Like all reptiles, all dinosaurs laid eggs, but fossil eggs are rare discoveries. The first recorded discovery of dinosaur eggs comes from France in 1859. Since then, important finds have been made in at least 220 different localities around the world, including Canada.
Dinosaur eggs vary in size, depending on the species. Among the largest are fossilized dinosaur eggs collected in the mid 1990s from Late Cretaceous rocks in China. These eggs are more than 60 cm (2 ft.) long and about 20 cm (8 in.) in diameter.
Until recently, whenever a dinosaur fossil was found in association with fossilized dinosaur eggs, palaeontologists thought the dinosaur had been stealing the eggs at the moment disaster (such as a sand storm) buried them. Palaeontologists started to change their minds in the mid-1990s, when an egg containing an embryonic oviraptorid skeleton was found in Mongolia. This egg was the same kind as was found in an egg clutch under an oviraptorid dinosaur skeleton. Later, two more fossils of an oviraptorid on top of a clutch of eggs were found (in different countries). These fossils suggest that the mother dinosaurs had just laid eggs when they were buried; they were not stealing them.
Dinosaurs laid eggs in either of two ways. The characteristics of one way are still found in modern reptiles. The other way resembles that of birds.
The most primitive form of egg-laying is still found in modern reptiles. The dinosaurs that laid eggs this way are the ornithischian (or "bird-hipped") dinosaurs. These dinosaurs existed until the great extinction. In fossils of eggs laid by ornithischian dinosaurs, the eggs are usually round, there are usually 20 to 30 in the nest, and they are all jumbled together. Palaeontologists think that the eggs in this kind of clutch were laid in one session, as living turtles and crocodiles do.
The other form of egg-laying evolved later, in certain groups of saurischian (or "lizard-hipped") dinosaurs, such as oviraptorosaurs. These dinosaurs also existed until the great extinction. Palaeontologists think that they represent an evolutionary transition between reptiles and birds. The many similarities in their egg-laying support this theory.
Oviraptorosaurs laid oblong eggs that were slightly ovoid (one end is slightly narrower than the other). In this group of dinosaurs, there can be 30 to 40 eggs in the nest, and they are arranged in pairs in a radial pattern, like spokes. In 2005, a fossil of an oviraptorosaur dinosaur with a pair of unlaid eggs was found. The evidence suggests that the eggs were laid a pair at a time, with an interval of at least a day between pairs. A clutch could therefore take days or weeks to be completed.
There are several points of resemblance to birds: birds lay markedly ovoid eggs, and there is an interval of at least a day between them until the clutch is completed. There are also differences: birds lay eggs one at a time, and they lay fewer eggs in a clutch.
Despite these differences, and when considered with other similarities (in skeletons and feathers) among oviraptorosaurs, other more birdlike (evolutionarily speaking) dinosaurs, and birds, many palaeontologists think that birds evolved from dinosaurs by way of these dinosaur groups.
Indeed, more and more palaeontologists would go so far at to say that birds are dinosaurs—flying dinosaurs. The implications are that dinosaurs are not extinct, and that birds should not be classified independently, as reptiles and mammals are. Rather, birds should be classified as a group of dinosaurs, which are themselves classified as reptiles. Many questions still need answering before the matter is settled.