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.