Amazingly well-preserved fossil egg shows link between dinosaurs and birds

University of Calgary paleontologist Darla Zelenitsky had seen well-preserved dinosaur egg fossils before, but nothing like this. "At one point, I'm pretty sure it winked at me," she laughs.

The fossil, the subject of a new paper published Wednesday, is so detailed it's revealing even more about the deep relationships between dinosaurs and birds, their modern-day descendants.

The fossil is from a type of theropod dinosaur called an oviraptorid, a species already on an avian evolutionary path.

First discovered in China about 20 years ago, the fossil is so completely preserved it reveals the posture of the soon-to-be theropod inside its shell. With its back curled up against the broad end of the shell and its head tucked in between its arms and legs, it looks much like an unhatched chicken.

"The skeleton is curled up in a birdlike embryonic pose," Zelenitsky said.

"Birds were thought to have a unique posture within the egg before hatching. It's evident from this fossil that some of these postures seem to have first evolved in their dinosaur ancestors."

It's the first time scientists have been able to see how dinosaur embryos were positioned inside their eggs. Previous egg fossils have been too fragmented.

"This reinforces the link between those theropod dinosaurs and birds," Zelenitsky said.

That link is growing stronger as more evidence comes in. Dinosaurs have long been known to have sported feathers. Their asymmetrical eggs, with one broad and one pointed end, is another development birds kept. So are eggshells.

"You almost can't tell the difference between the eggshell of a bird and an oviraptorid," Zelenitsky said.

Dinosaurs are now thought to have even sat on their nests before the nestlings hatched.

"There's been a number of skeletons of oviraptorid adults sitting directly on their eggs," said Zelenitsky. "Brooding behaviour is another one that preceded birds."

Dinosaur eggs aren't uncommon fossils. In 1997, 10 of them were found in southern Alberta at the Devil's Coulee site.

But Zelenitsky remains in awe of how well this particular fossil is preserved. The egg, she said, must have been buried quickly, completely and permanently under fine silts — perhaps from a riverbank mudslide or a flood.