What is generating the most excitement about the finds are joints in the fish'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 on Thursday in the journal
Nature.
"The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals," he said.
The sediment in which Tiktaalik was found has been dated to around 375 million years ago, in the swampy, pre-dinosaur 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.
Shubin said he believed that Tiktaalik's size and shape indicated that it was suited 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 tetrapods.
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.
