Thursday, April 20, 2006
This is not the first fossil snake with legs to be found. Previously, there had been Pachyrhachis, Haasiophis and Eupodophis. What's especially interesting about Najash is the presence of a sacrum: usually a modified rib that forms a strut between the vertebral column and the pelvis and helps support the body on land. This is a feature that is found even in the earliest tetrapods and tends only to be lost when limbs are lost or when tetrapods become more fully acquatic.
The authors analyzed the characters of various snakes (fossil and living) and generated the following tree:
The interesting thing to note is that Najash is the most primitive snake. However, the other three legged snakes find themselves deep inside snakes. In fact, they're what are known as the Macrostomata, the group that includes the things you most commonly refer to as modern snakes: boas, pythons, as well as the more derived vipers and the elapids (cobras and their kin).
Now, the last thing I want to get myself into is the sordid debate on snake origins. This new hypothesis will not go without contention since it requires several independent losses of limbs or even the independent re-development of limbs in order to account for the distribution. The debate on snake origins is a hot topic and the key players show no mercy with each other! So, I'll try to ride the fence here.
This phylogeny lies at the heart of the debate on snake origins. One issue here centers on whether or not the mosasauroids (large, extinct marine lizards) are the sister group of snakes and whether or not snakes have a marine origin. The mosasauroids (which includes the mosasaurs proper, as well as the smaller aigialosaurs) are not likely to include the ancestor of snakes, but they share a lot of features in common, such as body elongation and the way the teeth fit into sockets in the jaws -- a condition called "thecodonty" (tooth in hole). In other varanoid lizards (like the Komodo Dragon), the base of the tooth is sort of "squished" to the inside of the jaw, a condition called "pleurodonty" (see a diagram of these tooth types here).
Interestingly, however, a lower jaw referred to Najash apprears to present a pleurodont type condition, or something very similar. This means that the pleurodont condition might actually be ancestral for snakes, and therefore the condition seen in mosasauroids might have nothing to do with the condition seen in snakes.
A matter of outgroup:
The dichotomy that arises between a mosasauroid vs. 'other' sister-group hypothesis is also one between a terrestrial origin for snakes and a marine or aquatic origin. Najash is apparently a terrestrial snake and, in its position as the most 'primitive' snake of all would suggest that terrestriality is ancestral for snakes. But all of this is dependent on how we interpret their tree. This, of course, is further dependent on how the authors treat the data.
One of the potential problems with this new work is that it doesn't actually test the question of who is most closely related to the snakes. If you look at the tree they've obtained, one can see the most deeply stemming branch is labeled "Varanoid root". This is the outgroup, the taxon in the analysis against which allows the analysis to infer the "direction" of evolution inside the tree. The authors constructed a hypothetical varanoid from observations of different varanoids, including mosasauroids. However, since it's only a single terminal taxon, it is impossible for snakes as a total group to "move around" in the analysis. They're stuck there next to a hypothetical taxon. So, the question of mosasauroid relationships is unaddressed. Furthermore, the different outgroup will have different characters, resulting in
The other (and somewhat related) argument that may fall under contention is the inference of the ancestral condition for snakes as a whole based on the environment of Najash. This may be somewhat problematic, especially where they have the other snakes with legs higher up in the tree. If, instead, the other snakes with legs are more primitive (as one would initially think), and one were to test whether or not there is a marine outgroup, then terrestriality might be a specilaization of Najash rather than an ancestral feature. The placement of the legged snakes within the macrostomatans has been contested int he past and will continue to be, so watch for that!
The take-home message here is actually me 'busting down' the sacrosanctitiy of the concept of 'transitional forms'. Najash is almost certainly intermediate and, at this time, I hardly doubt that it is the most primitive snake. As such, it tells us about the early origins of snakes. But it is not an ancestor, and even as an approximation of an ancestor, it is still specialized in its own way. The same could be said for Tiktaalik which has its own specializations that are not necessarily intermediate between other fishes and tetrapods.
The other thing to bear in mind is that perhaps our problem with the re-elaboration of limbs is fraught with 'common sense' thinking -- often a danger in science. It is difficult to imagine how snakes could simple re-evolve their legs, but it would be unwise to use such an argument. The development of an embryo is a hierarchical process and all or most of the genes used in 'leg-making' in snakes probably never disappear (though perhaps somebody more up to speed on this could comment here). A few or a single 'up-stream' switch in the hierarchy of development could perhaps turn the whole leg on or off. Our personal preference or bias that snakes should not 'grow back' their legs must not be an argument. More robust phylogenetic arguments as well as developmental biology should be looked to for clues.
