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22-Oct-2004

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Egg-citing fossil discovery



Reconstruction of a precocial avian embryo from the lower Cretaceous of China. Image courtesy of Zongda Zhang.
Click here for a high resolution photograph.

A 121-million-year-old fossil bird frozen in time in a scrunched up position tells the story of a bird that grew feathers but never had the chance to fly. The bird became a fossil before it hatched from its shell, according to scientists reporting new research in the 22 October 2004 issue of the journal Science.

This fossil from northeast China is probably the oldest known unborn bird ever discovered. It had a large skull, feathers and a hardened skeleton. These are all signs of a bird that was pretty mature even though it never hatched from its shell. Baby birds (like this one) that are developed enough by the time they hatch to move and feed on their own are called "precocious."

The fact that this unborn bird is so well developed, or precocious, supports the idea that the young of Earth's first birds were already well developed by hatching time. If this is true, then baby birds that were born naked and helpless arose later in bird history, say Zhonghe Zhou and Fucheng Zhang from the Chinese Academy of Sciences in Beijing, China.

The preserved bird is enclosed in an egg-shaped space approximately 35 millimeters by 20 millimeters -- bigger than a robin's egg but smaller than most chicken eggs. The scientists did not find any fossilized shell pieces, however. The nails on the bird's feet are large and curved good for grasping tree branches.

Looking closer at the fossil, the scientists say that this bird's bill did not have an "egg tooth." An egg tooth is a top-of-the-bill tooth that some unborn birds use to break their egg from the inside. The lack of an egg tooth on this bird suggests that the egg tooth innovation arose later in bird history.

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