Dinosaur-era bird fossil found with unlaid egg

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.