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Old 10-01-2008, 12:33 PM   #189658  /  #1
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Default Luskin vs Tetrapod evolution - the rise and fall of Tiktaalik

Casey is at it again, this time in response to the latest paper that Per and colleagues have produced. Of course it wouldn't be a Luskin screed without some smug self-aggrandizement; he claims that Boisvert (the lead author of Per's recent paper) admits to an anti-Tiktaalik point he has made before:
Quote:
Moreover, now that we have Panderichthys, Darwinists are openly admitting that the orientation of Tiktaalik's radials do "not seem to match the way modern fingers and toes radiate from a joint." That's a good point, but it's old news for readers of ENV: in August, I observed that Tiktaalik’s radial bones could not be likened to tetrapod digits unless you "[d]ramatically repattern, reposition, and transform the existing radials by lining them up, separating them out."
he then moves on to shred Panderichthys

Quote:
Given the jagged and "peculiar" shape of these "radial" bones in the scan seen above, and the fact that they are flat like the other nearby bones in the fin, Michael Coates makes a good argument that these alleged radials are really just "fragments of damaged bone."

In the same NG article, one of the paper's co-authors Per Ahlberg said that if Tiktaalik were to remain the form that is closer to tetrapods, then "finger development took a step backward with Tiktaalik, and that Tiktaalik's fins represented an evolutionary return to a more primitive form." In other words, at least some the alleged similarities to tetrapods found in these fossils do not actually represent features that are homologous to tetrapods, i.e. they are convergent similarities, also called homoplasies. This means that similarities between these lobed-finned fish fossils and tetrapods imply homology, except for when they don't, making the Darwinian rationale for inferring "homology" appear weak and arbitrary.
Per, how can you sleep at night.

He ends to article with some mental gymnastics concerning Tiktaalik vs Panderichthys. It's all very half arsed, but there we go.

http://www.evolutionnews.org/2008/09...tiktaalik.html
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Old 10-01-2008, 01:32 PM   #189723  /  #2
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Yeah, so Tiktaalik is crap because Pandrichthys has "more tetrapod-like" fingers, and, at the same time, the distal radials of Panderichthys are too flat and irregular (compared to Tiktaalik's) to even be considered "fingers".

Riiiight. That's Casey Luskin for you.


Anyway, my guess was that Per was refering to the arrangement of the distal radials in Panderichthys: A basal line of bones that emegre palallel to each other. The following quote from this article (kindly provided by Rogue06 at TheologyWeb) seems to support that:
Quote:
Previous data from another ancient fish called Tiktaalik showed distal radials as well -- although the quality of that specimen was poor. And the orientation of the radials did not seem to match the way modern fingers and toes radiate from a joint, parallel to each other.

"The disposition of distal radials in Panderichthys are much more tetrapod-like than in Tiktaalik," Boisvert wrote. "Combined with fossil evidence from Tiktaalik and genetic evidence from sharks, paddlefish and the Australian lungfish, it is now completely proven that fingers have evolved from distal radials already present in fish that gave rise to the tetrapod."
However: Tiktaalik, besides having distal radials that look a lot more like digits in their structure, also has a proximal line of parallel bones. That arrangement, unlike Acanthostega, is lost in the distal lines (that Panderichthys lacks): That could be the "step backwards" that Per mentions, and it could be the result of a variety of conditions that we see in many animals (and even humans) today: Syndactyly, Complex Polydactyly, Duplication...

Perhaps that pattern might also make sense, if you consider that, for Tiktaalik, the "digits" were inside the 'fleshy' part of the fin- and were used, more or less, as a "pad" to support the animal when doing its "push-up". In Acanthostega, the bones were used to form the paddle-like structure of the limb, so a fan-like arrangement with single lines of parallel bones would be more beneficial.

I guess Per himself can clarify it all for us (I love Talk Rational! )
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Old 10-01-2008, 02:51 PM   #189857  /  #3
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Regarding the "fragments of damaged bone" comment (which may be out of context in any case: I haven't actually seen what Mike Coates said) it's a non-starter. The fin is lying undisturbed in soft sediment, the radials are embedded in an undisturbed and undistorted envelope of scales-plus-fin-rays, and none of the other endoskeletal bones shows any sign of breakage. If that's really what Mike said, he should have known better.

