Egg-Citing Discovery: Dinosaur fossil includes eggshells

For the first time, scientists have found eggs with shells inside a dinosaur fossil, strengthening previous conjectures about the ancient reptiles’ reproductive physiology.

READY TO GO. This fossil egg, one of two found in a female dinosaur’s pelvic cavity, retains its shell and external texture. Y.-N. Cheng

The dinosaur remains were unearthed in southern China from petrified sediments laid down between 100 million and 65 million years ago. The fragmentary fossil includes six back vertebrae, two adjacent tail vertebrae, and other bones from the dinosaur’s pelvic area, says Tamaki Sato of the Canadian Museum of Nature in Ottawa.

The remains were too scant to assign to a particular species but enough for Sato and her colleagues to identify the creature generally as an oviraptorosaur, a member of a group of dinosaurs that includes the feathered Caudipteryx (SN: 8/19/00, p. 119: Available to subscribers at Feathered fossil still stirs debate) and the vegetarian Incisivosaurus (SN: 9/21/02, p. 179: Veggie Bites: Fossil suggests carnivorous dinosaurs begat vegetarian kin). The newfound specimen probably would have measured about 3 meters from head to tail.

It’s crystal clear that the creature was a female. Inside its pelvis, paleontologists found two 17-centimeter-long, potato-shaped eggs, complete with shells. Because the eggs nearly filled the dinosaur’s pelvic cavity, they were ready to be laid, says Sato. The soft tissues inside eggs at that stage of development wouldn’t have readily fossilized, so the shells are probably all that’s preserved. Sato and her colleagues describe the fossil find in the April 15 Science.

The pelvis-filling volume of the eggs also suggests that the dinosaur could carry only two eggs at a time, says Sato. The similar size of the two eggs hints that the creature had two oviducts that produced one egg each and did so simultaneously.

That combination of traits suggests similarities with both reptiles and birds. Most modern reptiles, such as sea turtles, have two oviducts but can produce multiple eggs in each. Birds, in contrast, have single oviducts that produce only one egg at a time.

David J. Varricchio, a paleontologist at Montana State University in Bozeman, and his colleagues previously speculated that oviraptorosaurs laid eggs in pairs. That notion derived from their examination of oviraptorosaur nests in Asia. At those sites, the researchers noticed that the eggs in large clutches were grouped in twos, with the same end of each egg pointing in a similar direction, suggesting that the eggs were laid in one sitting. The new finding provides “pretty convincing evidence that our hypothesis was correct,” says Varricchio.

In two known cases, adult oviraptorosaurs died and were fossilized while sitting atop ring-shaped arrangements of at least 15 eggs. This suggests that egg laying occurred over an extended period, says Sato. Because the slightly pointed end of each egg was oriented rearward in the newfound oviraptorosaur fossil and eggs in the fossilized nests lay with their pointed ends toward the outside of the ring, Sato and her colleagues suggest that female oviraptorosaurs crouched over the centers of their nests to lay their eggs.

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