Unlaid egg discovered in ancient bird fossil

For the first time, researchers have found an unlaid egg inside a fossilized bird. The find—belonging to a sparrow-size flyer that lived in northwestern China 110 million years ago—is especially remarkable because fully formed eggs typically only stay within an adult bird for about 24 hours.

Researchers were initially puzzled by the discovery, as they never suspected the unusual, squashed mass within the headless fossil’s abdomen (seen as a flattened brown layer in the center of the picture) could be an egg. But a microscopic analysis of a fragment revealed it to be eggshell. Further study suggested structural abnormalities that hint that the egg may have been the cause of this bird’s demise, the paleontologists report today in Nature Communications .

The fossil eggshell’s structure doesn’t have the correct proportions seen in healthy eggs and consists of multiple layers of shell. This indicates a condition called “egg-binding,” where an egg becomes trapped inside a bird, the team argues. This can occur in chickens and small varieties of modern pet birds under stress and likely also led to the death of this long-lost, dinosaur-era relative.

This makes the discovery “the oldest documented case of this common reproductive disorder,” the researchers say. Intriguingly, the eggshell features microscopic spheres of calcium phosphate, which is seen today in birds that nest in humid, infection-prone environments. This waterproofing suggests it was a species that nested near water and buried its eggs in the ground.

The team has christened the bird Avimaia schweitzerae. (Avimaia means “mother bird”; and schweitzerae honors paleontologist Mary Schweitzer.)