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Earliest Unhatched-Bird Fossil Found—Old as Dinosaurs

John Roach
for National Geographic News
October 21, 2004
 
Paleontologists in China have unearthed a 121-million-year-old fossil bird embryo that is likely the world's oldest. (See pictures.) The bird was found scrunched in an oval-shaped space slightly smaller than a chicken egg—one of several clues that suggest the bird never hatched.

More important, scientists say, is the evidence that the embryonic bird had feathers, a large skull, and hardened bones. The findings support the notion that early birds, like dinosaurs, were well developed at birth and able to move and forage on their own from the get-go.

The same theory suggests that birds that give birth to helpless, naked young evolved much later.



Paleontologists Zhou Zhonghe and Zhang Fucheng discovered the fossil in Liaoning Province in northeastern China. Employees of the Institute of Vertebrate Paleo-anthropology at Beijing's Chinese Academy of Sciences, the researchers describe their find in tomorrow's issue of the journal Science.

The bird is preserved in a space that measures approximately 1.4 by 0.8 inches (35 by 20 millimeters), which is bigger than a robin's egg. No eggshell was preserved.

"We did think about the possibility of the fossil not in an egg. However, several lines of evidence appear to exclude this," Zhou said.

The fossil's egglike shape provides one bit of evidence. Another is that the fossilized bird rests in a "tucked" posture that is consistent with a late-stage embryo rather than a hatchling, the researchers said. They also note that the bird's feathers did not differentiate into barbs (the side structures of a feather that branch from the shaft), which is typical of a late embryo and not a hatchling.

"It is admittedly still a mystery why no eggshell was preserved," Zhou said.

Kevin Padian is a professor in the department of integrative biology and curator of the Museum of Paleontology at the University of California, Berkeley. He agrees that the fossil is likely that of a bird embryo and not a recent hatchling.

Common Bird

Zhou believes the bird is an enantiornithine, the most common bird type found during the early Cretaceous period in China. The Cretaceous period spanned from bout 145 million to 65 million years ago.

Zhou said several other enantiornithine species are known from the deposit where the latest fossil was found, but that it was difficult to link the embryo to a specific genus or species.

Padian, however, is less certain of the identification, noting that half of the fossil's characteristics are not exclusive to enantiornithines. He added that characteristics that would identify the fossil an enantiornithine are "either dubious or not well preserved on the specimen."

"But then, what else could it be?" Padian asked.

According to Zhou and Zhang, the bird embryo had a large skull, feathers, and a hardened skeleton. The features signal that the bird was precocial, or mature enough to move and feed independently upon hatching.

Finding such an ancient, well-developed, unhatched bird supports the idea that Earth's first birds were also precocial when young. Bird species with helpless, naked young, known as altricial species, evolved later, according to the paleontologists.

"The fact that there are bones at all at that stage of development is a basis for saying that the skeleton is relatively ossified [or hardened] and is therefore a precocial bird, said Padian, the Berkeley paleontologist.

He noted, however, that "it's not clear what degree of ossification [or skeletal hardening] would be expected in precocial versus altricial birds. It's a spectrum, not a dichotomy. But the inference seems justified."

Zhou and Zhang also suggest that the fact that early birds had well-developed young suggests it was a trait derived from dinosaur ancestors—that this species may have evolved from dinosaurs. For example, research suggests that Troodon, a fast moving, meat-eating theropod dinosaur from the Cretaceous, developed precocially.

"Several previously known theropod embryos and the late Cretaceous avian embryos all seem to be preocial animals, judged purely from skeletal evidence," Zhou said.

Egg Tooth

Some modern bird embryos have a special structure on the top of their bills known as an egg tooth, which they use to break open their eggs while they hatch. The egg tooth drops off soon afterward.

The unhatched bird in the newly discovered fossil lacks an egg tooth, which suggests the feature evolved later, according to the paleontologists. Instead of an egg tooth, the bird has long, curved nails, which it probably used to break open the shell, Zhou said.

The claws also suggest the bird was adapted to living in trees. A hundred and twenty-one million years ago, Liaoning Province in northern China was a forested landscape dominated by active volcanoes and sprinkled with lakes and streams.

According to Zhou, the unhatched bird likely lived in a tree near the water. Its nest, he suspects, fell directly into a lake and was quickly buried. The specimen's intact preservation excludes the possibility that it was transported for a distance before being buried.

"A volcanic eruption might have caused the fast burial, but it is difficult to imagine what really happened to this embryo," he said.

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