About 115 million years ago in what's now northwest China, a female bird was on the verge of motherhood. But somehow, her life on an ancient lakeshore took a traumatic turn, triggering a pregnancy complication that killed the egg forming inside her and may have even led to her death.

Entombed ever since, this mother bird is now a paleontology milestone: Named Avimaia schweitzerae, the newly described species is the first fossil bird known to science that contains an unlaid egg.

“We were not expecting anything interesting, but it turned out to be the first fossil bird ever found with an egg inside its body,” says lead study author Alida Bailleul, a postdoctoral researcher at China's Institute of Vertebrate Paleontology and