<i>Tiktaalik roseae</i>: Where's the Wrist? (Updated) - Evolution News & Views

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Tiktaalik roseae: Where's the Wrist? (Updated)

[UPDATE: I have responded to Carl Zimmer's critiques with updates and corrections, here.]

I recently picked up Your Inner Fish, a highly simplified science book written for a popular audience by paleontologist Neil Shubin that promotes the alleged intermediate fossil between fish and tetrapods, Tiktaalik roseae. On page 83, Shubin's book contains a nice diagram comparing the skull-components of a human head to the skull of a primitive craniate fish. It's a vague comparison that does little to convince that fish-heads formed the template for mammal heads. But that's not the focus of Shubin's book. The primary feature that excites Shubin and other evolutionary paleontologists about Tiktaalik isn't found in its head: it's that this fossil is allegedly "a fish with a wrist ... part fin, part limb." (pg. 38-39) What is conspicuously missing from Shubin's book is any diagram (like the one comparing fish heads to human skulls) comparing the bones of the "wrist" of Tiktaalik to a real tetrapod wrist, thereby demonstrating that Tiktaalik actually has a wrist.

Hoping to find a diagram that shows how the bones in Tiktaalik's fin are similar to a tetrapod wrist, I turned to Shubin's original paper in Nature, "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan." The abstract of this paper claims that Tiktaalik has "a functional wrist joint," so I presumed that there would be further discussion in the paper about the supposed wrist, perhaps with the diagram I sought. So I searched the paper: not only is there no diagram comparing wrists, but the word "wrist" is not found anywhere else in the paper; the only occurrence of the word "wrist" is the assertion in the abstract, quoted above.

Shubin has another paper in Nature specifically on Tiktaalik's fin, entitled, "The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb." In this paper, there's much more discussion of the "wrist," as the first sentence of the abstract contains a confession of retroactive ignorance that states, "Wrists, ankles and digits distinguish tetrapod limbs from fins, but direct evidence on the origin of these features has been unavailable." Shubin et al. go on to acknowledge, "Limb skeletons differ from those of fins mainly by the presence of bones that comprise mobile wrists, ankles and digits." It would thus indeed be very impressive to find a fish with a wrist, ankle, or digits in its fin. Does Tiktaalik have these bones?

When discussing Tiktaalik's "wrist," Shubin says he "invites direct comparisons" between Tiktaalik's fin and a true tetrapod limb. Surely this paper must have a diagram comparing the "wrist"-bones of Tiktaalik to a true tetrapod wrist, showing which bones correspond. So again I searched the paper. And again he provides no such diagram comparing the two. So we are left to decipher his jargon-filled written comparison in the following sentence by sentence analysis:

1. Shubin et al.: "The intermedium and ulnare of Tiktaalik have homologues to eponymous wrist bones of tetrapods with which they share similar positions and articular relations." (Note: I have labeled the intermedium and ulnare of Tiktaalik in the diagram below.)

Translation: OK, then exactly which "wrist bones of tetrapods" are Tiktaalik's bones homologous to? Shubin doesn't say. This is a technical scientific paper, so a few corresponding "wrist bone"-names from tetrapods would seem appropriate. But Shubin never gives any.

A diagram comparing Tiktaalik's bones to a true tetrapod wrist would have, at this point, been supremely helpful. But as I noted, Shubin doesn't provide one. In fact, the closest thing I can find to a description explaining which bones in Tiktaalik are similar to real tetrapod wrist bones is found in his book Your Inner Fish, where he asserts that Tiktaalik's fin has the basic bone pattern of tetrapod limb: "one bone--two-bones--lotsa blobs--digits arrangement." (pg. 39) So I will try to provide my own analysis.

Based upon the diagram below, it is obvious that Tiktaalik and true tetrapods have the "one bone" starting point (i.e. the humerus). But that's not very interesting, because many living fish have a "one bone" starting point (which one might call a "humerus") which just serves as a bone that articulates the rest of the fin. Let's continue on to see if the rest of Shubin's statement is defensible:

2. Shubin et al.: "In both Tiktaalik and early tetrapods, the ulnare is block-shaped and articulates with multiple radials or digits, whereas the intermedium is a simple rod."

This is simple enough to comprehend given the diagram I made below based upon his Nature paper. But this is just a descriptive sentence and doesn't talk about evolution.

tiktaalik2.jpg

(Adapted by permission from Macmillan Publishers Ltd: "The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb" (by Neil H. Shubin, Edward B. Daeschler, & Farish A. Jenkins Jr, Nature, Vol 440:764-771 (April 6, 2006); figure resized to fit the page except for the text; click for the original full figure)


On the left we see Tiktaalik's fin, and on the right we see the true limb of a true tetrapod, Tulerpeton. One looks like a fin, the other looks like a limb. Both have what one can call a "radius" and an "ulna." Except in Tiktaalik, the radius sticks off the end of the fin and does not articulate any other bones in the wrist fin, whereas in real tetrapods, including you and me, the radius (along with the ulna) articulates the wrist. One can claim that Tiktaalik has a "radius," but the bone that Shubin calls the "radius" in Tiktaalik clearly bears little relation in function, size, or arrangement to the radius of a true tetrapod with a real wrist (as is seen in the diagram from the Shubin's paper of the true limb of Tulerpeton).

3. Shubin et al.: "The formation of a mobile transverse joint at the distal margin of these bones in Tiktaalik presages the establishment of a functional proximal carpal joint."

