Dinosaur Eggs Discovered Inside Mother -- A First

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Oviraptorosaurians walked on two feet and were 6.5 to 10 feet (2 to 3 meters) long from head to tail, Sato said. Many were toothless and had beaklike mouths.

Reproductive Biology

Researchers have previously discovered oviraptorosaurian nests with more than 15 eggs, raising questions about how the dinosaurs laid their eggs. The new finding, Sato said, is important because it helps answer some of these questions.

For example, scientists have wondered whether dinosaurs laid all their eggs at once like crocodiles or one at a time like birds. Sato and colleagues analyzed the oviraptorosaurian pelvis and eggs and concluded that the dinosaur's reproductive anatomy was in some ways like crocodiles but that it produced and laid eggs like modern birds do.

Like a crocodile, the dinosaur had two ovaries for making eggs and each ovary was connected to a tube called an oviduct, where the eggshell hardened and through which the eggs traveled to the outside world.

Birds, by contrast, have only one functioning ovary-tube combination.

But unlike a crocodile—and like a bird—each of the dinosaur's oviducts produced only one egg at a time, according to the researchers. This condition "supports the dinosaur-bird relationship," Sato said.

To lay a nest full of eggs, the dinosaur would have made two eggs, laid them, and then repeated the process until the nest was full. The research also explains why eggs in dinosaur nests are paired—they were laid at nearly the same time.

In addition, the orientation of the egg inside the female dinosaur's body allowed Sato and colleagues to determine that the dinosaur would have come to the center of the nest to lay her eggs.

One end of dinosaur eggs is more pointed than the other. In Sato and colleague's dinosaur specimen, the pointed ends point toward the back end of the mother's body. In previously examined ring-shape groups of oviraptorosaurian eggs, the pointed ends of the eggs pointed outwards, indicating the mother was at the center of the nest to lay her eggs, Sato said.

Sues of the National Museum of Natural History is cautious about drawing conclusions about all dinosaurs from this fossil. He said very little is known about the reproductive biology of extinct archosaurs—the group of animals that included dinosaurs, crocodiles, birds, and pterosaurs.

"The problem is that we only can look at this [reproductive biology] in modern birds, crocodilians, and this one dinosaur specimen," he said.

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