Casey Luskin is not even worth addressing.
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Old 10-01-2008, 02:56 PM   #189867  /  #4
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Quote:
Originally Posted by Per Ahlberg View Post
Regarding the "fragments of damaged bone" comment (which may be out of context in any case: I haven't actually seen what Mike Coates said) it's a non-starter. The fin is lying undisturbed in soft sediment, the radials are embedded in an undisturbed and undistorted envelope of scales-plus-fin-rays, and none of the other endoskeletal bones shows any sign of breakage. If that's really what Mike said, he should have known better.

Casey Luskin is not even worth addressing.
The Coates quote comes from this Nat Geo article. There is a bit of quote-minery going on. Casey provided this quote:
Quote:
Michael Coates, an evolutionary biologist at the University of Chicago, called the new findings "intriguing" but is not convinced that the digit-like structures in Panderichthys's fin are the equivalent of our fingers.

For one thing, they seem unusually flat for radial bones, Coates said.

"Radials are generally cylindrical. When you look at [a] cross-section [of the digit], they're dumbbell-shaped."

The structures are so peculiar, they might just be fragments of damaged bone, he added.
the full quote is:

Quote:
Michael Coates, an evolutionary biologist at the University of Chicago, called the new findings "intriguing" but is not convinced that the digit-like structures in Panderichthys's fin are the equivalent of our fingers.

For one thing, they seem unusually flat for radial bones, Coates said.

"Radials are generally cylindrical. When you look at [a] cross-section [of the digit], they're dumbbell-shaped."

The structures are so peculiar, they might just be fragments of damaged bone, he added.

Coates agreed, however, that fingers and toes—or at least their precursors—were probably present in early fish.

"Nothing comes from nothing in evolution," he said.
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Old 10-03-2008, 10:44 PM   #193743  /  #5
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So Casey Luskin is lying for doctrine? Wow, I'd never have guessed.

[/sarcasm]
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Old 10-05-2008, 11:13 AM   #195051  /  #6
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Per, I was wondering about what you were quoted saying, about Tiktaalik being an evolutionary "step backwards" in digit formation. Does that mean that the row of distal radials in Panderichthys led to the creation of "proto-digits" with phalanges, and those digits were later fused in some places, creating the pattern we see in Tiktaalik?
Or is it possible that Tiktaalik's pattern was the first to be produced from Panderichthys' single proximal row, and divisions or duplications later led to the single-line parallel arrangement of digits in Acanthostega? Which is more probable in your opinion?

Also, do we have any clues as to how the distal radials in Ventastega were arranged?
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Old 10-06-2008, 08:37 AM   #195967  /  #7
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Hi Faid,

"Step backwards" is perhaps something of a simplification, but the fin skeleton of Tiktaalik is in certain respects less limb-like than that of Panderichthys.

From the shoulder out to the wrist (or from the hip out to the ankle) the tetrapod limb skeleton essentially consists of an axis - one element following another in a straight line - and a series of branches that come off towards the anterior. The humerus or upper arm bone is the first axial element; of the two forearm bones, the ulna (on the "little finger" side of your arm) is the second axial element whereas the radius (on the "thumb" side of your arm) is the first anterior branch. This pattern continues into the wrist, but is most easily seen there in primitive-ish living tetrapods such as salamanders; in ourselves, some of the wrist bones have fused in ways that obscure the pattern. Anyway, the most posterior wrist bone (i.e. just below the little finger) is the ulnare, which is the third axial element. Just anterior to it is the intermedium, which is the second anterior branch. Outside of the wrist bones lie the digits, arranged in a kind of fan shape.

We can summarise some of the characteristics of the tetrapod limb skeleton like this:

Ulna - long. Usually same length as radius.

Ulnare - short. A little wrist bone, much shorter than the ulna. The last element of the axis.

Digits - arranged in a fan shape.