The goal of the study of phylogeny is to sort out between three different kinds of characters: those that are ancestral and therefore ancient and general (plesiomorphy), those that are specialized and unique to a taxon (autapomorphy), and those that are the characteristics unique to a group that shares a unique common ancestor (synapomorphy). When we have sorted out the best and most heavily tested solution to this trichotomy, we can then infer what is the ancestral state. The data on the fish-tetrapod transition are so unequivocal that it is pretty easy to sort this out for Tiktaalik. The same can't yet be said for the origin of snakes, and Najash is by no means the last word. We'll be hearing a lot more about this soon.
Apesteguía, S. and Zaher, H. 2006. A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature 440: 1037-1040. <link>
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Sunday, April 09, 2006
I thought this was a rather telling remark on Tiktaalik posted over on Dembski's blog. We're treated to an excerpt of the pre-transformation version of the DI's original response that goes:
I especially like Crowther’s last sentence which I present in its original form (bold type included): “There’s a problem with the Darwinist position that runs even deeper than this, however: If Darwinian evolution is an undisputed fact, as its chief defenders routinely claim, why is this fossil find being billed as such an crucial piece of evidence?”What I love even more is all this rhetoric and absolutely no reference to the actual fossil material. So, I'll take that as meaning that these guys have nothing to say about its transitional status. The real icing on the cake is all this puff and no real substance.
Icing on the cake! I love it!!!
Unfortunately, the media's response to the discovery is not quite the same as the palaeontological community's interpretation of it. Therefore, by responding to these articles, creationists and their ilk are just blowing smoke. The importance of Tiktaalik has nothing to do with proving the fish-tetrapod transition. That's pretty much taken care of by a wealth of data from the past 100 years.
I would support this with a longer statement or references, but creationists are kindly providing the background for this fact by talking about Acanthostega and Ichthyostega as though we used to believe that were transitional forms and now somehow don't. All these clowns are doing is neatly summarizing growing list of "transitional forms" and making themselves look like asses in the process. These taxa are all still very much there, playing a critical role at the forefront of these reports on Tiktaalik:
From Daeschler et al. 2006
Most of the features used to support this grouping, however, are also seen in early tetrapods such as Acanthostega, Ichthyostega and Ventastega.
Tiktaalik retains primitive tetrapodomorph features such as dorsal scale cover, paired fins with lepidotrichia, a generalized lower jaw, and separated entopterygoids in the palate, but also possesses a number of derived features of the skull, pectoral girdle and fin, and ribs that are shared with stem tetrapods such as Acanthostega and Ichthyostega.
From Shubin et al. 2006
The glenoid [shoulder joint] is oriented posteroventrolaterally and partially exposed in lateral view, which is intermediate between the posterior orientation of the glenoid in Eusthenopteron and the lateral orientation of Acanthostega and other basal tetrapods
In both Acanthostega and Tiktaalik the appendage projects ventrolaterally from the body wall.
The endochondral bones [internal bones] of the pectoral fin of Tiktaalik combine features of Eusthenopteron and Acanthostega, and in some aspects are intermediate.
Need I continue?
If these folks had actually read the papers, they would've seen how comparisons with these early tetrapods figure so prominently in how we actually recognize what Tiktaalik is.
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Saturday, April 08, 2006
These fish are not neccesarily intermediates, explain Discovery Institute scientists I queried about the find. Tiktaalik roseae is one of a set of lobe-finned fishes that include very curious mosaics--these fishes have advanced fully formed characteristics of several different groups. They are not intermediates in the sense that have half-fish/half-tetrapod characteristics. Rather, they have a combination of tetrapod-like features and fish-like features. Paleontologists refer to such organisms as mosaics rather than intermediates.I wonder if they could explain how an animal with tetrapod-like features and fish-like features is not an intermediate. This is followed by
What is clear is that forms like Tiktaalik are a melange of primitive and more developed features.My word, this circus act just keeps getting better!
According to DI Fellows a number of these fishes—Ichthyostega, Elpistostege, Panderichthys—have been hailed in the past as the “missing link.” Maybe one is a missing link; maybe none are.Once again, these guys have rather nicely pointed out their own ignorance and a few extra transitional forms, to boot! Nobody hails anything as "the missing link". "Missing link" is a term that scientists don't use and we even try to ask that the media does not use the term, but they do anyway. We can't stop them.
