Tiktaalik had hind limbs!

by Greg Mayer

In a paper in press in the Proceedings of the National Academy of Sciences, Neil Shubin, Ted Daeschler, and the late Farish Jenkins describe the pelvis and partial hind limb of Tiktaalik roseae, the lobe-finned fish from the Canadian high arctic that they discovered in 2004 and described in Nature in 2006. Tikataalik is a transitional form from fish to tetrapods, and presents such a suite of advanced (for a fish) features that Neil dubbed it the “fishapod”. The newly reported finds show that Tiktaalik had a very substantial pelvic girdle and limb, which must have had a significant role in locomotion.

Of course, it’s not a surprise that Tiktaalik had hind limbs– most vertebrates do– but  the nature of the hind limbs in this, the most tetrapod-like of fish, is of especial interest. It’s been known for a while that Neil et al. had found the hind limb, and their publication on it has been eagerly awaited. The most important find, a pelvis and part of the associated limb, was actually found on the original holotype specimen (the one from which the species was described) found in 2004; four other isolated pelvises were found in later years. Since the first publication, preparator Fred Mullison has been working to free all the bones from the encasing rock.

Comparison of the girdles of Tiktaalik to those of Eusthenopteron  (a 'standard' lobe finned fish) and Acanthostega (one of the earliest known amphibians)

Comparison of the girdles of Tiktaalik to those of Eusthenopteron (a ‘standard’ lobe finned fish) and Acanthostega (one of the earliest known amphibians)

So, what have we found out? The pelvis is robust, with an ilium and pubis, and a large acetabulum for receiving what must have been a substantial femur. There’s no ischium (the third bone in a typical tetrapod pelvis). The Tiktaalik website has 3D scans of the pelvis which you can rotate to see the full morphology.

Tiktaalik pelvis from below: ilium on left, the rounded acetabulum for reception of the head of the femur, pubis on right.  The pubis is directed laterally.

Tiktaalik pelvis from below: ilium on left, the rounded acetabulum for reception of the head of the femur, pubis on right. The pubis is directed medially.

Only a portion of the hind limb was preserved: the intermedium, two radials, and several bony fin rays (lepidotrichia). We can tell from the acetabulum though that the femur must have been robust.

Hind limb of Tiktaalik from Shubin et al. 2014. The thin rays are lepidotrichia; the upper rectangular bone is the intermedium, the lower pair are radials.

Hind limb of Tiktaalik from Shubin et al. 2014. The thin rays are lepidotrichia; the upper rectangular bone is the intermedium, the lower pair are radials.

Here’s how the team summarized their findings:

Although no femur was found, Tiktaalik‘s fin rays and several other bones suggest the hind fin was comparable in size and complexity to the front fin. The shape and size of the hip socket reveal that the fin was capable of a wide range of movements, from swimming to supporting weight and rotating more like a tetrapod limb. But the overall structure of the pelvis is still more fish-like. Whereas tetrapods have a pelvis made of three parts, Tiktaalik‘s pelvis is still made of one, like fish. …

Overall, the mix of fish and tetrapod characteristics show us that the structures and mechanisms necessary for the invasion of vertebrate life on land evolved in the water first. Not only that, but before this discovery, we thought the front fins held the key to how vertebrates began to walk on land. The “front wheel drive” theory that fish dragged themselves out of the water with strong front fins and puny hind fins no longer holds. It appears that an “all-wheel” or even a “rear-wheel drive” system is a more appropriate analogy as the hind fins were just as important and may have even been involved in a walking behavior first.

____________________________________________________________

Shubin, N.H., E.B. Daeschler and F.A. Jenkins, Jr. 2014. Pelvic girdle and fin of Tiktaalik roseae. Proceedings of the National Academy of Sciences in press. pdf

First two Images from the Tiktaalik website 2014 New Discovery page.

44 Comments

  1. Posted January 14, 2014 at 9:54 am | Permalink

    I highly reccommend ‘Your Inner Fish’. Neil Shubin came to the Royal Institution last year to do a talk & showed a cast of Tiktaalik’s head. He signed the book for me :) Nice man.

    I think he is at the University of Chicago.

    • Posted January 14, 2014 at 11:55 am | Permalink

      “Robert R. Bensley Professor of Organismal Biology and Anatomy, Associate Dean of Organismal Biology and Anatomy and Professor on the Committee of Evolutionary Biology at the University of Chicago along with being the Provost of the Field Museum of Natural History.”