In Acanthostega (d below), which is a very primitive tetrapod, the radius is still longer than the ulna but all the other characteristics seem to be there.

Now, in Panderichthys, we find that the ulnare is much shorter than the ulna and really looks like a wrist bone. Furthermore, the ulnare is the last axial element, and beyond it the distal radials are arranged in something of a fan shape. But in Tiktaalik the ulna and ulnare are equal in size, there are two more axial elements beyond the ulnare, and the distal radials are arranged bipinnately (i.e. like the leaflets of a palm leaf) on either side of this distal axis. In all these respects Tiktaalik's fin skeleton (c) is less limb-like than that of Panderichthys (b) and compares more closely with lobe-finned fishes (e.g. Eusthenopteron, a):



The interesting question is whether this means that:

The detailed similarities between Panderichthys and tetrapods are convergent, or -

The seemingly more primitive fin skeleton of Tiktaalik represents an evolutionary reversal, or -

The current phylogenetic hypothesis is wrong and Panderichthys is actually more closely related to tetrapods than Tiktaalik.

One of these three explanations must be correct, but it is not yet possible to tell which one.

Sadly, we don't have any data at all on the limb structure of Ventastega. A reasonable guess is that they resembled those of Acanthostega, because the limb girdles are similar, but no limb bones have been found.
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Old 10-06-2008, 10:44 PM   #196871  /  #8
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Regular Pandas Thumb creationist FL has been happily spouting forth about this latest research, starting from here:

http://pandasthumb.org/archives/2008...comment-169203

I have invited him to Talk Rational to discuss these issues. He claims he will post this weekend (I'll believe it when I see it). In advance, he relies heavily on this article on Tiktaalik:

http://www.earthhistory.org.uk/techn...taalik-roseae/

and this more general one on tetrapod evolution:

http://www.earthhistory.org.uk/trans...-to-amphibian/

If you've got time at the moment Per, I'd appreciate your thoughts on these.
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Old 10-08-2008, 01:32 PM   #199053  /  #9
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Quote:
Originally Posted by Per Ahlberg View Post
One of these three explanations must be correct, but it is not yet possible to tell which one.
I'll ask the awkward question (I already have some possible thoughts, so I want to see if I am right) - how might you be able to tell? You imply that it may be possible in future...
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Old 10-08-2008, 04:31 PM   #199338  /  #10
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Please don't invite FL here. He is an onerous douche with a very bad grasp of anything remotely resembling reality. Although if you could get him and Dave to square off in the same thread, that might be entertaining.
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Old 10-08-2008, 04:36 PM   #199345  /  #11
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Already invited I'm afraid. I doubt very much that he will come.
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Old 10-08-2008, 06:43 PM   #199510  /  #12
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Quote:
Originally Posted by Jet Black
I'll ask the awkward question (I already have some possible thoughts, so I want to see if I am right) - how might you be able to tell? You imply that it may be possible in future...
More complete information on Tiktaalik (which has not yet been fully described) and Panderichthys (which has not been terribly well described in some respects) will help us to determine whether the current phylogenetic picture is correct. Discoveries of additional transitionals may also help.
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Old 10-13-2008, 02:59 PM   #205071  /  #13
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Well, I'm here. And thanks for inviting me, SteveF, it's a nice cozy place you got here.

I use the handle Mellotron for this forum, but I'm also the "FL" that SteveF spoke of.

I have no intention of doing any long-drawn-out debating in this thread, and I know that Dr. Ahlberg's time is restricted and valuable, but I was asked to present some arguments to Dr. Ahlberg for his commentary.

So I'll just ask about a couple of items a little later today, and that should be all.

(For this thread, anyway.... heh-heh-heh.)

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Old 10-14-2008, 06:49 AM   #206019  /  #14
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Hi Mellotron, and welcome to TalkRational!

As luck would have it I have just started my main teaching block for the autumn (I'm lecturing 9 am to 2 pm today - hooray!!) but I'll try to find time to answer your points.

Cheers, Per
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Old 10-20-2008, 01:08 AM   #213408  /  #15
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No problem! And my apologies for my own delay in getting back to this forum.