It was nice to see that they tossed in a mention of Elpistostege. As you can see, Elpistostege is a lot like Tiktaalik, which is why Tiktaalik is dubbed an "elpistostegalian". Elpistostege is known only very incomplete remains, the limbs have been entirely unknown. However, it's tetrapod affinities have been recognized since its discovery in 1938, when T.S. Westoll actually called it the earliest known tetrapod! In 1985, Hans-Peter Schultze and Marius Arsenault recognized it for what it was, a very tetrapod-like fish, similar to Panderichthys. In 1996, Schultze described a short pice of the trunk that has rhomboid-shaped schales and a few vertebrae. What little is known of its skull and trunk of Elpistostege is scarcely different from Tiktaalik. However, after decades of searching no new data on Elpistostege ever came to light. Shubin and Daeschler wanted more of this animal, but knew that the original locality wasn't going to give up its secrets. So, they went looking in similar-aged rocks elsewhere in the world. Their discovery of Tiktaalik essentially confirms the prediction of what we have long thought Elpistostege to be (and shows that Westoll was not too far off!)
Again, like AiG's response, the DI is simply responding to the media and not the original material itself.
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Friday, April 07, 2006
It's sort of amusing and sad, all at the same time. This is no surprise, as the response is co-authored by Dr. David Menton, a captial clown. He and co-author Mark Looy have produced a terrible mess: all their basic facts are wrong, dead-wrong. There is the clear impression that they have not even looked at the original reports (a point further evidenced by the fact that they cite only one of the two back-to-back articles published in the same issue).
Most of the article discusses the report in the NY Times rather than the original reports in Nature. This has the clear implication that the authors are more interested in the public's perception of the matter and have no scientific interest in the actual fossil material itself. As if that needed to be said. It is another manifistation of that odd species of thought that is creationist solipsism.
Their opening shot is the fact that the discoverers, when interviewed, used tentative wording to describe their interpretations. This is a rather alien concept for creationists: the idea of proceeding cautiously towards conclusions, rather than brazenly starting with immutable revealed Truth.
Here's the pinnacle of dishonesty, though. Menton and Looy write: "[the use of tentative language implies that] the find is not as firm as evolutionists would lead you to believe". You only wish. I'm not sure how the use of tentative language could result in Daeschler and Shubin overblowing the implications of their discoveries. Creationists are no strangers to contradiction.
But as I said, they've got their facts dead-wrong, to the point of outright dishonesty. So, let me back that up. Here's the breakdown (break-down?) of some of their thoughts. I've taken some bits out of order.
There is the coelacanth fish, found in the same geological system (Devonian it is called) as this Tiktaalik discovery, that also has lobed fins. These lobed fins were once thought to enable the coelacanth to walk on the ocean floorThanks for that careful bit of analysis there, guys. You should get a prize for that. Sorry that we didn't circulate a memo beforehand to point out that tetrapods are widely regarded to have desceded from lobe-finned fishes for maybe the past 50 years. Somehow, I think it might be odd if a bona fide fish-tetrapod intermediate were not allied with the lobe-finned fishes. If it had been anything else, it would have been a problem! But I'm glad these gentlement have conceded the point on behalf of AiG.
However, this business with the coelacanth is a pretty nasty equivocation: 'coelacanths are lobe-fins, Tiktaalik is a lobe-fin'. Hmm... I'm not sure if that really captures all the details, so perhaps a little comparison might help us evaluate this claim:
The authors' cluelessness takes them to previously unexplored depths of hoplessly impotent arguments. But these intrepid crusaders in the name of ignorance soldier on!
(in fact it was, like “Tiklaalik,” once considered by evolutionists to be a type of transitional form). Later, it was determined that the coelacanth fins were used for better maneuvering through the water, and not for walking.These guys have their own wild and weird theory of evolution that has no basis in reality. Coelacanths are still considered a plesion (branch) on the tetrapod tree. Coelacanths, as demonstrated by a large amount of molecular and morphological data, are closely related to tetrapods. What happened was, J.L.B. Smith who was an ichthyologist and not a palaeontologist or evolutionary biologist, took the accepted relationships of coelacanths and tetrapods to infer that coelacanths might walk around on the bottom. He was shown to be wrong. Science moved on. This is quite different from how the authors of the Tiktaalik papers reached their conclusions.
Please consider for a moment the fin of an extant (i.e. living) coelacanth, on the left; and the fin skeleton of Tiktaalik on the right.
