'Link' Between Fish and Land Animals Found
Thursday, April 6, 2006
Scientists yesterday reported discovering an evolutionary "missing link" between fish and land animals -- an ancient, river-dwelling predator with arm joints in its fins, an alligator-like head and ribs heavy enough to support its body on dry land.
Researchers found several fossils between four and nine feet long. The creature was a fish -- with scales, fins and gills -- but it moved its head independently of its body, could drag itself along on land as today's seals do, and may have walked, although the research team did not find fossil hindquarters to test that hypothesis.
The discovery provides the best evidence yet that fish emerged from the oceans and rivers of the early Earth between 380 million and 360 million years ago and evolved into terrestrial vertebrates beginning with amphibians and reptiles, and ending up with mammals and, ultimately, humans.
"This is extremely significant, because while we have been amassing evidence for years on the link between fish and tetrapods [four-legged animals], there was still a gap," said Hans Sues, associate director of research and collections at the Smithsonian's National Museum of Natural History. "This link is one we would have predicted, but it's nice to see that it really exists."
A research team led by paleontologist Neil H. Shubin of the University of Chicago and Edward B. Daeschler of Philadelphia's Academy of Natural Sciences found the fossils locked in red siltstone in the windblown Arctic wilderness of Canada's Ellesmere Island, about 600 miles from the North Pole.
The fish lived 375 million years ago in what had been an equatorial river delta before continental drift moved the land mass northward. The team dubbed it Tiktaalik roseae . "Tiktaalik" is an Inuktitut word for "large, shallow water fish," and Shubin said "roseae" refers to one of the patrons of the project who wants to remain anonymous. The research was reported today in the journal Nature.
Scientists for decades have been gathering evidence that four-footed, vertebrate land animals evolved from fish sometime in the latter part of the geologic period known as the Devonian, after the emergence of insects and spiders.
Researchers for years had collected Devonian fossil fish with bones and muscles in their fins. The fish were regarded as forerunners of the land animals that soon made their way to solid ground. Today's coelacanth is such a "lobe-finned fish," a holdover from this early period.
" Tiktaalik is adapting," Shubin said in a telephone interview. "It could stand on the water bottom in the shallows, or it could stand up in the mud flats. It's a fish tetrapod or a 'fishtopod.' "
Sues, also speaking in a telephone interview, said scientists first subscribed to a theory that the red sedimentary rock where most of the transitional fossils were found indicated an ancient desert climate, and that legs evolved because fish were trapped in evaporating ponds and "had to move out or die."
"That became passe, when scientists in recent years found good lakes for the creatures," Sues said. "It seemed likely that they never left the water, and instead evolved limbs for the purpose of running along under the surface. One idea is that they developed limbs to navigate lakes choked with vegetation."
Shubin said he and Daeschler set out to find transitional animals in the far North, because large Devonian deposits are exposed to the weather there without civilization, vegetation or dirt to conceal them. "Up there, it's either rock or ice," he said.
They discovered Tik taalik embedded in a deposit in the southwestern part of Ellesmere: "There were all kinds of carnivorous fish -- big and small" -- and Tiktaalik was one of the top-line predators, Shubin said. "This was a fish-eat-fish world -- you're either big with giant teeth, or you're small and armored."
Shubin said the team understood immediately that they had found what they were looking for. Tiktaalik may have been a fish, but it had a remarkable set of land-animal characteristics, from its flat, alligator-like head with eyes on top to its ribs -- suitable for both heavy lifting and, perhaps, breathing with lungs.
"But the truly remarkable thing is the internal skeleton preserved within the fin," vertebrate paleontologist Hans-Peter Schultze of the University of Kansas said in a telephone interview. "It looks like a tetrapod limb, and you can see that it was used to walk on the ground."
By examining the pectoral fins extending from Tiktaalik like arms, the team could easily see the outlines of bones and joints -- shoulder, elbow and wrist -- powerful enough to support the animal on land.
And once on land, "opportunities were available," Sues said. "They've found [fossil] scorpions about two feet long in Scotland, and that would have made a good meal for this animal. Like eating lobster."