At best, I'll only be asking a couple questions. The first one is simply, do you agree with Casey Luskin's main observation regarding Tiktaalik?

Quote:
My main observation is this: if Panderichthys is dethroning Tiktaalik as the icon of the fish-to-tetrapod transition, what does that say about all the hype we’ve seen surrounding Tiktaalik? It says that “poor” and “primitive” Tiktaalik was never all it was hyped up to be.

http://www.evolutionnews.org/2008/09...tiktaalik.html
****

As I await your answer to that question, I'll go ahead and share mine. I agree with Luskin's statement. I think his main observation is correct. Here's a couple examples of the hype Luskin spoke of, items that I googled on my own:

Quote:
U.S. researchers say they have found the missing evolutionary link between fish and land animals: fossils of a strange creature that crawled onto the shore abou 375 million years ago.

....."This really is what our ancestors looked like when they began to leave the water", according to an editorial accompanying the report.

.....Also, at the ends of the powerful fins, the team found wrists and bones similar to fingers. But the fins also contained the thin rods found in fish fins. "Here is a creature with fins that can do push-ups," Shubin said.

---Los Angeles Times; also reprinted in Seattle Times
http://seattletimes.nwsource.com/htm..._fossil06.html
Quote:
NEW YORK - Scientists have caught a fossil fish in the act of adapting toward a life on land, a discovery that sheds new light on one of the greatest transformations in the history of animals.

.......About 375 million years ago, the creature (Tiktaalik)looked like a cross between a fish and a crocodile. It swam in shallow, gently meandering streams in what was then a subtropical climate, researchers say. A meat-eater, it lived mostly in water.
Yet, its front fins had bones that correspond to a shoulder, upper arm, elbow, forearm and a primitive version of a wrist, (Neil) Shubin said. From the shoulder to the wrist area, “it basically looks like a scale-covered arm,” he said.

---- MSNBC, "Fossil shows how fish crept onto land"

http://www.msnbc.msn.com/id/12168265/
There are other examples of hype, such as the National Academy of Science's 2008 book Science Evolution and Creationism stating that Tiktaalik had simple lungs and a flexible neck. (The creationist website "Earth History A New Approach" points out that lungs (simple or otherwise) were not mentioned in the scientific report at all, and that a flexible neck is not a "uniquely tetrapod feature". See http://www.earthhistory.org.uk/trans...-to-amphibian/ ).

These examples lead me to conclude that indeed the Tiktaalik hype seriously does NOT match the actual reality, and thus Luskin is therefore correct in his main observation.

So, back to the question: Do you agree or disagree with Luskin's main observation?

****

There is a second question based on a snippet from the creationist website Earth History A New Approach (this is separate and different from Casey Luskin's Evolution News & Views article), and I'll place that question on the table now.
Just asking to see if you agree or disagree, and why.

In an April 12, 2008 article, "Tiktaalike roseae -- a missing link?", the Earth History website wrote:

Quote:
As it happens, a very explicit prediction was made in the pages of Nature four months earlier (22 December 2005), when Catherine Boisvert was discussing the pelvic fin and girdle of Panderichthys:

"The pelvic girdle is even less tetrapod-like than that of … Eusthenopteron, but the pelvic fin … shares derived characteristics with basal tetrapods despite being more primitive than the pectoral fin of Panderichthys. The evolution of tetrapod locomotion appears to have passed through a stage of body-flexion propulsion, in which the pelvic fins played a relatively minor anchoring part, before the emergence of hindlimb-powered propulsion in the interval between Panderichthys and Acanthostega."

What Boisvert is saying here is that Panderichthys had ‘front-wheel drive’: its front fins were bigger and more powerful than its rear fins. However, the early tetrapods were ‘rear-wheel drive’. Consequently, evolution theory predicted that the emergence of hindlimb-powered propulsion would be seen in the interval between Panderichthys and Acanthostega. Tiktaalik fails that prediction. Indeed, it was more of a ‘front-wheel drive’ animal than Panderichthys was.
So here's my second question, and again it's a simple one.
Do you agree or disagree that Tiktaalik failed that prediction?

