Dinosaurs did not give birth to live young in the way that mammals do. Like most reptiles, and like birds, female dinosaurs laid a number of eggs. These eggs were usually deposited in a nest made of the twigs and leaves, or in a hole that the female had dug in the soil or sand. Dinosaur eggs could be oval or round and as small as a tennis ball or as large as a cannonball. A baby dinosaur, such as this Troodon, may have pecked its way out of its protective shell with a specially designed egg tooth, just as baby birds do.
Some dinosaurs laid about 20 eggs, and carefully arranged them in a circle inside the nest. Female sauropods may have laid up to 40 eggs. It is likely that dinosaurs laid eggs once a year, so a single female dinosaur probably hatched hundreds of babies in her lifetime. Experts think that between 50-90 percent of eggs survived to hatching.
Not surprisingly the biggest dinosaurs – the plant-eating sauropods laid some of the biggest eggs, although these were surprisingly small. A sauropod egg weighed about 5kg (11lb) and was about 30cm (12in) long. This is only about twice the size of an ostrich egg, and tiny compared to the size of the mother.
This 80 million-year-old fossilized Oviraptor nest proved to be an amazing discovery. The mother Oviraptor had died with her arms spread over her eggs, as if protecting them. This suggests that Oviraptor, and probably other bird-like dinosaurs, were brooders – they sat on their nests to keep the eggs warm or cool.
Oviraptor on its nest
Fossil evidence suggests that a female Maiasaura brought food to her babies after hatching, and may even have looked after her young for up to a year. There is no evidence that other dinosaurs cared for live young, but it is like that many stayed near the nest to guard their eggs, like their closest living relatives, crocodiles.
Young Maiasauras with mother
After hatching, dinosaur babies, like these Trodons, grew quickly – most had to learn to defend themselves fast if they were to survive. Fossil evidence of hadrosaur young shows that they doubled in size in about six weeks. A one-year-old Maiasaura was about 3m (10ft) long. Depending on their type, dinosaurs were generally fully grown after about 14 years.