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 ancestorsthat 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.
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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