The fossil of an ancient bird contains evidence of a non-smeared egg

the Avimaia schweitzerae fossil. The egg fragments are too small to be seen with the naked eye.
Image: Barbara Marrs

Paleontologists in China have detected traces of an unvolled egg in a 110 million-year-old Cretaceous bird fossil, which is considered the first such discovery. And in an ironic twist, it seems that the egg has killed the mother bird.

Say hello to Avimaia schweitzerae, a newly described species of Cretaceous bird. The fossil is remarkable in that it still contains traces of an unposed egg and a medullary bone – a particular type of bone tissue associated with oviposition. This is the first time that paleontologists have found these two elements in a fossil, and the first time that fragments of eggshell are found in an ancient fossil. These findings were published today in an article by Nature Communications written by Alida Bailleul and Jingmai O'Connor of the Institute of Paleontology and Paleoanthropology of Vertebrates of the Chinese Academy of Sciences.

As a species, Avimaia schweitzerae (meaning "mother-bird" and a wink at the paleontologist Mary Higby Schweitzer) may be a new science, but it belonged to a well-researched group of ancient birds known as 39; Enantiornithes. This family of birds has been very successful, living more than 100 million years ago alongside dinosaurs such as Tyrannosaurus and Triceratops. An interesting thing about Enantiornithes is that they were born with flying feathers, suggesting that they could fly very quickly after hatching.

Interpretation of the artist Avimaia schweitzerae, including the dead mother in the water.
Illustration: Michael Rothman

The fossil, identified as IVPP V25371, was found in northwestern China and it was very well preserved, even though it was ground as a pancake. The partial skeleton includes the lower half of the vertebral column, the pelvis, the hind limbs and traces of feathers.

The shell fragments, which surprised the paleontologists, were detected in the abdomen of the bird following a microscopic examination. Fragment analysis showed parts of the egg membrane and cuticle (the outermost protective layer of the shell), the latter being covered with proteins and other organic matter . Interestingly, the researchers also found traces of small spherical substances composed of calcium phosphate. Similar minerals are found in birds that partially bury their eggs, suggesting Avimaia schweitzerae do the same thing – not a huge revelation, since the Enantiornithes are already suspected of having buried their eggs.

Scanning electron microscope images of cuticle and shell shell membrane
Image: Alida M. Bailleul et al., 2019 / Nature Communications

However, the state of the shell fragments strongly suggests that the bird's reproductive system is not functioning properly. And in fact, the non-stripped egg itself has probably resulted in the death of this specimen, argued the authors. The shell was exceptionally thin and consisted of two layers instead of one. Together, this suggests that this bird was suffering from egg binding, an often deadly condition in which birds are unable to lay a developing egg.

The analysis of the fossil has also revealed traces of a single reproductive tissue called medullary bone, which acts as a calcium reservoir for the developing egg.

"This Mesozoic bird specimen is the only one I know that keeps both an egg and a bone marrow," said Michael Pittmann, a paleontologist at the University of Hong Kong, to Gizmodo. "It provides the best evidence to date of an early female who was active for breeding. The fossil also provides invaluable information on the reproduction of enantiornithines, a diverse group of early birds that dominated the Cretaceous and were born "out of the egg" and may also be ready to fly.

[Nature Communications]

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