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Fossilized Egg Found Preserved Inside Ancient Bird In China

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Scientists have unearthed the remains of a female bird in China and found something unexpected — when it died, the bird was expecting.

In a new study, a team of paleontologists from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing described how they found a fossil of an ancient bird species they called Avimaia schweitzerae and noticed that it was carrying a well-preserved egg, which they believe may have been the cause of its demise.

"We were not expecting anything interesting, but it turned out to be the first fossil bird ever found with an egg inside its body," said Alida Bailleul, lead researcher of the study, which was issued in the journal Nature Communications.

The discovery of this fossil specimen could shed more light on avian reproduction as well as on possible reproductive disorders that could harm the female bird and its egg. Researchers added that the fossil also provides more reproductive information than any other Mesozoic fossil bird in history.

The Tragic Story Of The Avimaia Schweitzerae

About 11 years ago, Bailleul and her teammates dug up fossils from the Xiagou formation in northwest China, and this mission yielded many fossil discoveries of an extinct bird group called enantiornithines. Jingmai O'Connor, Bailleul's supervisor and co-author, noticed the weird membrane-like structure on one of the fossils but did not investigate further.

Years later, O'Connor combed through the IVPP's storage for unstudied specimens and remembered the special fossil. Since O'Conner's specialty is soft tissues, she pursued and sampled the specimen. With the help of teammate Shukang Zhang, O'Connor realized that the soft tissue was in fact a developing embryo.

What they found was that the egg had up to six more eggshell layers. This meant that when the mother bird was still carrying it, trauma must have caused delay in laying the eggs, and so the mother bird had to encase the egg in several more eggshell layers. As a result, this "egg-binding" suffocated the embryo and highly likely killed the mother bird.

Finding A Medullary Bone

After examining the specimen, the team found that the Avimaia bird had a medullary bone, a calcium reservoir, which could help understand how the bird formed the egg in the first place.

Finding such a bone in fossils is quite tricky because it is a temporary structure, and bone diseases can form similar-looking tissues. This makes it difficult to confirm whether ancient medullary bones are linked to reproduction.

In 2005, a groundbreaking study by paleontologist Mary Schweitzer found signs of the medullary bone in Tyrannosaurus rex. The team in Beijing decided to name the discovered species after her.

In the Avimaia bird, Bailleul and her team found the medullary bone where it should be, and the bone surface do not look diseased. This is enough evidence to support that the fossil is female, which could help scientists investigate hypotheses regarding the differences between genders.

Furthermore, scientists are ecstatic over the discovery because it could yield more scientific investigations in the future. If the fossilized egg preserves pigments, it could even reveal how the Avimaia bird nested and its relationship with its youngling.

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