The fish that walked out of water
Fossils of a species of fish in the act of adapting to life on land have been found by scientists, shedding new light on one of the most momentous events in evolution.
The well-preserved remains of creatures with a crocodile-like head and flattened body neatly fill a gap between fish and the first creatures to walk on land.
Previous fossils representing this milestone have essentially been fish with a few land characteristics, or slightly fishy land vertebrates. The newly-found fossils show an animal that sits between the two.
Dr Ted Daeschler, of the Academy of Natural Sciences, in Philadelphia, said: "The find is a dream come true."
The Tiktaalik roseae fossils were found on Ellesmere Island, in Arctic Canada, which was subtropical at the time they were alive.
They provide the missing link between fins and limbs, when animals first walked out of water on to land about 375 million years ago. They have a skull, a neck, ribs and parts of the limbs that are similar to four-legged animals known as tetrapods, as well as fish-like features such as a primitive jaw.
In two articles published in the journal Nature today, Prof Neil Shubin, Dr Daeschler and colleagues describe the fossil that could become an icon of evolution as potent as Archaeopteryx, a transitional fossil between reptiles and birds.
"Tiktaalik blurs the boundary between fish and land-living animals, both in terms of its anatomy and its way of life," said Prof Shubin, of the University of Chicago. "This animal is both fish and tetrapod; we jokingly call it a fishapod."
The fossils were recovered from the layered rock of the so-called Fram Formation - deposits of meandering stream systems formed when North America was part of a super-continent that straddled the equator.
They were collected during four summers of exploration in Nunavut Territory, 600 miles from the North Pole. Members of the team reached the tundra site by a series of aircraft, including a helicopter, and had to endure freezing temperatures and high winds. They carried guns for protection from polar bears.
The skeletal material from several specimens shows that Tiktaalik, ranging from 4ft to 9ft long, had the scales and fins of a fish but the ribs, neck, head and appendage bones like those of a land animal.
Prof Shubin said: "As each piece of Tiktaalik's anatomy was exposed, we began to see how wonderfully intermediate this animal's features were between land and water.
"Most of the major joints of the fin are functional in this fish. The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals."
The deposits where the fossils were found and the skeletal structure of Tiktaalik suggest that it lived in shallow water and perhaps could survive out of the water for short periods.
Instead of using the traditional Latin or Greek to name the fossil, the scientists asked the Nunavut Inuit people to propose a name. The elders council suggested Tiktaalik (pronounced tic-TAH-lick), meaning a large, shallow water fish.
Dr Richard Lane, of the United States National Science Foundation, said: "Human comprehension of the history of life on Earth is taking a major leap forward.
"These exciting discoveries are providing fossil Rosetta Stones for a deeper understanding of this milestone from fish to land-roaming tetrapods."
Dr Marcello Ruta, of the University of Bristol, said: "This amazing discovery has joined many of the dots of a complex picture."
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