Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) at the Chinese Academy of Sciences have discovered an incredible fossil. They have found the first fossilized bird with an unlaid egg preserved inside its body.

As reported in Nature Communications, the fossil belongs to a new species named Avimaia schweitzerae. It lived 110 million years ago in what is now northwest China. It belongs to a group of birds known as Enantiornithes or "opposite birds", which were common across the world during the Cretaceous period.

The researchers were able to analyze both the egg and its mother in detail. And there were several surprises. The female bird's reproductive system was not behaving as it should. The egg fragments show that the shell was made of two layers instead of just one and both were extremely thin, too thin for proper development.

If an egg doesn’t get laid, it can get coated in a second layer. This is seen in modern birds during periods of stress and also occurred in the sauropod dinosaurs, as well as extinct and living turtles. It is possible that the egg was what caused the bird's death. It might have gotten stuck, a condition known as egg-binding.

While unfortunate for the animal, this is an incredible find for palaeontologists. The egg is incredibly well preserved. The researchers were able to see not just the shell but also traces of the egg's membrane and cuticle, the protein coating that covers the shell as the egg matures. The team was able to establish that the cuticle was made of small spherules (tiny spheres) of minerals. This is expected for birds that partially bury their eggs and is consistent with the idea that protective spherules were a standard component of the cuticle of ancient bird eggs.

Another important finding is linked to the mother bird's skeleton. Female birds about to lay eggs can accumulate calcium in a special deposit within the empty space of their skeletons. This deposit is called medullary bone and has previously been seen in fossilized birds, dinosaurs, and pterosaurs.

Knowing that this specimen is equivocally female also allowed scientists to test current ideas about sexual dimorphism, the way animals of different sexes but the same species can look very different. This small bird has given scientists a unique view of the reproductive cycle of birds and their relatives during the late Mesozoic epoch.

Avimaia’s name comes from the Greek for mother bird (Maia = mother, Avi = bird), while schweitzerae honors Dr Mary Higby Schweitzer for her crucial work on medullary bones and molecular palaeontology.