The pregnant pterosaur was just trying to make it home in time to lay her egg. But when a sudden storm assaulted the 3-foot-long flying reptilian, the wind broke her wing, sending her spiraling down into a lake. As the mud sucked her under, the pressure expelled her egg, which fossilized beside her as the mud turned to rock.
"It was a tragedy," says paleontologist David Unwin of the University of Leicester in the United Kingdom, whose analysis of the fossil was published  today in Science. He and colleagues say the find could answer longstanding questions about differences between pterosaur sexes.
Researchers began studying the fossil after a farmer in the Liaoning Province of China sold it to the Zhejiang Museum of Natural History in 2009. There, Junchang Lü of the Chinese Academy of Geological Sciences dated the fossil as 160 million years old and identified it as Darwinopterus, a mid-Jurassic pterosaur which Lü, Unwin, and others had first identified in 2009. Affectionately known as Mrs. T (a misspelling of pterosaur), the new fossil is a rare example among extinct animals of a nearly complete skeleton with an egg.
It also represents only the fourth pterosaur egg ever discovered. Even more unusual, Mrs. T's egg appears to have just been laid. The researchers say that the eggshell appears crumpled and soft like a reptile's, indicating that pterosaurs buried their eggs to soak up nutrients from the ground and left the hatchlings to fend for themselves, rather than caring for them as a bird would.
If the structure is indeed Mrs. T's egg, there can be no doubt that the fossil is a female, says Unwin. And that would mean that features such as her large pelvis and lack of a bony headcrest may be defining characteristics of Darwinopterus females. It also means that headcrests, which Unwin believes were most likely used by males to intimidate rivals or woo a mate, likely varied in size and boniness between individuals and among the approximately 130 species of pterosaur currently classified.
Paleontologist Kevin Padian of the University of California, Berkeley, is skeptical about using headcrests to define pterosaur gender. Mrs. T may lack a bony headcrest not because she was female, but because she was young, he says. "Anything horny, antlery, or feathery gets more that way as an animal gets older," says Padian, adding that too little is known about the size of mature pterosaurs to be able to tell whether the fossil is full-grown.
He's also not convinced that the round structure found with the pterosaur is an egg. Padian says it's far too large to be an egg as it would take up much of the pterosaur's body cavity. Plus, he says, reptiles lay dozens of eggs at a time.
Still, paleontologist Mark Witton of the University of Portsmouth in the United Kingdom, says the discovery sheds light on these enigmatic animals. Unlike dinosaurs, whose ancestral features are preserved in birds, pterosaurs were an evolutionary dead end. "They're intriguing—and confusing—animals to study," he says. "When Darwinopterus was first discovered, it bridged the gap between two anatomically distinct pterosaur groups. It will be interesting to see what else this animal reveals."