Arctic fossils link sea, land life
Scientists on Wednesday 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 4 and 9 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 like today’s seals and may have walked, although the research team did not find fossil hindquarters to test that hypothesis.
The discovery provided the best evidence yet that fish emerged from the oceans and rivers of the Earth between 385 million and 360 million years ago and evolved into terrestrial vertebrates that began with amphibians and reptiles and ended 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 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 University of Chicago paleontologist Neil Shubin and Edward 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 a Inuktikuk word for “large, shallow water fish,” and Shubin said “roseae” refers to one of the patrons of the project, who wanted 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 insects and spiders.
Researchers for years had collected Devonian fossil fish with bones and muscles in the fins, 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. “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 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 legs evolved because fish were trapped in evaporating ponds and “had to move out or die.”
“That became passé 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.”