A 121 million year-old baby arboreal bird,
fossilised while still curled in its egg, was found in China
in 2004. The fossil was thought to be the most ancient unborn
bird discovered to date.
It piqued researchers' interest because it had
feathers, whereas many modern flying birds are naked and helpless
when they first hatch. The authors behind the report said this
supported the view that birds developed the strategy of hatching
featherless later in history.
This fossil was interesting because its
preservation was so exceptionally fine, so much so that even soft
tissue such as feathers had been preserved. Dr Angela Milner of
London's Natural History Museum thought that for an embryo that
was still inside the egg, it was surprising how advanced the
The researchers knew that the bird, found in
north-eastern China, was an embryo because the fossil was tucked
up in very characteristic way for an unhatched chick. Report authors
Zhonghe Zhou and Fucheng Zhang from the Chinese Academy of Sciences
in Beijing, China, stated that this tucked-in posture was consistent
with a late-stage embryo rather than with a hatchling, in which case
the head would have raised beyond the vicinity of the feet.
Apart from the chick's posture, it was otherwise
not very babyish at all. But despite being a bird which had not yet
hatched, it was almost fully formed. All of its bones were formed
and its feathers were very well developed. This maturity meant that
the bird must have been 'prococial'. Prococial birds - chickens,
ducks, and ostriches, for example - produce young which are
immediately competent: they have downy feathers, and can run about
and feed themselves almost as soon as they hatch.