Mesozoic World

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 which was still inside the egg, it was surprising how advanced the feathers were.

Fully formed

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.

'Altricial' birds on the other hand, such as all songbirds, are born completely helpless, naked, and blind. They require devoted care from their parents in order to survive.

Risky business

Most modern arboreal (tree dwelling) birds are altricial so that they can grow to almost full-size in a protected environment, like a nest, before they must attempt the risky business of flying. The fact that this bird - which lived in the Lower Cretaceous period - was prococial could suggest that it did not have the same luxury as its modern day counterparts. It may have been forced to make its own way in the world much sooner. The fact that its feathers were so well developed could mean that these things could fly quite soon after they hatched, according to Dr Milner.

This fossil supported the notion that the planet's first birds had not yet developed the altricial strategy, with all the intensive parental care which it entails. It is generally believed that precociality is ancient and altriciality is derived, according to Dr Zhou and Dr Zhang in their paper.

Four-winged bird

In a separate development, a new fossil bird discovered in China may have been able to shed light on a controversial theory of the origin of flight. Some scientists thought that birds went through a 'four-winged' stage in their evolution before the tail evolved its current aerodynamic shape, freeing the legs from flight duties.

In the journal Nature, Fucheng Zhang and Zhonghe Zhou described a fossilised bird from Early Cretaceous times which had very long feathers on its legs. The bird, which belonged to a group of early birds known as enantiornithines, was between 145 million and 125 million years old.

It was hypothesised that the feathers could be remnants of earlier long, aerodynamic leg feathers, in keeping with the theory that birds went through a four-winged stage during the evolution of flight.

Palaeontologists had previously uncovered evidence of a four-winged feathered dinosaur called Microraptor. This squirrel-sized creature used the long feathers on all four of its limbs to glide or parachute from tree to tree.

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