Dinosaur eggs aren't exactly a dime a dozen yet. But scientists have been finding lots of them - some uncovered singly, some still neatly arranged in their nests, or even beneath the fossil remains of their nesting mothers.
Yesterday, a team from Canada and Taiwan reported they have identified a pair of 7-inch dinosaur eggs still tucked inside the fossilized pelvis of an "oviraptorosaurian" dinosaur discovered in China.
Although some fossil eggs have revealed traces of embryos inside, these have not.
But the find, reported in the journal Science, is shedding fresh light on the reproductive anatomy and egg-laying habits of dinosaurs, and on the evolutionary links between ancient reptiles and modern birds.
"It is remarkable to find a fossil that has the eggs up inside of it still," said Mark A. Norell, of the American Museum of Natural History in New York, who was not part of the team reporting in Science. "There have been other such cases reported, but this is the first one I would say is clearly definitive. ... It's a cool fossil."
The mama dinosaur's pelvis and femurs were discovered in 2002 by a geologist in China's southern Jiangxi province, according to Tamaki Sato, a post-graduate researcher at the Canadian Museum of Nature in Ottawa and co-author of the paper in Science.
The bones were all that was left of the 8-foot carnivore, a member of a group of odd-looking dinosaurs that walked on their two hind legs and lived about 100 million years ago, during the Cretaceous period.
Discovery of two large, potato-shaped eggs inside the fossil won it a place at the National Museum of Natural Science in Taiwan, for it added weight to the idea that dinosaurs had two functioning sets of ovaries and oviducts, and laid their eggs in pairs. It's a transitional design, between that of primitive reptiles such as crocodiles, and modern birds.
Paleontologists have long noted that dinosaur eggs found in their nests were apparently laid side by side, in pairs. The patterns suggested they were produced two at a time.
Crocodiles, too, have two functioning ovaries. But they produce many eggs, which are stored inside the body and laid all at once. Birds, on the other hand, have just one working ovary and oviduct (the other is vestigial). They produce and lay single eggs over a period of days.
Sato said scientists don't know why evolution shut down half of the twinned reproductive system birds inherited from their reptilian ancestors. Some have suggested that the weight demands of flight may have played a role. "But I don't think it's too good an explanation," she said.
For evolutionary biologists, the more interesting question is just where in the evolution of birds did half of their equipment became non-functional.
"To find two fully formed eggs lying next to each other in the body cavity tells us it [the loss of one working ovary and oviduct] had to evolve sometime between when oviraptors appeared and modern birds appeared," Norell said.
That doesn't mean there is a direct evolutionary line from primitive reptiles, to oviraptorosaurian dinosaurs to the first birds. There isn't. "By the time this animal lived there were already true birds," Norell said.
Sato said the production of the single egg in each of two oviducts "must have happened in a common ancestor of dinosaurs and modern birds, and must [have occurred] after the lineage of crocodiles diverged from the lineage to birds and dinosaurs. But we don't know when it happened."
The researchers have plenty more questions, too.
Sato said there is not enough left of the animal to determine which species of oviraptorosaurian dinosaur it belonged to.
But all of them had very peculiar skulls with beak-like, toothless mouths. Scientists aren't sure what they ate, but eggs, insects and shellfish have all been suggested.
Sato's team also noted that the eggs inside their oviraptor showed clear evidence of eggshell formation - something that occurs between the time the eggs form and the time they're laid.
But measurements suggest that the shell on these eggs was thinner than on those found in nests. "We are wondering whether it's because of incomplete shell deposition or if it's because of decomposition" after the animal died, she said.
The critter's death is another mystery. Although much of the animal is missing, what is left remained remarkably intact - its bones still arranged mostly as they were in life. "We think it's not likely it was attacked by a scavenger," Sato said.
Somehow, the dinosaur was quickly buried in sand, which turned to sandstone and protected the remains as they fossilized.
Elsewhere in China, scientists have suggested that sand avalanches caused by heavy rains might account for such quick burial and preservation in sandstone, Sato said. But "we don't have the firm evidence to say anything at this stage, I'm afraid."