Thanks again for your time.

Mellotron
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Old 10-20-2008, 02:27 AM   #213567  /  #16
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Um, I think we should wait for Per to respond, so I'll refrain from commenting, even though I'll have to fight the urge...
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Old 10-20-2008, 08:49 AM   #213871  /  #17
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Quote:
Originally Posted by Mellotron View Post
No problem! And my apologies for my own delay in getting back to this forum.

At best, I'll only be asking a couple questions. The first one is simply, do you agree with Casey Luskin's main observation regarding Tiktaalik?

Quote:
My main observation is this: if Panderichthys is dethroning Tiktaalik as the icon of the fish-to-tetrapod transition, what does that say about all the hype we’ve seen surrounding Tiktaalik? It says that “poor” and “primitive” Tiktaalik was never all it was hyped up to be.

http://www.evolutionnews.org/2008/09...tiktaalik.html
****

As I await your answer to that question, I'll go ahead and share mine. I agree with Luskin's statement. I think his main observation is correct. Here's a couple examples of the hype Luskin spoke of, items that I googled on my own:

Quote:
U.S. researchers say they have found the missing evolutionary link between fish and land animals: fossils of a strange creature that crawled onto the shore abou 375 million years ago.

....."This really is what our ancestors looked like when they began to leave the water", according to an editorial accompanying the report.

.....Also, at the ends of the powerful fins, the team found wrists and bones similar to fingers. But the fins also contained the thin rods found in fish fins. "Here is a creature with fins that can do push-ups," Shubin said.

---Los Angeles Times; also reprinted in Seattle Times
http://seattletimes.nwsource.com/htm..._fossil06.html
Quote:
NEW YORK - Scientists have caught a fossil fish in the act of adapting toward a life on land, a discovery that sheds new light on one of the greatest transformations in the history of animals.

.......About 375 million years ago, the creature (Tiktaalik)looked like a cross between a fish and a crocodile. It swam in shallow, gently meandering streams in what was then a subtropical climate, researchers say. A meat-eater, it lived mostly in water.
Yet, its front fins had bones that correspond to a shoulder, upper arm, elbow, forearm and a primitive version of a wrist, (Neil) Shubin said. From the shoulder to the wrist area, “it basically looks like a scale-covered arm,” he said.

---- MSNBC, "Fossil shows how fish crept onto land"

http://www.msnbc.msn.com/id/12168265/
There are other examples of hype, such as the National Academy of Science's 2008 book Science Evolution and Creationism stating that Tiktaalik had simple lungs and a flexible neck. (The creationist website "Earth History A New Approach" points out that lungs (simple or otherwise) were not mentioned in the scientific report at all, and that a flexible neck is not a "uniquely tetrapod feature". See http://www.earthhistory.org.uk/trans...-to-amphibian/ ).

These examples lead me to conclude that indeed the Tiktaalik hype seriously does NOT match the actual reality, and thus Luskin is therefore correct in his main observation.

So, back to the question: Do you agree or disagree with Luskin's main observation?

****

There is a second question based on a snippet from the creationist website Earth History A New Approach (this is separate and different from Casey Luskin's Evolution News & Views article), and I'll place that question on the table now.
Just asking to see if you agree or disagree, and why.

In an April 12, 2008 article, "Tiktaalike roseae -- a missing link?", the Earth History website wrote:

Quote:
As it happens, a very explicit prediction was made in the pages of Nature four months earlier (22 December 2005), when Catherine Boisvert was discussing the pelvic fin and girdle of Panderichthys:

"The pelvic girdle is even less tetrapod-like than that of … Eusthenopteron, but the pelvic fin … shares derived characteristics with basal tetrapods despite being more primitive than the pectoral fin of Panderichthys. The evolution of tetrapod locomotion appears to have passed through a stage of body-flexion propulsion, in which the pelvic fins played a relatively minor anchoring part, before the emergence of hindlimb-powered propulsion in the interval between Panderichthys and Acanthostega."

