains why they were missed. The team concluded that the very first dinosaurs probably laid soft eggs, and that hard shells evolved several times in different groups. That conclusion is still debated, but the discovery changed how palaeontologists search for eggs.

From dinosaur eggs to bird eggs

Many features we see in birds, such as open nests, brooding, colourful eggs and a single functioning oviduct, appear step by step in the dinosaur family tree. Studies of pigments in fossil eggshells suggest that some dinosaurs, like many birds today, even laid blue-green or speckled eggs. When you look at a robin’s nest, you are seeing a reproductive strategy refined over more than 150 million years of dinosaur evolution.

In short

• As far as we know, all dinosaurs laid eggs, like their descendants, the birds.
• Even giant sauropods laid small eggs, about 12–15 cm across, in large clutches.
• Some dinosaurs buried their eggs; others, like Citipati, brooded them like birds.
• Oviraptor, the “egg thief”, was actually guarding its own nest.
• Embryo teeth show incubation took about 3 to 6 months.
• Some early dinosaurs laid soft-shelled eggs; hard shells may have evolved more than once.