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Tiktaalik

2007 Schools Wikipedia Selection. Related subjects: Birds

   iTiktaalik

                           Fossil range: Devonian

             Scientific classification

   Kingdom:   Animalia
   Phylum:    Chordata
   Subphylum: Vertebrata
   Class:     Sarcopterygii
   Subclass:  Tetrapodomorpha
   Genus:     Tiktaalik
   Species:   T. roseae

                                Binomial name

   Tiktaalik roseae
   Daeschler, Shubin & Jenkins, 2006

   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 several lines of ancient sarcopterygian fish developing
   adaptations to oxygen-poor shallow-water habitats at that time , which
   led to the evolution of amphibians. Excellent fossils were found in
   2004 on Ellesmere Island in Nunavut, Canada.
   Tiktaalik skull showing spiracle holes above the eyes.
   Enlarge
   Tiktaalik skull showing spiracle holes above the eyes.

   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" .

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
   Enlarge
   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 lobe-finned 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 . The
   incomplete specimens found thus far suggest animals that ranged from 4
   to 9 feet (1.2 to 2.75 meters) in length.
   In Late Devonian vertebrate speciation, descendants of pelagic
   lobe-finned fish – like Eusthenopteron – exhibited a sequence of
   adaptations: Panderichthys, suited to muddy shallows; Tiktaalik with
   limb-like fins that could take it onto land; Early tetrapods in
   weed-filled swamps, such as: Acanthostega which had feet with eight
   digits, Ichthyostega with limbs. Descendants also included pelagic
   lobe-finned fish such as coelacanth species.
   Enlarge
   In Late Devonian vertebrate speciation, descendants of pelagic
   lobe-finned fish – like Eusthenopteron – exhibited a sequence of
   adaptations:
     * Panderichthys, suited to muddy shallows;
     * Tiktaalik with limb-like fins that could take it onto land;
     * Early tetrapods in weed-filled swamps, such as:
          + Acanthostega which had feet with eight digits,
          + Ichthyostega with limbs.

   Descendants also included pelagic lobe-finned fish such as coelacanth
   species.

   The fossils were found in the " Fram Formation", deposits of meandering
   stream systems near the Devonian equator, suggesting a benthic 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 . 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.. Neil Shubin and Ted Daeschler,
   the leaders of the team, have been searching Ellesmere Island for
   fossils since 1999. 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."

   The name Tiktaalik is an Inuktitut word meaning " burbot", a
   shallow-water fish. 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
   Enlarge
   Landsat 7 photo of Ellesmere Island with the discovery site marked

   The three fossilized Tiktaalik skeletons were discovered in rock formed
   from late Devonian 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.

   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

   Skull front view

                   Skull rear view

                                  Limb fin to shoulder

                                                      Limb shoulder to fin

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