Dinosaur eggs

While the dinosaurs have been extinct for millions of years, there are clues that can help us to piece together how they reproduced. Evidence includes fossil eggs and nests and rare discoveries of embryonic skeletons and soft-tissue. Birds descended from theropod dinosaurs and crocodilians are related to dinosaur ancestors, so by looking at modern day animals we can make some informed guesses about how dinosaurs reproduced and looked after their young.

As far as we know, all dinosaurs laid hard-shelled eggs rather than giving birth to live young. Eggs belonging to most groups of dinosaurs have been identified, except those of the armoured stegosaurs and ankylosaurs. Identifying which dinosaur laid a particular egg is not always easy and there have been cases of mistaken identity.

Fossilised dinosaur eggs - their size and spherical shape suggest they came from a sauropod dinosaur.
Source: Museum Victoria

Discovery of fossil eggs

The first dinosaur egg discoveries were made in France in the mid-1800s. Fossil eggs have since been discovered on every continent except Antarctica and Australia. The most spectacular finds include entire nesting grounds caught in sandstorms, floods and volcanic eruptions that rapidly buried the eggs and babies and fossilised them in their original positions.

In the early 1920’s, expeditions to the Gobi Desert in Mongolia led to the discovery of skeletons of Protoceratops associated with fossilised eggs in circular nests. The skeleton of a carnivorous dinosaur was discovered poised above one clutch of eggs, implying it was caught in the act raiding the nest for food, earning it the name Oviraptor (egg thief). It was later realised that these eggs were instead those of Oviraptor itself, and the skeleton was that of a brooding parent protecting her own eggs rather than stealing them to eat.

Spectacular discoveries of dinosaur nest sites of hadrosaurs and theropods have been uncovered in north-western USA and southern Canada. These hadrosaur nests provided the first evidence of parental care among dinosaurs, and led to the naming of one genus as Maiasaura, meaning ‘good mother lizard’.

The largest known dinosaur nesting site was discovered at Auca Mahuevo in Argentina in the 1990s. Here thousands of clutches of eggs from a vast sauropod hatchery were buried in mud after a flood in the Cretaceous period. Some of these fossil eggs contain preserved embryos with skin impressions.

Dinosaur nesting behaviour

Just as different types of birds show varying degrees of care for their eggs and young, it is likely dinosaurs did the same, using different strategies in nest construction, incubation and parental care.

Evidence from the Auca Mahuevo site suggests that Sauropods laid 20-40 eggs into a simple depression in sand. They nested in groups, with hundreds of animals producing nests close together. There is no evidence of burial in sand or vegetation so they may have relied on the heat of the sun to incubate the nests. While it is possible that the adults protected the nesting area, the closely packed clutches make it unlikely that parents tended individual nests.
 
The hadrosaur Maiasaura nested in large colonies with well-spaced nests; enough room for parents to tend to their offspring. Mothers laid 30-40 eggs in a raised circular bowl. They covered the incubating eggs with vegetation that heated the nest as it rotted. Like modern parrots, the newly hatched Maiasaura stayed within the nest and parents brought them food and protected them.

Theropods such as Troodon built dish-shaped nests with 16-24 eggs in a spiral pattern. Fossils of adult skeletons of theropods on top of egg-clutches suggest they directly sat on their nests, incubating them with warmth from their feathery bodies. As the shells of newly-hatched Troodon eggs show no evidence of trampling, the hatchlings were seemingly precocious, leaving the nest at a young age. They were probably able to disperse themselves quickly from the nest after hatching and were likely capable of feeding themselves, in the manner of baby chickens and crocodiles. 

Shapes and sizes

Dinosaur eggs clearly came in a variety of shapes and sizes. Theropod eggs varied from long and elongate to teardrop-shaped, In contrast, many herbivorous dinosaurs had round, almost spherical eggs. Dinosaur eggshells were made of calcite with microscopic pores allowing the developing embryo to ‘breathe’.

Dinosaur eggs came in a variety of sizes. The smallest dinosaur eggs were just a few centimetres across. At the other end of the scale, sauropod eggs could be 30 cm long and 25 cm wide. Biggest of all are enormous 70 cm long eggs from a giant oviraptor from China.

Despite their impressive size, even the largest dinosaur eggs are small compared with the adults who laid them If the eggs were any larger, the eggshell would need to be very thick and would be impossible for sufficient air to diffuse through the shell. The shell thickness of such an egg would also be difficult for a hatchling to break.

Artist’s reconstruction of a hatchling ornithopod dinosaur emerging from an egg.
Source: Artist: Peter Trusler / This material has been reproduced with the permission of the Australian Postal Corporation. The original work is held in the National Philatelic Collection.