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 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.

...

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.

... Previous digs have unearthed the oldest known fossil belonging to the tyrannosaur group and the oldest horned dinosaur among several others.

This time, the ancient mire has yielded a primitive ceratosaur, a theropod that often had horns or crests, many of whom had knobbly fingers without claws.

"It's a really weird animal - it's got no teeth, had a beak and a very long neck, and very wimpy forelimbs," Professor Clark told BBC News.

"Then when we looked closely at the hand, we noticed it was relevant to a very big question in palaeontology."

The fossil has a first finger which is barely present, made up of just one small bone near the wrist. The fifth finger is gone altogether.

It is a fossil that appears to offer a snapshot of evolution, proving that the more modern three-fingered hand is made up of the middle digits of the ancestral hand, with the outer two being shed.

The third finger is made up of the four phalange bones that the second should have, and it is presumed that the second would lose one bone to become like the first finger that was missing in the fossil.

via BBC NEWS | Science & Environment | New dinosaur gives bird wing clue.