'Missing link' fossil unearthed
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Scientists have unearthed the preserved remains of what is described as the "missing evolutionary link" between fish and tetrapods, the four-footed animals that emerged from water, from river sediments on Ellesmere Island in Canada's Nunavut.
The newly-discovered species, dubbed Tiktaalik roseae, looks superficially like a crocodile, with a skull around 20 centimetres long.
Its flattened body, which probably grew to a length of between 1.25 and 2.75 metres, was covered in diamond-shaped bony scales.
It is like a fish, with its primitive jaw and fins, but has a tetrapod's neck and ribs.
In the beginning, there was the sea and little by little, the fish in the sea moved on to the land, evolving into the myriad species of animals that have lived, eaten or been eaten, fought and died on terra firma for more than 300 million years.
But when did these extraordinary events take place? And how did they happen? How did fish develop the ability to support the weight of their body out of water and to move around on land?
The answers, say American palaeontologists, lie in astonishingly detailed fossils that have been coaxed out of ancient rock just 1,000 kilometres from the North Pole.
What is generating the most excitement are joints in Tiktaalik's pectoral fins, which have bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals.
"Most of the major joints of the fin are functional in this fish," said Neil Shubin, a professor of organismal biology at the University of Chicago and co-leader of the expedition, whose findings appear in Nature, the British science weekly.
"The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals."
The sediment in which Tiktaalik was found has been dated to around 375 million years ago, in the swampy primeval era known as the Devonian.
At that time, what is now frigid Arctic Canada had a balmy, sub-tropical climate, for it was part of a mega-continent that straddled the equator.
Professor Shubin believes that Tiktaalik's size and shape indicates that it was fitted for living in small streams in a delta system - an environment that probably encouraged the fish to venture into shallow water or even make forays onto land in search of food or shelter from predators.
"The skeleton of Tiktaalik indicates that it could support its body under the force of gravity, whether in very shallow water or on land," said co-author Farish Jenkins, a professor of organismic and evolutionary biology at Harvard University.
"This represents a critical early phase in the evolution of all limbed animals, including humans - albeit a very ancient step."
In a commentary also published in Nature, evolutionary biologists Per Erik Ahlberg of Sweden's Uppsala University and Jennifer Clack of the University of Cambridge in Britain hail the find as being quite literally ground-breaking.
They say Tiktaalik fills a knowledge gap about a key transition period spanning around 20 million years in the Devonian era, between fish and the first tetropods.
Tiktaalik comes between a torpedo-shaped fish called Panderichthys that lived around 385 million years ago and whose pectoral fins and shoulder skeleton suggest it could walk in shallow water but not on land; and the first unambiguous tetrapods, Acanthostega and Ichthyostega, which lived around 365 million years ago and, despite having fishy tails, had limbs with digits.
The newly-discovered species owes its name to the Elders Council of Nunavut, which was invited by the scientists to propose a formal name.
Tiktaalik is the word in the Inuktikuk language for "a large, shallow-water fish," the University of Chicago said in a press release.
-AFP
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