110-million-year-old fossil sheds light on bird reproduction

A team of scientists led by Alida Bailleul and Jingmai O’Connor from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences reported the first fossil bird ever found with an egg preserved inside its body. Their findings were published in Nature Communications. The new specimen, representing a new species, Avimaia schweitzerae, was discovered in 110-million-year-old deposits in northwestern China. It belongs to a group called the Enantiornithes (“opposite birds”), which were abundant all around the world during the Cretaceous and co-existed with the dinosaurs.

The new fossil is incredibly well preserved, including the remains of an egg inside its abdomen. Because the specimen is crushed flat, it was only after a small fragment was extracted and analyzed under the microscope that the team realized that the unusual tissue was an egg.


Photograph of the holotype of Avimaia schweitzerae. CREDIT: Barbara Marrs

Detailed analysis of the eggshell fragment revealed a number of interesting facts indicating the reproductive system of this female bird was not behaving normally: The egg shell consists of two layers instead of one as in normal healthy bird eggs, indicating the egg was retained too long inside the abdomen. This condition often occurs in living birds as a result of stress. The unlaid egg then gets coated in a second layer (or sometimes more) of eggshell. This abnormality has also been documented in sauropod dinosaurs, as well as in many fossil and living turtles.

In addition, the eggshell preserved in Avimaia was extremely thin – thinner than a sheet of paper – and did not show the correct proportions of healthy eggs. These abnormalities suggest that the preserved egg may have been the cause of death of this “mother bird.” Egg-binding, in which the egg becomes stuck inside the body causing death, is a serious and lethal condition that is fairly common in small birds undergoing stress.

The preserved egg allows the specimen to be unequivocally identified as female, allowing scientists to test current hypotheses regarding sexual dimorphism (differences between sexes). This new specimen is therefore arguably one of the most interesting Cretaceous fossil birds yet discovered, providing more reproductive information than any other Mesozoic fossil bird.

Research article: An Early Cretaceous enantiornithine (Aves) preserving an unlaid egg and probable medullary bone

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