New dinosaur gives bird wing clue

A new dinosaur unearthed in western China has shed light on the evolution from dinosaur hands to the wing bones in today's birds.

The dinosaur fossil, from about 160 million years ago, has been named Limusaurus inextricabilis.



The find contributes to a debate over how an ancestral hand with five digits evolved to one with three in birds.

The work, published in Nature, suggests that the middle three digits, rather than the "thumb" and first two, remain.

Theropods - the group of dinosaurs ancestral to modern birds and which include the fearsome Tyrannosaurus rex - are known for having hands and feet with just three digits.

It has been a matter of debate how the three-fingered hand developed from its five-fingered ancestor. Each digit among the five was composed of a specific number of bones, or phalanges.

Palaeontologists have long argued that it is the first (corresponding to the thumb), second, and third fingers from that ancestral hand that survived through to modern birds, on grounds that the three fingers in later animals exhibit the correct number of phalanges.

However, developmental biologists have shown that bird embryos show growth of all five digits, but it is the first and fifth that later stop growing and are reabsorbed.

The remaining three bones fuse and form a vestigial "hand" hidden in the middle of a bird's wing.

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