Thursday, April 06, 2006

Science News: Fossil Fills Land-Sea Animal Gap

If you trace our family tree 370,000,000 years back to the Late Devonian, this could well be the face of our long-lost great grand mother.
Meet Tiktaalik roseae, the newly described lobefinned fish from Nunavut, Canada. The name Tiktaalik comes from an Inuit word for the fish we call burbot, after local elders noted the similarity between the fossil and the large living fish they catch.

Unlike its namesake, however, Tiktaalik is a member of the great fish clan Sarcopterygia. Sarcopterygians achieved enduring fame and glory after certain members evolved the means to crawl out of the water and taking the first steps (literally) to a totally landlubbing lifestyle. Tiktaalik fits into this story quite well, sporting a set of characters more amphibian-like than all other fish, but more fish-like than the earliest amphibians.

The astonishingly detailed fossils were coaxed out of ancient rock just a thousand kilometres (600 miles) from the North Pole, and are helping to explain how, little by little, 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.

Buried in river sediments on Ellesmere Island, the preserved remains of
Tiktaalik have already been described as the "missing evolutionary link" between fish and tetrapods.

The newly discovered species looks superficially like a crocodile, with a skull around 20 centimetres (eight inches) long. Its flattened body, which probably grew to a length of between 1.25 and 2.75 metres (four to nine feet), was covered in diamond-shaped bony scales. It was like a fish, with its primitive jaw and fins, but had a tetrapod's neck and ribs.

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."

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.

Acanthostega and Panderichthys

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