What Boisvert is saying here is that Panderichthys had ‘front-wheel drive’: its front fins were bigger and more powerful than its rear fins. However, the early tetrapods were ‘rear-wheel drive’. Consequently, evolution theory predicted that the emergence of hindlimb-powered propulsion would be seen in the interval between Panderichthys and Acanthostega. Tiktaalik fails that prediction. Indeed, it was more of a ‘front-wheel drive’ animal than Panderichthys was.
So here's my second question, and again it's a simple one.
Do you agree or disagree that Tiktaalik failed that prediction?

Thanks again for your time.

Mellotron
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Old 10-20-2008, 11:19 AM   #213924  /  #18
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Hi Mellotron,

That's a pretty relevant question you're asking, and I am going to give the best and most nuanced answer I can. Please pay careful attention to every part of the argument.

Is there a problem with the way Tiktaalik has been hyped? Yes, there is. Does this mean that it has been "dethroned" by Panderichthys, or that it is no longer relevant to the question of tetrapod origins? No it doesn't.

The underlying issue is that research into major evolutionary transitions (from fish to tetrapod, from reptile to bird, that kind of thing) does not and should not focus on Unique Missing Links, but is frequently portrayed that way in the media. Nor is this a straightforward case of the wicked sensationalist media distorting the views of the innocent scientists: there is always a temptation for the scientists to boost the profile of their particular find, and in the case of Tiktaalik I do feel that the authors have not always been careful enough to ensure that the animal was placed in its proper context by news reporters. By allowing it to be portrayed as a unique missing link devoid of context, standing all alone in the beam of the spotlight as it were, they have inadvertently obscured some of its real significance. Note, however, that I am speaking here of popular media reports written by others, not the actual scientific papers by the discoverers.

Two things need to be understood in relation to the study of major transitions: the first is that these are not simple two-step affairs from a starting point via a "missing link" to an end point ("normal" fish -> Tiktaalik -> fully developed land vertebrate) but rather very gradual processes involving lots of steps, and the second is that it is extremely difficult to identify direct ancestor-descendant sequences in the fossil record. The first of these points is I think intuitively obvious, but the second needs a bit of consideration. In essence it boils down to the fact that we have in the fossil record, in all likelihood, only a very small sample of the animals that lived at any one time, and the time sequence is full of gaps as well. With very recent fossils such as those from the Ice Age, where the time sampling is densest, we can be reasonably confident that we are retrieving some genuine ancestor-descendant chains: for example, it really does seem to be the case that Mammuthus primigenius (the woolly mammoth) is descended from Mammuthus trogontherii, which in turn is descended from Mammuthus meriodinalis, and that Homo sapiens is directly or indirectly descended from Homo erectus. However, with fossils from the Devonian we don't have much hope for that kind of resolution, so we don't look for direct ancestors but relatives. The characteristics shared by the different fossil forms that we find, with each other and with now living groups, tell us about how they are related to each other and what their common ancestors must have been like.

Think about it this way: Consider a person you know, that you are not related to - a colleague at work or whatever. You belong to the same species and have numerous features in common, both morphological (people all basically look the same apart from gender differences, only the incidentals differ) and molecular (all people have very similar genes). This means that you actually are related, despite having no known family relationship; somewhere back in time, maybe thousands of years ago, the two of you share a last common ancestor. Now here's the key point: you have virtually no hope of ever finding the bones of that ancestor, but you can actually say quite a lot about him/her because every attribute shared by the two of you (Presence of nose? Check! Presence of the gene Hoxd13? Check! Etc. etc.) must also have been there in your common ancestor. Do you see how it works?

