Re: Fossil Fish With "Limbs" Is Missing Link, Study Says


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http://news.nationalgeographic.com/news/2006/04/0405_060405_fish.html

James Owen
for National Geographic News

April 5, 2006
Fossil hunters may have discovered the fish that made humans possible.

Found in the Canadian Arctic, the new fossil boasts leglike fins,
scientists say. The creature is being hailed as a crucial missing link
between fish and land animals-including the prehistoric ancestors of
humans.

Researchers say the fish shows how fins on freshwater species first
began transforming into limbs some 380 million years ago. The change was
a huge evolutionary step that opened the way for vertebrates-animals
with backbones-to emerge from the water.

"This animal represents the transition from water to land-the part of
history that includes ourselves," said paleontologist Neil Shubin of the
University of Chicago.

Shubin was co-leader of a team that uncovered three nearly complete
fossils measuring up to nine feet (three meters) long on Ellesmere
Island in 2004.

The new species, Tiktaalik roseae, had a flattened, crocodile-like head
and strong, bony fins.

The large fish probably flexed and extended these fins like legs to help
it move through shallow, subtropical waters or even on land, the team
says.

The discovery marked the culmination of a five-year, 400-mile
(650-kilometer) fossil hunt across the Arctic's frozen tundra. The
National Geographic Society partially funded the project, which is to be
detailed tomorrow in the journal Nature.

The fish shows other features characteristic of land animals, including
ribs, a neck, and nostrils on its snout for breathing air.

The previously unknown creature is the closest known fish ancestor of
land vertebrates, Shubin said.

It likely used its fins "to prop its body, much like we do when we do a
push-up," he said.

Likewise, the animal's broad ribs would have supported its long, scaly
trunk, adds team member Farish Jenkins of Harvard University in
Cambridge, Massachusetts.

Land Excursions

Water supports the bodies of submerged fish, making strong ribs largely
unecessary, "so this animal must have developed these structures for
life in the shallows and making excursions on to land," Jenkins said.

Shubin says the fish's wide head and sharp teeth suggest it hunted much
like a crocodile and that it also breathed air.

"Look at the side of the snout. It has a nice big pair of external
nostrils," he said.

Tiktaalik could become an icon of evolution in action, write
paleontologists Per Ahlberg of Sweden's Uppsala University and Jennifer
A. Clack of the University of Cambridge in the United Kingdom in an
accompanying commentary.

The paleontologists say the new fish form goes a long way toward filling
the evolutionary gap between fish and the earliest amphibians.

"Our remote ancestors were large, flattish, predatory fishes," they
write. "Strong limblike pectoral fins enabled them to haul themselves
out of the water."

Evolutionary scientists agree that all four-limbed land vertebrates,
including dinosaurs and mammals, are descended from lobe-fins, a group
of primitive fishes with fins suggesting limbs.

__________________________________

One question: If it had the ability to make excursions on land like a
crocodile, possibly to hunt, what was it hunting?

George

Well, that's the strange thing, because according to the fossil record at
least, almost all of the early land animals were carnivores. Possibly
the smaller carnivores first started leaving the water to feed on insects
and/or sun themselves, and the larger carnivores left to prey on the
smaller carnivores.

Exactly. So somewhere out there, there should be some of these smaller
critters, theoretically speaking.

George


.

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