Tiktaalik

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Tiktaalik (IPA pronunciation: [tikta:lik]) is a genus of extinct sarcopterygian (lobe-finned) fishes from the late Devonian period, with many features akin to those of tetrapods (four-legged animals) . It is an example from a succession of ancient fish with adaptations to oxygen-poor shallow-water habitats developing at that time Jennifer A. Clack, Scientific American, [Getting a Leg Up on Land] Nov. 21, 2005., which led to the evolution of amphibians. Excellent fossils were found in 2004 on Ellesmere Island in Nunavut, Canada.

Tiktaalik skull

Tiktaalik lived approximately 375 million years ago. Paleontologists suggest that it was an intermediate form between fish such as Panderichthys, which lived about 385 million years ago, and early tetrapods such as Acanthostega and Ichthyostega, which lived about 365 million years ago. Its mixture of fish and tetrapod characteristics led one of its discoverers, Neil Shubin, to characterize Tiktaalik as a "fishapod" John Noble Wilford, The New York Times, [Scientists Call Fish Fossil the Missing Link], Apr. 5, 2006..

Contents

1 Description
2 Discovery
3 Images: casts of ''Tiktaalik'' fossils
4 See also
5 References
6 External links

Description

Tiktaalik is a transitional fossil; it is to tetrapods what Archaeopteryx is to birds.

Its mixture of both fish and tetrapod characteristics include:

Tiktaalik limb, looking from fin towards shoulder

fish
*fish gills
*fish scales
fishapod
*half-fish, half-tetrapod limb bones and joints, including a functional wrist joint and radiating, fish-like fins instead of toes
*half-fish, half-tetrapod ear region
tetrapod
*tetrapod rib bones
*tetrapod mobile neck
*tetrapod lungs

Tiktaalik generally had the characteristics of a fish, but with front fins featuring arm-like skeletal structures more akin to a crocodile, including a shoulder, elbow, and wrist. The rear fins and tail have not yet been found. It had the sharp teeth of a predator, and its neck was able to move independently of its body, which is not possible in other fish. The animal also had a flat skull resembling a crocodile's; eyes on top of its head, suggesting it spent a lot of time looking up; a neck and ribs similar to those of tetrapods, with the latter being used to support its body and aid in breathing via lungs; well developed jaws suitable for catching prey; and a small gill slit called a spiracle that, in more derived animals, became an ear . Specimens found thus far range from 4 to 9 feet (1.2 to 2.75 meters) in length.

Late Devonian vertebrate speciation saw lobe-finned fish like Panderichthys having descendants such as Eusthenopteron which could breathe air in muddy shallows, then Tiktaalik whose limb-like fins could take it onto land, preceding the first tetrapods such as Acanthostega whose feet had eight digits, and Ichthyostega with developed limbs, negotiating weed-filled swamps. Lobe-finned fish evolved into coelacanth species which survive to this day.

The fossils were found in the "Fram Formation", deposits of meandering stream systems then near the equator, suggesting an animal that lived on the bottom of shallow waters and perhaps even out of the water for short periods, with a skeleton indicating that it could support its body under the force of gravity whether in very shallow water or on land The Academy of Natural Sciences, Philadelphia, [press release] April 3, 2006. (doc). At that period, for the first time, deciduous plants were flourishing and annually shedding leaves into the water, attracting small prey into warm oxygen-poor shallows that were difficult for larger fish to swim in. The discoverers said that in all likelihood, Tiktaalik flexed its proto-limbs primarily on the floor of streams and may have pulled itself onto the shore for brief periods . In an interview, Ted Daeschler stated that "we're making the hypothesis that this animal was specialized for living in shallow stream systems, perhaps swampy habitats, perhaps even to some of the ponds. And maybe occasionally, using its very specialized fins, for moving up overland. And that's what is particularly important here. The animal is developing features which will eventually allow animals to exploit land."NewsHour, [Fossil Discovery], April 6, 2006.

The name Tiktaalik is an Inuktitut word meaning "burbot", a shallow-water fish. [Nunavut Living Dictionary]. Entry for [tiktaalik] The "fishapod" genus received this name after a suggestion by Inuit elders of Canada's Nunavut Territory, where the fossil was discovered .

Discovery

Landsat 7 photo of Ellesmere Island with the discovery site marked

The three fossilized Tiktaalik skeletons were discovered in river sediments on Ellesmere Island, Nunavut, in northern Canada. At the time of the species' existence, Ellesmere Island was part of the Laurentia continent, which was centered on the equator and had a warm climate. [Neil Shubin]and [Ted Daeschler], the leaders of the team, have been searching Ellesmere Island for fossils since 1999.

The remarkable find was made by a paleontologist who noticed the skull sticking out of a cliff. On further inspection, the ancient animal was found to be in fantastic shape for a 383-million-year-old specimen .

The discovery was published in the April 6 2006 issue of Nature and quickly recognized as a classic example of a transitional form. Jennifer A. Clack, a Cambridge University expert on tetrapod evolution, said of Tiktaalik, "It's one of those things you can point to and say, 'I told you this would exist,' and there it is." According to a New Scientist article,

"After five years of digging on Ellesmere Island, in the far north of Nunavut, they hit pay dirt: a collection of several fish so beautifully preserved that their skeletons were still intact. As Shubin's team studied the species they saw to their excitement that it was exactly the missing intermediate they were looking for. 'We found something that really split the difference right down the middle,' says Daeschler."

Images: casts of Tiktaalik fossils

Image:Tiktaalik skull full front.jpg|Skull front view Image:Tiktaalik skull rear.jpg|Skull rear view Image:Tiktaalik limb.jpg|Limb fin to shoulder Image:Tiktaalik limb2.jpg|Limb shoulder to fin

References

External links

[University of Chicago website dedicated to the discovery]
Associated Press, [Fossil shows how fish made the leap to land], Apr. 5, 2006.
Alok Jha, The Guardian, [Discovered: the missing link that solves a mystery of evolution], Apr. 6, 2006.
BBC news, [Arctic fossils mark move to land], Apr. 5, 2006.
NewsHour, [Fossil Discovery], April 6, 2006. (Interview with Ted Daeschler)
Quirks and Quarks, [Missing Link Fish Fossil], April 8, 2006. (Interview with Ted Daeschler)
Time Magazine, ["Our Cousin the Fishapod"], April 10,2006
Science Museum (London), ["Fish fingers: how our limbs came from fins"] April 6, 2006.
[A Devonian tetrapod-like fish] (pdf)
Harvard Gazette [Missing link crawls out of muck]

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