      And he has time to do research!

      /@

    • Merilee
      Posted January 14, 2014 at 12:47 pm | Permalink

      One of the best books I read last year!

    • Posted January 14, 2014 at 3:03 pm | Permalink

      I too enjoyed that book. I was fascinated by the little known detail that the trigeminal cranial nerve has tiny extra branches that innervate the teeny stabilizing muscles for certain ear bones that we have. These bones and nerves correspond to the additional bones and trigeminal nerve branches seen in the lower jaw of reptiles. I did not know that!

    • Diana MacPherson
      Posted January 14, 2014 at 6:02 pm | Permalink

      It’s on my massive to-reads list!

    • darrelle
      Posted January 15, 2014 at 7:21 am | Permalink

      One of my favorite popular science books as well. Brief , easy to read, full of information, and just a darn good story too. Instead of feeling like a task that you are determined to complete, as is often the case, this book is hard to put down once you start it.

      It is informative enough that I hesitate to call it a “popular science” book, but this is only because of how it compares to the typical offering in that genre. ‘Your Inner Fish’ is an example of how “popular science” books should be done.

  2. Christine Janis
    Posted January 14, 2014 at 9:54 am | Permalink

    That should put paid to the creationists who claim that it’s just an
    alligator that swallowed a fish (and choked on it, which is why the
    fins are coming out of its “neck”). I kid you not.

    • Achrachno
      Posted January 14, 2014 at 8:34 pm | Permalink

      It gets discouraging at times, doesn’t it? I’m sure that, like everything else, they’ll ignore this.

  3. Maple
    Posted January 14, 2014 at 10:34 am | Permalink

    Of course “Tiktaalik had hind limbs!” Its not evolution on to land, but onto ice (it is northern Canada after all), you have to have hind limbs for your skates, and fins up front for your stick, how else can you play hockey.

    We really need a team in the NHL from Tuktoyaktuk so we can call them the Tiktaaliks.

    • George
      Posted January 14, 2014 at 11:09 am | Permalink

      During the Devonian era – the period when Tiktaalik existed, the land where it lived (now Ellesmere Island) was close to the equator. So no ice. However, it is still a great name for a hockey team. Maybe when Seattle gets a team – the Tiktaaliks! But not shortened to the Tiks.

      • gravelinspector-Aidan
        Posted January 15, 2014 at 5:32 am | Permalink

        the Tiktaaliks! But not shortened to the Tiks.

        I don’t know … some of those ticks can be pretty fearsome looking. Imagine painting that decor onto the helmets.

    • Posted January 14, 2014 at 11:11 am | Permalink

      Now *there’s* a small-market team. ;)

    • Merilee
      Posted January 14, 2014 at 12:48 pm | Permalink

      +1

  4. Posted January 14, 2014 at 11:06 am | Permalink

    §

    b&

    • Diane G.
      Posted January 15, 2014 at 1:13 am | Permalink

      //

  5. Steven Obrebski
    Posted January 14, 2014 at 11:08 am | Permalink

    I read this with a sense of great satisfaction. The details of the anatomy of an intermediate stage between fish and amphibian has been discovered and its structure elucidated. I wonder if Shubin and his colleagues plan to go back to find more material. It would be fabulous if some kind of younger (? pretadpole, juvenile fish like ?) metamorphic stages could be found. One can dream.

    • Posted January 14, 2014 at 11:57 am | Permalink

      Ah, but where is the transitional fossil between fish and Tiktaalik? :-D

      /@

      • Achrachno
        Posted January 14, 2014 at 9:23 pm | Permalink

        I think tadpoles make dandy transitional forms between fishy creatures and tetrapods. One can even watch them evolve.

        • gravelinspector-Aidan
          Posted January 15, 2014 at 5:33 am | Permalink

          But that leaves no room for god! Not even a sauce-dripping spaghettiform one!!

    • Posted January 14, 2014 at 3:04 pm | Permalink

      I think they go back whenever they can.

  6. eveysolara
    Posted January 14, 2014 at 11:21 am | Permalink

    Lungs, that provide neutral buoyancy, are game changers for locomotion in aquatic environments. Walking in aquatic environments is possible in creature that have morphologies, including tiny pelves, large fin rays, etc, that would make such behaviors impossible on land.