Now we can return to Tiktaalik, Panderichthys and their kin. The real story here is that there exists a set of fossils that are more similar to the extant Tetrapoda (land vertebrates) than to any other extant vertebrate group, but which nevertheless do not have the full set of land vertebrate characteristics. We hypothesise that these fossils belong to the lineage that gave rise to land vertebrates, and refer to them collectively as the tetrapod stem group. Some members of the stem group, such as Eusthenopteron, are pretty conventional-looking fishes that have a few anatomical features in common with land vertebrates (for example internal nostrils and a humerus-radius-ulna limb skeleton inside the pectoral fin) but show no actual adaptations for life on land. These would appear to form the lower part of the tetrapod stem group. Other members of the stem group such as Ichthyostega and Acanthostega have all those same land vertebrate characteristics plus additional ones like limbs with toes, and evidently had some degree of terrestrial capability, but retain a few fish characteristics (for example a real tail fin with fin rays) that are not seen in any extant tetrapods. These appear to form the upper part of the stem group. So, from this information we can infer the existence of two common ancestors: one very ancient common ancestor of Eusthenopteron (and other similar fishes), Acanthostega + Ichthyostega and extant tetrapods, and another somewhat more recent common ancestor of just Acanthostega + Ichthyostega and extant tetrapods. The second of these common ancestors must be a descendant of the first. Furthermore, we can infer that the first of these common ancestors possessed internal nostrils and a humerus-radius-ulna complex in the pectoral fin, while the second ancestor also possessed these characteristics plus digits, a pelvis attached to the backbone, etc.etc. This begins to give us a breakdown of the evolutionary transformation from fish to tetrapod.

Panderichthys and Tiktaalik both belong in the middle part of the stem group, somewhere between Eusthenopteron and Acanthostega. In most respects, Tiktaalik is marginally more tetrapod-like than Panderichthys: for example, it lacks a bony gill cover, which is still present in Panderichthys. For this reason the original discoverers argued that Tiktaalik occupies a somewhat higher position in the stem group. In most respects, P. and T. are very similar. At a minimum (i.e. without attempting for the moment to resolve exactly how they are related to each other), they allow us to infer one new common ancestor in our scheme: the last common ancestor of Panderichthys, Tiktaalik, Acanthostega + Ichthyostega and extant tetrapods. This common ancestor will lie between the two previously discussed ones in the stem group, being a descendant of the first common ancestor and an ancestor of the second. This new common ancestor shows us an intermediate step in the transformation: it has, for example, already lost the dorsal and anal fins (unlike Eusthenopteron which retains these fins) but has not yet acquired digits (unlike Acanthostega, Ichthyostega and extant tetrapods).

If we accept the hypothesis that Tiktaalik goes above Panderichthys in the stem group, things get a bit more interesting because the new common ancestor that I just described splits into two: a slightly older common ancestor of Panderichthys, Tiktaalik, Acanthostega + Ichthyostega and extant tetrapods, and a slightly younger common ancestor of just of Tiktaalik, Acanthostega + Ichthyostega and extant tetrapods. These two common ancestors will be very similar, and this is their value: they begin to define in great detail the small evolutionary steps occurring in this part of the tree.

You should now be able to see that the importance of Tiktaalik lies very largely in its context, and at least partly in the fact that is is not very different from Panderichthys. Together, all these fossil forms help to map out the transition from fish to land vertebrate that occurred in the tetrapod stem group. So what about the pectoral fin business? Well, the pectoral fin skeleton of Tiktaalik is in many ways a good prototype for a tetrapod forelimb skeleton. It has the same basic humerus-radius-ulna architecture, and the shape of the humerus in particular is very like that of a tetrapod. The humerus and radius of Panderichthys compare closely with those of Tiktaalik. Surprisingly, however, the pectoral fin skeleton proves to be somewhat more limb-like than that of Tiktaalik in having a lung ulna, short ulnare (a wrist bone), and distal radials (digit equivalents) that are arranged in a transverse row rather like toes. In Tiktaalik the ulna and ulnare are about the same size and the distal radials are arranged differently. What does this mean? Well, it obviously clashes with the other character distributions (gill cover etc.) that place Tiktaalik closer to tetrapods than Panderichthys, so three interpretations appear plausible: either 1) the similarities between the fin skeleton of Panderichthys and the limb skeletons of tetrapods are due to parallel evolution, or 2) the non-limb-like characteristics of the fin skeleton of Tiktaalik are just unique specialisations of that animal, or 3) the currently accepted phylogeny is wrong and Panderichthys is in fact closer to tetrapods than Tiktaalik. Personally I believe that 1) and 2) are more probable than 3), but we are not yet in a position to settle this question - and may never be. However, this has only a minor negative impact on our understanding of tetrapod origins.