The fin skeleton of Tiktaalik is much more robust and has a number of prominent ridges and scars for muscle insertions. The joints have surfaces that permit a range of motion in the fin elements suggesting flexibility and even elbow joint movement. The shoulder bones (not shown -- I'll try to get an image up soon) are like a tetrapod's: it has a robust, plate like region where the limb attaches. It has grooves and passages for more muscle attachments and suggests that there was a very sturdy shoulder region. All of these are features that set Tiktaalik apart from a coelacanth or any other lobe-finned fish and ally it closely with tetrapods. Moreover, the distinct structure of the two types of fin implies radically different functions, nonetheless. Whatever functional argument you want to make for that is immaterial, the fact remains that the equivocation of Tiktaalik's fin skeleton with that of a modern coelacanths is pretty absurd. But you guys looked pretty funny doing it! Nice try!
This hopelessly optimistic appraisal kind of made me feel sad on the inside:
The new creature uncovered in the Arctic might be something similar.If only they knew how wrong they were, 'maybe, just maybe the editors of Nature were having a slow newsday and decided to publish two back-to-back articles on another generic lobe-finned fish!'. I hate to break your heart, Tiny Tim, but it ain't similar to a coelacanth -- that's why it's newsworthy!
After completely embarassing themselves in the domain of diversity, they move on to anatomy, the area where Menton is supposed to be an expert. In the process, they manage to discover the nexus of pathetic and hilarious:
[T]he bones for Panderichthys, Tiktaalik and the coelacanth are imbedded in the muscle, and are not attached to the axial skeleton, which you would find in a reptile or amphibian (and which would be necessary for weight-bearing appendages). [emphasis added - MB]Presumably this is about how the shoulder girdle lacks a direct structural connection to the vertebral column. In fishes, the front fins attach to the shoulder girdle which is embedded in the tissues and muscles of the body. Well, guess what? It's exactly the same way in almost all primitive tetrapods, including Ichthyostega and Acanthostega, the taxa widely held to be the next plesions up from Tiktaalik. In fact, this is general for most tetrapods, where a bony structural link to the ribs or vertebral column is the exception, not the rule. Do we laugh or cry here? I'm not sure.
On the other hand, if they're talking about the pelvic limbs, then Menton and Looy are just blowing smoke, because there is no report on the pelvic girdle here.
Also, there are other creatures (e.g., the Panderichthys) that are thought to be fish and yet appear to be similar in lobe and fin structure to Tiktaalik.Panderichthys is widely regarded as a close relative of tetrapods and it is only one node below Tiktaalik in the evolutionary tree presented by the authors of the Tiktaalik papers. It would, in fact, be somewhat of a problem if these two animals were not similar. Thanks for pointing this out, guys! It's amazing that these are the same people who say there aren't any transitional forms, but they've nicely admitted that both Panderichthys and Tiktaalik are transitional.
As we often state on this website, keep in mind that evolutionists and creationists have the same facts (e.g., fossils), but interpret the facts uncovered today differently in regard to the past.Well, apparently you don't have all the same facts. According to your "facts", the shoulder girdles of tetrapods generally have a bony connection to the vertebral column. Apparently your "facts" did not include the facts of tetrapod anatomy and diversity at all. Your "facts" are convenient selections of observations, untruths, and equivocation of terms like "lobe-fin". These are not facts at all, but the framework of a deceptive fantasy that helps insulate you from reality.
Because evolutionists want to discover transitional forms, when they find a very old fish with leg-bone-like bones in its fins, they want to interpret this as evidence that it is some sort of transitional creature. However, other fish seem to have the same sort of structure as stated above, and these bones are not constructed as one would expect for weight-bearing legs.So, there we have it again, further evidence that the authors have not read the paper. What appears to be more important than reality to these men is the term "lobe-fin", a term of convenience used to describe a general condition for a certain type of fish: the sarcopterygians. However, fin morphology is radically diverse in this group, and there are many ways of being a "lobe-finned fish". To deny the tetrapod-like nature of the fins of Tiktaalik on the basis of the fact that it happens to be a lobe-finned fish as well as is a coelacanth is a shameful advertisement of one's ignorance.
This is my favourite:
For the moment, we can confidently state that evolutionists have no examples of mutations or evolutionary processes that can lead to an increase in genetic information in a creature that would, for example, develop the appendage of a land animal from the fin of a fishHere are two men, claiming that they will be able to provide a scientific opinion on this discovery, and they haven't the foggiest clue about anything they're talking about. One of the most remarkable things about limb and fin development is the extraordinary conservation of molecular genetic mechanisms. For the most part, fins and limbs employ exactly the same set of genes to make their structure. The main difference is the timing, level, and position of the deployment of these genes. Until AiG specifies exactly what they mean by "new information" whenever they say this, it'll be uncertain as to what "new genetic information" is that they're talking about. Presumably, they've somehow measured "information" in fishes and tetrapods and found that tetrapods had "novel information"? I thought not.
