Community College (LRCCD)

Geology & Earth Science Instructor: Arthur Reed, P.G.



Happy Fossil Friday!

Friday May 14, 2021


Unlaid egg discovered in ancient bird fossil


In the early Cretaceous in what is now northern China, a small bird (Avimaia schweitzerae) died, sank to the bottom of a shallow lake, was buried by sediment, and survived as a fossil. A thorough investigation into the cause of its death was carried out 110 million years after it died by several researchers which included experts in the filed of avian reproduction. You see, the fossil of this bird was found with an unlaid egg still inside its body. Characteristics of the eggshell indicated its death was likely the results of carrying the egg too long…for some reason it was not able to lay the egg. This fossil was found in the mid-2000’s, and the resulting research was published in March 2019 in the journal Nature Communications.


A brief overview of the study is given by the American Association for the Advancement of Science is provided below, and the complete study is available in Nature Communications (1st link below). Seven of the authors of the original study are researchers in Beijing, China, and one is a researcher at the Carnegie Museum of Natural History in Pennsylvania. It is refreshing to me to see this type of international collaboration in science.





Article from scienceMag.org


In this illustration, a colony of Avimaia schweitzerae nests on a lakeshore in what would become northwest China. Somehow, one female ended up dead in the water—eventually yielding the first fossil bird ever found with an unlaid egg inside.



Photograph and line drawing of the holotype of Avimaia schweitzerae, IVPP V25371



Unlaid egg discovered in ancient bird fossil

By John PickrellMar. 20, 2019 , 6:00 AM

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.


Bottom of Form

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.)



Location in China where fossil was uncovered.






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