Fossil Fish With "Limbs" Is Missing Link, Study Says
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 animalsincluding 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 vertebratesanimals with backbonesto emerge from the water.
"This animal represents the transition from water to landthe 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.
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.
(Related reading: "Was Darwin Wrong?")
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.
( See pictures of a later "missing link" fish.)
Living lobe-fins include lungfish, which have gills but can also breathe air using modified swim bladders that act as lungs.
Tiktaalik would have breathed like a lungfish, says Clack, senior assistant curator at Cambridge's University Museum of Zoology.
"It's increasing its reliance on air, so it's not purely a gill-breather," she said.
This freshwater fish needed a large, wide head to pump air into its simple lungs.
"It's a sort of bellows arrangement," Clack explained. "The more air you can get in with a mouthful, the better."
Like a Croc
The creature's long snout seems to be adapted for snapping at prey and hunting with its head above water like a crocodile, Clack said.
"Snapping underwater is less efficient, because water pressure gets in the way," she added. "There would probably have been some large invertebrates around the water margins, or it might still have been feeding on small fishes and things in the water."
The creature also lacked the rigid bony covering over the head and shoulders that most fish have, effectively giving Tiktaalik a neck.
"The animal would have been able to lift its head from the water," Clack said.
The new fossils are so complete and well preserved that they "answer questions that previous material has been unable to answer," she added.
Ted Daeschler, co-leader of the fossil hunt, called the discovery "a dream come true."
"We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and were formed in the right kinds of environments to provide the potential for finding [such] fossils," said Daeschler, curator of vertebrate biology at the Academy of Natural Sciences in Philadelphia, Pennsylvania.
Ellesmere Island is more than 600 miles (970 kilometers) north of the Arctic Circle in Canada's Nunavut territory.
Polar bears roam the now frigid region. But Nunavut's fossil-bearing rocks were formed when North America was part of a giant supercontinent that straddled the Equator.
Huge predators would have lurked in Tiktaalik's rivers and lakes, study co-leader Shubin saysperhaps one reason why Tiktaalik appears to have been headed for land.
"Land had no predators, and it also had food in the form of invertebrates," Shubin said.
"Put this all together and the shallows and mudflats might have been a good place to make a living."
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