Dinosaur-era bird fossil found with unlaid egg

A group of scientists in Gansu province have discovered a fossilized bird dating back about 110 million years and the first ever to have an unlaid egg in its abdomen.

The fossil represents a new species, Avimaia schweitzerae, belonging to a group called the Enantiornithes, which was abundant around the world and coexisted with dinosaurs in the Cretaceous period.

The well preserved fossil was discovered in the city of Yumen.

Because the specimen was crushed flat, it was only after a small fragment was extracted and analyzed under a microscope that the research team at the Chinese Academy of Sciences’ Institute of Vertebrate Paleontology and Paleoanthropology realized the unusual tissue was an egg, said Alida Bailleul, the lead author of the article.

Detailed analysis of the fragment indicated that the bird’s reproductive system was not behaving normally. The eggshell consists of two layers instead of one as in normal, healthy bird eggs, indicating the egg was retained too long inside the abdomen, Bailleul said.

This condition occurs in living birds as a result of stress. Unlaid eggs may be coated with a second layer, or sometimes more, of shell. This abnormality has also been documented in sauropod dinosaurs, and in fossil and living turtles.

In addition, the eggshell was extremely thin, thinner than a sheet of paper, and did not have the proportions of a healthy egg, Bailleul said.

The abnormalities suggest the egg may have been the cause of death of the mother bird, Bailleul said, adding that egg-binding, in which an egg becomes stuck inside the female’s body, is a serious and lethal condition that is fairly common in small birds experiencing stress.

Despite being malformed, the egg is well-preserved, including parts of the shell rarely seen in the fossil record, such as traces of egg membrane and the cuticle, mostly made of proteins and other organic material.

And electron microscope revealed the cuticle (the outermost protective layer of shell) was made of small spherules of mineral. Although never before reported in a fossil egg, in living birds that nest in environments prone to infections, the minerals protect developing chick embryos from microbes.

The find was published online in the journal Nature Communications.