Merriam-Webster: Presage:

1. something that foreshadows or portends a future event : omen
2. an intuition or feeling of what is going to happen in the future

Note that presage does not mean "equivalent to." So when we come to Shubin's technical analysis, he admits that Tiktaalik does not have a real "wrist," but at best he says that it has some bones that foreshadow a wrist. But does Tiktaalik's fin really foreshadow a wrist, and how closely do its bones resemble a real wrist?

Let's go back to Shubin's claim that Tiktaalik has a "one bone--two-bones--lotsa blobs--digits arrangement" pattern in its fin, just like a tetrapod limb. Digits are part of fingers or toes that have a grasping capability. It's tough to grasp something with one bone in your finger, so these don't deserve to be called digits. In fact, Clack and Ahlberg explain that Tiktaalik's radial bones are clearly not digits:

Although these small distal bones bear some resemblance to tetrapod digits in terms of their function and range of movement, they are still very much components of a fin. There remains a large morphological gap between them and digits as seen in, for example, Acanthostega: if the digits evolved from these distal bones, the process must have involved considerable developmental repatterning.

(Ahlberg & Clack, "A firm step from water to land," Nature, Vol. 440:747-749 (2006).)

So we'll call these bones exactly what they are: radial bones that are "very much components of a fin," not digits, unless you undergo "considerable developmental repatterning." If we trace from the humerus to the radials of Tiktaalik, then the bone structure of Tiktaalik's fin goes like this: "One bone--one bone--two bones with various radials--one bone with various non-digit radial bones--one bone with various non-digit radial bones." Compare that with the pattern of true tetrapods given by Shubin: "one bone--two-bones--lotsa blobs--digits arrangement." There are considerable differences, and as far as wrist-function goes, the similarities seem superficial.

Also conspicuously missing from Tiktaalik is the "lotsa blobs" section, i.e. in less simple terms, it lacks anything resembling the group of 5 or so carpal bones found in the true wrist of Tulerpeton. Tulerpeton clearly has a wrist--the mass of bones between the radius, ulna, and the digits. Where is this in Tiktaalik? It isn't there, unless one imagines severe readjustments of bones that would have been necessary to effect such a transformation.

But there is another living organism that has a bone-structure resembling Tiktaalik's "One bone--one bone--two bones with various radials--one bone with various radials--one bone with various radials," etc. pattern. But it isn't a tetrapod; it's a fish. The lungfish has a bone structure very similar to "One bone--one bone--two bones with various radials--one bone with various radials--one bone with various radials" (see page 35 of Shubin's Your Inner Fish and compare). But the lungfish conspicuously doesn't have a wrist. And given its non-tetrapod, fish-like morphology, and lungfish are of virtually no use in explaining the origin of limbs:

The limbs, of course, occupy pride of place in any analysis of tetrapod origins. The pattern of internal structure of the osteolepiform limb as in Eusthenopteron and Sterropterygion is clearly homologous with that of tetrapods with respect to the humerus/femur or ulna and radius/tibia or fibula, but little else. It would also be a mistake to exaggerate the extent to which osteolepiform fishes actually used their fins as arms and legs; the fins in the forms that we know are all small and feeble (compared even with the large fins of porolepiforms, coelacanths, and the modern lungfish Neoceratodus, which have a different internal structure). These fishes obviously could not live out of water because they would suddenly be unsupported and feel the force of gravity."

(Keith Stewart Thomson, "The origin of the tetrapods," American Journal of Science (1993) 293-A:33-62 (January).)

So if Tiktaalik "presages" anything, perhaps it's the lungfish--not a tetrapod: Were one to start with a Tiktaalik-like arrangement of bones, lose the "radius," and then add a few more "one bone--various non-digit radial bones" to the end, one would have a fin not unlike a lungfish. Very little repositioning of bones would be necessary to convert Tiktaalik's fin into the fin of a lungfish, meaning this appears to be far simpler of an evolutionary story than what would be required to transform Tiktaalik's fin into a true tetrapod limb. But that's no fun because Darwinists are eager to use Tiktaalik to explain how fish evolved into tetrapods. That's far more difficult, evolutionarily speaking, because here's what would be required to convert Tiktaalik's fin into a tetrapod limb:

  • Shrink the radius and reposition it so that it articulates other bones further down the limb.

  • Dramatically repattern, reposition, and transform the radials by lining them up, separating them out, and adding additional parallel bones in sequence so they can be called digits.

  • Add a bunch of small bones between the radius & ulna, and the now-aligned digits, to form a wrist.

In short, one would have to imagine severe readjustments of bones that would have been necessary to effect such a transformation--readjustments that Shubin assumes in any of his claims of homology. Without trying to force-fit the fin of Tiktaalik into a pre-conceived evolutionary story, the living species that Tiktaalik's fin seems to bear a much closer relationship to is the lungfish. Perhaps Shubin's book should have been called The lungfish's inner fish rather than Your inner fish. Sounds a little less impressive, doesn't it?

4. Shubin et al.: "As in the digits and phalanges in a tetrapod limb, the inter-radial joints distal to this primordial wrist are more or less transversely aligned and capable of flexion and extension. ... the distal endoskeleton of both Tiktaalik and tetrapods contains multiple joints capable of extensive degrees of flexion and extension."

In other words, the joints can flex or straighten. Shubin may be correct, but this is nothing special: the same could be said for living fish species that are capable of using their fins to prop themselves up. And they certainly don't have wrists.

In the end, it's no wonder Shubin chose not to provide a diagram comparing Tiktaalik's fin-bones to the bones of a real tetrapod limb. Where's the wrist? From what I can tell, Tiktaalik doesn't have one.


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