Fossil Fish (named "Tiktaalik") Sheds Light on Transition

Fossil Fish Sheds Light on TransitionBy MALCOLM RITTER, AP Science Writer 2 hours, 14 minutes agoNEW YORK - Scientists have caught a fossil fish in the act of adapting toward a life on land, a discovery that sheds new light one of the greatest transformations in the history of animals.Scientists have long known that fish evolved into the first creatures on land with four legs and backbones more than 365 million years ago, but they've had precious little fossil evidence to document how it happened.The new find of several specimens looks more like a land-dweller than the few other fossil fish known from the transitional period, and researchers speculate that it may have taken brief excursions out of the water."It sort of blurs the distinction between fish and land-living animals," said one of its discoverers, paleontologist Neil Shubin of the University of Chicago.Experts said the discovery, with its unusually well-preserved and complete skeletons, reveals significant new information about how the water-to-land evolution took place."It's an important new contribution to (understanding) a very, very important transition in the history of life," said Robert Carroll of McGill University in Montreal.The new find includes specimens, 4 to 9 feet long, found on Ellesmere Island, which lies north of the Arctic Circle in Canada. It is reported in Thursday's issue of the journal Nature by Shubin, Ted Daeschler of the Academy of Natural Sciences in Philadelphia and Farish A. Jenkins Jr. of Harvard.Some 375 million years ago, the creature looked like a cross between a fish and a crocodile. It swam in shallow, gently meandering streams in what was then a subtropical climate, researchers say. A meat-eater, it lived mostly in water.Yet, its front fins had bones that correspond to a shoulder, upper arm, elbow, forearm and a primitive version of a wrist, Shubin said. From the shoulder to the wrist area, "it basically looks like a scale-covered arm," he said."Here's a creature that has a fin that can do push-ups," he said. "This is clearly an animal that is able to support itself on the ground," probably both in very shallow water and for brief excursions on dry land. On land, it apparently moved like a seal, he said.It might have pulled itself onto stream banks, perhaps moving from one wet area to another, and even crawled across logs in swamps, said Daeschler.The researchers have not yet dug up any remains from the hind end of the creature's body, so they don't know exactly what the hind fins and tail might have looked like.The creature was dubbed Tiktaalik (pronounced "tic-TAH-lick") roseae, and also had the crocodile-shaped head of early amphibians, with eyes on the top rather than the side. Unlike other fish, it could move its head independently of its shoulders like a land animal. The back of its head also had features like those of land-dwellers. It probably had lungs as well as gills, and it had overlapping ribs that could be used to support the body against gravity, Shubin said.Yet, the creature's jaws and snout were still very fishlike, showing that "evolution proceeds slowly; it proceeds in a mosaic pattern with some elements changing while others stay the same," Daeschler said.If one considers adaptation as a process of collecting tools to live in a new environment, the new finding offers "a snapshot of the toolkit at this particular point in this evolutionary transition," Daeschler said.In fact, much of its value comes from this insight into the order in which those tools appeared in fish, said Jennifer Clack of Cambridge University, an expert unconnected with the study.Knowing that detail about the transition from fish to land-dweller, she said, "might help us to unravel why it happened at all. Why did creatures come out of the water and get legs and walk away?"It's impossible to tell if Tiktaalik was a direct ancestor of land vertebrates, she said, but if a scientist set out to design a plausible candidate, "you'd probably come up with something like this."Shubin said the researchers plan to return to the small rocky outcropping that yielded the fossils and recover more material. "We've really only begun to sort of crack that spot," he said.The site is in Nunavut Territory, and "Tiktaalik" in the creature's name comes from the traditional language used in the area. It refers to a large freshwater fish seen in the shallows. SOURCE

The 383 million-year-old specimens are described as crocodile-like animals with fins instead of limbs that probably lived in shallow water.Before these finds, palaeontologists knew that lobe-finned fishes evolved into land-living creatures during the Devonian Period.But fossil records showed a gap between Panderichthys, a fish that lived about 385 million years ago which shows early signs of evolving land-friendly features, and Acanthostega, the earliest known tetrapod (four-limbed land-living animals) dating from about 365 million years ago."The really remarkable find came when one of the crew found a snout of a flat-headed animal sticking out of the side of a cliff - that is totally what you want to find because if you are at all lucky the rest of the skeleton is back in the cliff," said Professor Shubin.The team found three near-complete, well-preserved fossils of the new species, Tiktaalik roseae, in an area of the Arctic called the Nunavut Territory. The largest measures almost 3m (9 ft) in length.The creature shares some characteristics with a fish; it has fins with webbing, and scales on its back.But it also has many features in common with land animals. It has a flat crocodile-like head with eyes positioned on top and the beginnings of a neck - something not seen in fish."When we look inside the fin, we see a shoulder, we see an elbow, and we see an early version of a wrist, which is very similar to that of all animals that also walk on land," said Professor Shubin."Essentially we have an animal that is built to support itself on the ground."

Tiktaalik - the name means "a large, shallow-water fish" in the Inuit language Inuktikuk - shows that the evolution of animals from living in water to living on land happened gradually, with fish first living in shallow water.The animal lived in the Devonian era lasting from 417m to 354m years ago, and had a skull, neck, and ribs similar to early limbed animals (known as tetrapods), as well as a more primitive jaw, fins, and scales akin to fish.The near-pristine fossil was found on Ellesmere Island, Canada, which is 600 miles from the north pole in the Arctic Circle.Scientists from the Academy of Natural Sciences in Philadelphia, the University of Chicago, and Harvard University led several expeditions into the inhospitable icy desert to search for the fossils.
The find is the first complete evidence of an animal that was on the verge of the transition from water to land. "The find is a dream come true," said Ted Daeschler of the Academy of Natural Sciences.
"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 fossils documenting this important evolutionary transition."When Tiktaalik lived, the Canadian Arctic region was part of a land mass which straddled the equator. Like the Amazon basin today, it had a subtropical climate and the animal lived in small streams. The skeleton indicates that it could support its body under the force of gravity.

"Most of the major joints of the fin are functional in this fish," Professor Shubin said."The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals."
Dr Clack said that, judging from the fossil, the first evolutionary transition from sea to land probably involved learning how to breathe air. "Tiktaalik has lost a series of bones that, in fishes, covers the gill region and helps to operate the gill-breathing mechanism," she said. "The air-breathing mechanism it had would have been elaborated and having lost the series of bones that lies between the head and the shoulder girdle means it's got a neck, it can raise its head more easily in order to gulp the air.
"The flexible robust limbs appear to be connected with pushing the head out of the water to breathe the air."

RAZD writes:The scientists looked in an area that is inhospitable to most living things today, because they knew that (a) the area dated to the right time in the geological past to be in the transitional period, and (b) at that time it had the right kind of environment for such life - it was not a lucky find in a rock quarry (like Archaeopteryx), but one based on research and prediction, in turn based on previous evidence and evolutionary theory. Where and when, consistent with geology and evolution.

The scientists looked in an area that is inhospitable to most living things today, because they knew that (a) the area dated to the right time in the geological past to be in the transitional period, and (b) at that time it had the right kind of environment for such life - it was not a lucky find in a rock quarry (like Archaeopteryx), but one based on research and prediction, in turn based on previous evidence and evolutionary theory. Where and when, consistent with geology and evolution.