http://www.msnbc.msn.com/id/20074727/

Fin fossil gives clue to evolution of limbs
400-million-year-old find sheds light on how fins evolved into
appendages

Updated: 2:40 p.m. ET Aug. 1, 2007
[live science]

A 400 million-year-old fossilized fin from a strange-looking,
primitive fish is shedding light on how fins evolved into limbs that
enabled animals to walk on land.

The fossil fin comes from a coelacanth, a type of lobe-finned fish,
and provides the only skeletal fin remains to date from the extinct
relatives of today's living coelacanths. Scientists spotted the
four-inch-long (10 centimeter-long) specimen at Beartooth Butte in
northern Wyoming and have dubbed the fish Shoshinia arctopteryx after
the Shoshine people and the Shoshone National Forest. When alive, the
fish would have been about 18 to 24 inches (46 to 62 centimeters) in
length.

The finding, detailed in the July/August issue of the journal
Evolution & Development, shows the arrangement of bones within the
fossil fin match the fin patterns found in primitive, living
ray-finned fishes, such as sturgeons, paddlefishes and sharks.

Surprisingly, however, the patterns don't match the lobe-finned fish's
living relative. Until now, scientists had assumed the living
coelacanths and their relatives, the lungfish, served as accurate
models of their ancestors dating back hundreds of millions of years
ago.

"Two living fossils, coelacanths and lungfishes, are in fact not
primitive," said lead author Matt Friedman of the University of
Chicago. "They are specialized, and they are not particularly good
models for understanding the origin of limbs."

In fact, living coelacanths are adapted for deep-water habitats off
the coasts of Africa and Indonesia where they use a specialized organ
in their noses to detect weak electrical signals from prey hidden in
the mud along the seafloor.

Unlike fins on living coelacanths and lungfishes, the fossil fin has
an asymmetrical pattern in which there are more bones on the front of
the central shaft than the back. It has more in common with the
anatomy of four-limbed vertebrates, called tetrapods, and even humans
than it does with the anatomy of living coelacanths.

The discovery of the new fossil means scientists can no longer make
inferences about the evolution of limbs based on living coelacanths
and lungfishes.

“To understand the developmental evolution of the limbs of tetrapods,
we shouldn’t be looking at the fins of our nearest living fish
relatives — lungfishes and coelacanths — because they’re far too
specialized,” said co-author Michael Coates, a University of Chicago
biologist.