    • Achrachno
      Posted January 14, 2014 at 8:32 pm | Permalink

      Are we counting limbs as large fin rays here? Otherwise, I’m thinking about dippers walking along on the stream bottom, finless.

    • gravelinspector-Aidan
      Posted January 15, 2014 at 5:38 am | Permalink

      Lungs, being open to ambient pressure gases, do not provide neutral buoyancy. Swim bladders, which are closed to bulk movement of gas, can be adjusted to provide neutral buoyancy by changing the proportion of fluid to gas within the (approximately constant volume) swim bladder.
      It’s actually a normal part of SCUBA training to practice performing fine adjustments to one’s buoyancy by varying the volume in your lungs using the costal muscles, then ventilating with the diaphragm. given that, it’s not necessary to get your buoyancy accurate to much better than a couple of litres/ kilos. Which is good, since on a dive you’ll lose about that much mass through your exhaust valves. (Another reason that the military initially used closed-circuit systems.)

  7. George
    Posted January 14, 2014 at 11:25 am | Permalink

    A great example of how science works. The fossils were first found in 2004. The first paper was not published until 2006. Those fossils have been studied constantly ever since. So ten years after the first fossils were found, new discoveries are still being made.

    Science is a careful, exacting, ongoing process – always subject to revision. Compare and contrast to religion.

    • mordacious1
      Posted January 14, 2014 at 12:55 pm | Permalink

      In this case, science would probably work a lot faster if it didn’t have to chip all that extraneous rock away.

      • Matt G
        Posted January 14, 2014 at 7:46 pm | Permalink

        Why did god bury it so darn deep?

        • Achrachno
          Posted January 14, 2014 at 8:29 pm | Permalink

          He was trying to hide the evidence, no doubt.

        • mordacious1
          Posted January 14, 2014 at 11:47 pm | Permalink

          It’s a mystery, always a mystery.

        • gravelinspector-Aidan
          Posted January 15, 2014 at 5:39 am | Permalink

          It could have been worse. Didn’t some gods bury their chosen people in volcanos?

  8. Reginald Selkirk
    Posted January 14, 2014 at 11:36 am | Permalink

    “but it’s still a fishapod.”

  9. g2-a302f5ba5123fb53501f7ec0ace37b96
    Posted January 14, 2014 at 12:15 pm | Permalink

    I actually knew this was coming. Back in 2008, Ted Daeschler gave a talk to our local Skeptics group here in Philly. During the Q&A, he told us a “secret” as to what the next big finding would be (it was the ball joint in the pelvis, aka. hind legs). The fact that it took this long to get published means that there must have been A LOT of work left to do to “prove” the result.

  10. lkr
    Posted January 14, 2014 at 12:22 pm | Permalink

    A good time to revisit the YouTube classic “Tiktaalik (Your Inner Fish”, credited to the UPenn Reading Project. Perhaps someone can add a new verse [and pelvic fins to the fishapod model used in the video]!

    ti-tic-taalik

  11. Kevin Alexander
    Posted January 14, 2014 at 1:07 pm | Permalink

    This is why religion is superior to science. You have to go thousands of miles, spend thousands and thousands of dollars and then spend years of hard work to get the truth while believers only have to look in a drawer at the local motel while waiting for their date to arrive.

    • Achrachno
      Posted January 14, 2014 at 8:27 pm | Permalink

      Are believers allowed to wait for their date in a motel room?

      • nicky
        Posted January 14, 2014 at 9:01 pm | Permalink

        No they’re not supposed to, but they can always repent and be forgiven…

      • darrelle
        Posted January 15, 2014 at 7:28 am | Permalink

        As long as they are wearing two wet suits.

  12. Matt G
    Posted January 14, 2014 at 3:50 pm | Permalink

    I sure hope it had hind limbs! It would be a sorry excuse for a tetrapod if it didn’t.

    • Achrachno
      Posted January 14, 2014 at 8:26 pm | Permalink

      Look up Bipes. I think they’re OK and not sorry at all.

  13. microraptor
    Posted January 14, 2014 at 10:33 pm | Permalink

    It’s a sport-utility fish!

  14. Sigmund
    Posted January 15, 2014 at 12:41 am | Permalink

    I’m not a paleontologist and apart from some amateur bashing of old rocks with a hammer I’m pretty ignorant of the mechanics of exposing fossils distinguishing them from their surrounding rock, but I was curious about the following part of the article:

    “Since the first publication, preparator Fred Mullison has been working to free all the bones from the encasing rock.”

