The first dinosaur eggs found complete with shells in the body of the mother has solved the long-standing mystery of how dinosaurs laid their eggs. The evidence shows they laid a clutch in a series of sittings, like birds, rather than all at once like crocodiles and other living reptiles.
The pair of eggs come from a fossil found in the Jiangxi province of China which includes the pelvis and part of a leg of an oviraptor - a two-legged dinosaur that roamed between 100 and 65 million years ago.
Dinosaur eggs are a relatively common find, and some have been unearthed still containing the skeletons of unhatched babies. And the discovery of a brooding mother on the top of her nest in 1993 showed that at least some dinosaurs cared for their eggs. Yet the only other eggs found inside a dinosaur, from the feathered Sinosauropteryx, were immature and lacked shells, leaving the laying process unclear.
To confuse matters further, the closest living relatives of dinosaurs - birds and crocodiles - lay eggs in different ways. Crocodiles and other modern reptiles have a pair of functional oviducts and lay their clutch of eggs in a single sitting. In contrast, birds have only one functional oviduct and lay one egg at a time.
Modern alligators take about 3 weeks to form the shells on a whole clutch of eggs whereas birds deposit the shell layer on their single egg in one to two days. Producing and laying only one egg at a time saves on weight, making flight easier.
Eggs by the dozen
Oviraptors are part of the wide-ranging group called theropods, which also includes Tyrannosaurs and the ancestors of birds. Their fossilised nests are well known in China, typically including over a dozen elongated oval eggs in two rings.
Too little is preserved of the new find to identify the particular species, but the newly eggs looked ready to be laid, says Tamaki Sato of the Canadian Museum of Nature in Ottawa.
The best-preserved egg of the find is nearly 20 centimetres long and 6 to 8 cm wide, although somewhat deformed. Sato says the egg's shape and microscopic structures match those of some previously found eggs.
The pair of eggs show the oviraptor developed one egg at a time in each of its two oviducts, most probably laying one pair at a time in the nest, Sato suggests.
That puts it somewhere between the primitive reptilian form of crocodiles and the more advanced form of the birds, consistent with the theory that birds evolved from dinosaurs related to oviraptors.
The fossil "is absolutely stunning", says Ken Carpenter of the Denver Museum of Nature and Science in the US. Capturing a moment so close to when a female dinosaur was to lay her eggs makes it "one of the most remarkable discoveries yet".
Journal reference: Science (vol 308, p 375)
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