A Pair of Shelled Eggs Inside A Female Dinosaur. Tamaki Sato, Yen-nien Cheng, Xiao-chun Wu, Darla K. Zelenitsky, and Yu-fu Hsiao. 2005. Science 15 April 2005: 375.
From National Geographic News.Com:
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.
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.
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.