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Ancient Bird Fossil Includes Evidence of an Unlaid Egg



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

Paleontologists in China have detected traces of an unlawful egg in a 110-million-year-old bird fossil from the Cretaceous period, which is considered the first discovery of its kind. And in an ironic twist,

Say hello to Avimaia schweitzerae a newly described bird species from the Cretaceous. The fossil is remarkable in that is still contains traces of an unlawful egg and a medullary bone-a special type of bone tissue related to egg-laying. It's the first time paleontologists have found those two elements together in a fossil, and the first time bits of egg shell have been found in such an ancient fossil. These findings were released in a Nature Communications paper authored by Alida Bailleul and Jingmai O'Connor of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences.

As a species, Avimaia schweitzerae ( meaning "mother bird" and a paleontologist Mary Higby Schweitzer) may well be a member of a well-documented group of ancient birds known as Enantiornithes. Along with 100 million years ago alongside dinosaurs such as Tyrannosaurus and Triceratops. A neat thing about Enantiornithes is that they were born with flight feathers, which seems to fly very soon after hatching.

Artist's interpretation of 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. The partial skeleton consists of the bottom half of the spine, pelvis, hind limbs, and traces of feathers.

The bits of eggshell, which came as a surprise to the paleontologists, were detected in the bird's abdomen following microscopic examination. Analysis of the fragment of parts of the egg membrane and cuticle (the outer layer of the protective layer of the shell), the latter of which is coated with proteins and other organic materials. Interestingly, the researchers also discovered evidence of small sphere-like minerals. Similar animals are found among the beasts of their eggs Scarlet spider [1969019] Scanning electron microscope images of eggshell's cuticle and shell membrane

Image: Alida M. Bailleul et al., 2019 / Nature Communications

The condition of the eggshell fragment, however, strongly suggest the bird's reproductive system was not functioning properly. And in fact, the unlaid egg itself is likely to be the death of this specimen, the authors argued. The shell was unusually thin and it featured two layers instead of one.

Analysis of the fossil so-called yielded traces of a unique reproductive tissue called the medullary bone, which acts as a calcium reservoir for the developing egg.

"This Mesozoic bird specimen is the only one that preserves both an egg and the medullary bone," Michael Pittman, a paleontologist at the University of Hong Kong, said Gizmodo. "It provides the best evidence yet of a female early bird that is reproducively active. The fossil thus provides invaluable insight into the reproduction of enantiornithines, a diverse group of early birds that dominates the Cretaceous and hatched 'out of the egg running' and possibly ready to fly too. "

[Nature Communications]


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