Paleontologists find the first fossilized egg inside an ancient bird

The unlaid egg may have killed its mother 110 million years ago

This ancient bird is Avimaia schweitzerae. It lived about 110 million years ago in what is now northwestern China. When the bird died, its body held an unlaid egg (the lighter brown smudge).

A.M. BAILLEUL ET AL/NATURE COMMUNICATIONS 2019

When a sparrow-sized bird died about 110 million years ago, she had an egg inside her body. Over time, pressure crushed and flattened that egg. Now scientists report it’s the first unlaid bird egg found inside a fossil. 

Scientists unearthed the fossil 11 years ago in northwestern China. Last year, paleontologists led by Alida Bailleul took a closer look. Bailleul works in China at the Key Laboratory of Vertebrate Evolution and Human Origins in Beijing. 

At once the researchers noticed something odd: The bird had a strange sheet of tissue between her pubic bones. Bailleul examined a piece of the tissue under a microscope. She found that it had come from an egg. The researchers shared their discovery March 20 in Nature Communications.

The egg was a first. The bird was too — a new species. The researchers named it Avimaia schweitzerae in honor of Mary Schweitzer. She’s a paleontologist who works on fossilized soft tissues. 

Further analyses turned up more surprises. The mother bird’s skeleton holds traces of medullary bone. This is a calcium-rich tissue that helps to form eggshells. Modern birds make this tissue while they’re producing eggs. The new finding is the strongest evidence yet that ancient birds did the same thing. 

The scientists also found tiny mineral spheres in the shell’s outermost layer, or cuticle. There are similar spheres in the egg cuticles of modern water birds, such as quails and ducks. The spheres may protect embryos from microbial infections. Until now, no one had ever seen them in a fossilized egg.

This bird and her embryo had some problems, though. The eggshell has two layers instead of the usual one. That suggests that the egg had stayed in the bird’s body for too long. And the egg’s layers are extremely thin — thinner than a sheet of paper. 

In today’s birds, these symptoms can point to a deadly condition called egg-binding. That’s especially true in small birds that are under extreme stress. In egg-binding, a bird is unable to lay the egg. The researchers think this ancient, unlaid egg may have been what killed its mother.

Carolyn Gramling is the earth & climate writer. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.

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