As regards the pelvis of Tiktaalik, the situation is somewhat different. The pelvis has not been described, so Luskin is either seriously confused or dishonest in making claims about it. There is no basis for claiming that Tiktaalik "was more of a ‘front-wheel drive’ animal than Panderichthys was". However, if we assume for the moment that Tiktaalik will prove to have exactly the same fish-like kind of pelvis that we see in Panderichthys, this still does not mean that Tiktaalik fails any sort of prediction. The reason is simple: your prediction assumes, invalidly, that any fossil falling between Panderichthys and Acanthostega on the tree will be neatly intermediate between them in every respect. There is no reason to believe that that will be the case. You are basically arguing that, because 5 is intermediate between 2 and 9, any randomly picked number that is not 5 is not an intermediate between 2 and 9. I trust you can see the fallacy.
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Old 10-20-2008, 12:05 PM   #213951  /  #19
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Per, he's talking about panderichthys' pelvis for the second question isn't he, not Tiktaalik's.?
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Old 10-20-2008, 12:11 PM   #213956  /  #20
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No, he's asserting that the pelvis of Tiktaalik is even less tetrapod-like than that of Panderichthys.
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Old 10-20-2008, 12:16 PM   #213961  /  #21
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Quote:
Originally Posted by Martin.au View Post
Per, he's talking about panderichthys' pelvis for the second question isn't he, not Tiktaalik's.?
Nope, aririsng from that comparison is the following.

Quote:
Tiktaalik fails that prediction. Indeed, it was more of a ‘front-wheel drive’ animal than Panderichthys was.
That's just not true, as no pelvis has been described (despite previous assertions otherwise) there is no way of determining whether Tiktaalik was more "front-wheel" or "back-wheel" so it cannot have failed the prediction. So misinformed or lying is the correct response.
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Old 10-20-2008, 12:17 PM   #213962  /  #22
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Ahh yep, sorry. I misread his question. It's been a long day.
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Old 10-20-2008, 12:52 PM   #213993  /  #23
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Ahh yep, sorry. I misread his question. It's been a long day.
I'm betting the creationist crap site that he got his unformation (intentional spelling) from probably got it from the site that Martin B dissected long ago. But we all know how creationists won't let little things like facts and reality get in the way of spreading their crap.
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Old 10-20-2008, 01:04 PM   #214007  /  #24
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"unformation" - I love it.
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Old 10-21-2008, 05:49 PM   #216291  /  #25
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I'm betting the creationist crap site that he got his unformation (intentional spelling) from probably got it from the site that Martin B dissected long ago. But we all know how creationists won't let little things like facts and reality get in the way of spreading their crap.
Well, it's not my job to talk anybody out of any personal hatreds or bigotries against creationists, so I offer no response to that one (but insert shake of head here.)

Meanwhile, sincere thanks to all who responded, and particularly Dr. Ahlberg.

Dr. Ahlberg, I have seen that "context" argument once or twice before (but not in as much detail and honesty as your explanation), and it seems clear that the "context" line of argument will be the direction in which evolutionists continue to maintain that Tiktaalik is part of the fish-to-tetrapod transition.
But again, I appreciate your detailed explanation and "nuances", and will save it for my own study, to compare and contrast it with the Luskin and Earth History articles.

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One other thing. You said.....

Quote:
Is there a problem with the way Tiktaalik has been hyped? Yes, there is.
.....and you were willing to point out that this problem wasn't entirely the media's fault, but of some of the scientists to a degree, given "the temptation for the scientists to boost their profile of their particular find".

In three different discussion forums, you're the ONLY evolutionist I've met who has displayed such honesty and candor about the situation.

(Just want to say a sincere thanks for that. Most refreshing.)

So, like I said, I don't have any plans for long-drawn-out debate here. It's enough for me to put forth the two questions and to receive your receive your response. Thanks once again.

Mellotron

Last edited by Mellotron; 10-21-2008 at 05:57 PM.
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