March 20 (UPI) -- Scientists have discovered a preserved egg shell trapped inside the fossilized body of a Cretaceous-era bird. Analysis of the ancient egg suggests the would-be mom died as a result of reproductive abnormalities.
Researchers at the Chinese Academy of Sciences determined the specimen represents a new species, Avimaia schweitzerae. The species belongs to a group known as Enantiornithes, or "opposite birds." Scientists found the opposite bird species among 110-million-year-old deposits in northwestern China.
Close examination of the preserved egg under a microscope revealed two layers of egg shell, suggesting the egg was retained for too long. Healthy eggs feature just one layer.
When small birds are under stress, they sometimes retain their egg or eggs for too long, allowing for another layer to form. Scientists also determined the two layers were thinner than a sheet of paper.
In the paper describing their discovery, published this week in the journal Nature Communications, researchers suggest the reproductive abnormalities led to the bird's death.
Small birds under undue stress regularly suffer from what's known as "egg-binding."
The analysis didn't just reveal abnormalities. The researchers also confirmed the presence of mineral spherules in the egg's outer protective shell, the cuticle. The mineral makeup of the egg's cuticle suggests Avimaia schweitzerae partially buried its eggs.
Researchers also confirmed the presence of medullary bone, a unique type of bone tissue that scientists estimate was used as a calcium reservoir for developing eggshells. The new fossil marks the only identification of medullary bone and an egg within Mesozoic bird fossil.
"This new specimen is arguably one of the most interesting Cretaceous fossil birds yet discovered, providing more reproductive information than any other Mesozoic fossil bird," researchers wrote in a news release.