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Dinosaur Eggs Discovered Inside Mother -- A First

John Roach
for National Geographic News
April 14, 2005
 
Scientists have discovered for the first time a dinosaur with shelled eggs inside her belly. The find yields insight into how dinosaurs made babies and supports the theory that modern birds and dinosaurs are close relatives.

"I don't think too many people had expected [us] to discover a specimen that actually had eggs inside its body. It's something we wanted to have, but it's very surprising we actually got it," said Tamaki Sato, a paleontologist at the Canadian Museum of Nature in Ottawa.

Sato and her colleagues will report the find in tomorrow's issue of the journal Science.

Many scientists believe birds evolved from dinosaurs. In their effort to prove this hypothesis, scientists appreciate hard evidence of similarities between the two types of creatures, including their reproductive biology.

"We can give a hypothesis, but it's often very difficult to confirm the hypothesis," Sato said. "Our specimen gives direct, undoubted evidence" that dinosaurs shared with birds some aspects of reproductive behavior.

Hans-Dieter Sues is the associate director for research and collections at the Smithsonian Institution's National Museum of Natural History in Washington, D.C. He said the find is "very interesting" but not unexpected, as it was predicted by previous studies.

"Still, it is neat to find such a fossil," he said. Sues is a member of the National Geographic Society's Committee for Research and Exploration.

Birdlike Dinosaur

The research is based on a dinosaur pelvis that contains a single pair of shelled eggs inside the body cavity. The dinosaur specimen was discovered in China's Jiangxi Province, a few hundred miles north of Hong Kong.

This is the first time shelled eggs have been found inside a dinosaur. Previously, only egglike structures have been found in dinosaur skeletons, Sato said.

The researchers describe the eggs as looking like pineapple-size potatoes. "Compared to a chicken egg, they are much more elongated," Sato said. Measured lengthwise, the eggs are each 7.9 inches (20 centimeters) in diameter. They measure 2.4 to 3.1 inches (6 to 8 centimeters) in diameter at the "waist."

Based on an analysis of the pelvis, Sato and colleagues identified the dinosaur as an oviraptorosaurian, a subgroup of the theropods. Theropods, which include Tyrannosaurus rex, are considered the most birdlike of the dinosaurs.

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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