    I know that in archaeology the trend has been towards avoiding any physical disruption of sites and instead using imaging techniques (sonar, etc) to visualize what is buried beneath the surface – hence you will not destroy objects that might be fragile or difficult to see during a standard dig. Not only that but you will leave material underground that can be reimaged when techniques have improved or new methodological approaches are invented in the future.
    Does the same thing apply to paleontology? The idea of slowly scaping away the rock to reveal the fossilized bones conjures up the classic image of the paleontologist at work but at the same time I wonder if technology may have superceded this approach.
    Perhaps they do this already (I seem to recall somewhere hearing of this) but in the days of computer imagery and 3D printing I suspect it’s not necessary to physically clean up fossils when it can be done quicker by computer and an accurate cast made even while the original is still encased in rock.

    • gravelinspector-Aidan
      Posted January 15, 2014 at 6:09 am | Permalink

      Two interlinked questions:

      The idea of slowly scaping away the rock to reveal the fossilized bones conjures up the classic image of the paleontologist at work but at the same time I wonder if technology may have superceded this approach.

      Field palaeontology, similarly to field archaeology, consists mainly of recovering the fossils, with as much related “contextual” evidence as possible, as quickly as possible. There’s a nice “Palaeocast” describing this for Isotelus rex, where complicating factors included the rising tide, hungry polar bears and a complete absence of power tools. (I’ve worked with the interviewer, Dave Marshall. Relatively sane, for a bugs man. He uses sieves instead of power tools, while my power tools are over a hundred metres long and have a crew of 180 – when they’re not damned broken!)

      Perhaps they do this already (I seem to recall somewhere hearing of this) but in the days of computer imagery and 3D printing I suspect it’s not necessary to physically clean up fossils when it can be done quicker by computer and an accurate cast made even while the original is still encased in rock.

      Certainly 3-d photography and scanning are becoming more prominent. It’s a field I’ve been doing some experimentation with myself, and it’s trickier than it looks. Until you drop the camera and snap your good lens in two, when it becomes easier.
      To get imaging of fossils without having to “clean them up” is also possible, in some cases, using CT scanning (Computerised Tomagraphy, using X-rays). But it depends on there being an adequate contrast in properties between the fossil’s body and the matrix rock. This can lead to spectacular results – I recall some pyritised insects, though I forget the names, and is non-destructive. OTOH, it does require a very high intensity X-ray source, but they’re becoming more common and several continents have such.
      An older technique – non-computerised tomography – has been around since the 1880s (-ish).
      01 – Take your (suspected) fossil-bearing rock and
      10 – cut a thin section off one end ;
      20 – trace the outline of your fossil ;
      30 – align the tracing with previous stack of tracings
      40 – if you’ve run out of fossil, goto 100 ; else goto 10
      100 – figure out some way of presenting your data
      Obviously very time-consuming. But it works. And if you’ve got a fossil that is little more than a smudge of lighter-grey limestone in a matrix of darker-grey limestone, it works better than composition-dependent remote sensing methods.
      The BGS are in the process – a multi-decade process, I suspect – to 3-d scan many of the “type fossils” in British museums and publish files for printing high-resolution copies of the fossils. As I think about it, this is the sort of project that has the potential to thin down the morass of differing file formats for such things. I’ve been trying to persuade the wife to let me build a 3-d printer for a while now …

  15. Karl Boyd
    Posted January 15, 2014 at 9:10 am | Permalink

    Here is a question: Why did creatures develop legs and not wheel type limbs?

    • Posted January 15, 2014 at 9:46 am | Permalink

      For the same reason that the nerve that connects your brain to your voicebox loops under your heart.

      Wheel-type limbs would be a sign of intelligent design, not of stepwise evolution.

      Cheers,

      b&

      • Sigmund
        Posted January 15, 2014 at 1:54 pm | Permalink

        While we have plenty of examples of species adopting a circular shape for movement (woodlouse rolling up and tumbling away from danger, tumbelweed, etc) there are no examples of a circular part rotating on an axle.
        Indeed the closest to that would probably be the bacterial flagellum (a rotating axle attached to a whip-like appendage.)
        But don’t worry, who on earth would resort to using the bacterial flagellum as a sign of intelligent design!


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