Offering a rare insight into sex and the single dinosaur, researchers announced Thursday the discovery of two unlaid fossil eggs inside a female dinosaur unearthed in China.

The unusual find of a single pair of intact eggs within a dinosaur’s body cavity – remains of a mother who died before she could give birth – shows that in matters of reproduction, these extinct creatures had as much in common with modern birds as with more primitive reptiles, the scientists reported.

“It gives direct evidence of dinosaur reproduction,” said paleontologist Tamaki Sato at the Canadian Museum of Nature in Ottawa who led the study. “Now we have the evidence that the dinosaur laid two eggs at once.”

Since dinosaur diggers first ventured into the bleak deserts of Inner Mongolia in the 1920s, scientists have known that dinosaurs gave birth to their young by laying eggs. Fossilized dinosaur eggs and egg-filled dinosaur nests are not uncommon. In 1993, researchers in the Gobi desert even found the skeleton of a dinosaur that died brooding over its clutch of eggs.

Until now, however, paleontologists had only indirect evidence of dinosaurs’ egg-laying habits and their reproductive biology.

The researchers found the potato-sized eggs nestled within the fossilized pelvis of a dinosaur called “Oviraptor” excavated near the city of Ganzhou in Jiangxi province. They published their find today in the journal Science.

The 12-foot-long, beaked dinosaur belonged to group called the theropods, from which the “Tyrannosaurus rex” and modern birds are thought to be descended. It is between 65 million and 100 million years old, Sato said.

Researchers knew from other fossils that these dinosaurs laid clutches of up to 15 eggs, but they did not know how many eggs were produced at once or how long it took these dinosaurs to fill their nests.

It takes a modern chicken about 26 hours to produce a single egg. A crocodile may take three weeks or more to produce one. While chickens usually lay their eggs one at a time, crocodiles lay from 20 to 60 eggs at once. Turtles lay between 80 and 180 eggs at a time.

Like a bird, but unlike a crocodile, this dinosaur could not lay all its eggs at once.

The fossil suggests that the dinosaur had two ovaries and oviducts, a primitive anatomical feature found in crocodiles and other reptiles. Modern birds only have one set.

Like birds, but unlike crocodiles, however, each of the dinosaur’s oviducts produced only one large, shelled egg at a time. The researchers believe the dinosaur made two eggs at once, laid them, and then repeated the process until her nest was full.