The interesting thing about all this is that the Menton and Looy are simply pointing out that Tiktaalik has attributes of a fish, but doing nothing to dispute the observed similarities with tetrapods. Has it ever dawned on them that an animal somewhere between fishes and tetrapods might actually have some attributes of a fish? What makes me wonder is why AiG didn't post a picture of the specimen. There are, by now, tons of pictures on media websites all over the place. A Google image search for "tiktaalik" turns out four pages of results. Here's why: they're scared, deathly scared. The implications of Tiktaalik are so bloody obvious that they have a lot of work to do in order to deal with this one.
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Thursday, April 06, 2006
However, I did find ´this. It's a Q&A from John Morris at the Institute for Creation "Research", answering the question "what's a missing link?"
With almost prophetic timeliness, Morris gives the following example:
If some type of fish evolved into some type of amphibian, there should have been distinct steps along the way of 90% fish/10% amphibian; then 80% fish/20% amphibian; etc., leading to the 100% amphibians we have today.
does this count:
a, Left lateral view; b, dorsal view with enlargement of scales; and c, ventral view with enlargement of anterior ribs. See Fig. 3 for labelled drawing of skull in dorsal view. Abbreviations: an, anocleithrum; bb, basibranchial; co, coracoid; clav, clavicle; clth, cleithrum; cbr, ceratobranchial; ent, entopterygoid; hu, humerus; lep, lepidotrichia; mand, mandible; nar, naris; or, orbit; psp, parasphenoid; ra, radius; suc, supracleithrum; ul, ulna; uln, ulnare. Scale bar equals 5 cm. From Daeschler et al. 2006?
Hee hee, just wanted to do that one more time.
Apart from that, however, Morris is still making a ridiculous caricature of evolution that's couched in his own typological thinking. In other words, as opposed to recognizing that there are probably about 35,000 species of things we call "fish", all of them quite different from one another in many ways, he thinks there's a "fish kind" and an "amphibian kind" and one should morph into the other, with something that's an "average of all fish" and an "average of amphibians". The reality is that some lineages of fish, very distinct in their own right, ought to acquire progressively more tetrapod-like features, as we see presented in an animal like Tiktaalik.
So, I'll wrap it up with this little prediction from Dr. John
Creation says they never existed, and agree that we have no record of them.
Well, does that settle it then?
Somehow, I think I can hear the sounds of distant goalposts scraping...
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Wednesday, April 05, 2006
a, Left lateral view; b, dorsal view with enlargement of scales; and c, ventral view with enlargement of anterior ribs. See Fig. 3 for labelled drawing of skull in dorsal view. Abbreviations: an, anocleithrum; bb, basibranchial; co, coracoid; clav, clavicle; clth, cleithrum; cbr, ceratobranchial; ent, entopterygoid; hu, humerus; lep, lepidotrichia; mand, mandible; nar, naris; or, orbit; psp, parasphenoid; ra, radius; suc, supracleithrum; ul, ulna; uln, ulnare. Scale bar equals 5 cm. From Daeschler et al. 2006
This is Tiktaalik roseae, a lobe-finned fish from the Late Devonian Fram Formation of Arctic Canada. The key features that make this animal a lobe-finned fish are the limbs which clearly have a humerus that branches out to a radius and ulna, like our own limbs. But Tiktaalik is clearly different. It has foregone a lot of the other lobe-finned fish conditions for characteristics that are much more like a tetrapod. In fact, Tiktaalik is without question the most tetrapod-like sarcopterygian known to date, and it fills an important gap in the fossil record. Clearly, it's can no longer be safely jammed into that "fish" category.
Here's the fin skeleton and a picture of one of the specimens to go with. The important thing to note is that the dashed lines are not inferred elements. There are several specimens of the fin and shoulder skeleton (I've seen them personally), and the dahsed lines represent gaps that are filled by an overlapping specimen. The authors were able to study the specimens in articulation as well as prepare them 'in the round'
a, Dorsal view; b, ventral view. Elements with stipple shading were preserved in articulation in NUFV 109 and prepared in the round. Elements with a dashed outline are reconstructed based on their presence in the articulated distal fin of NUFV 110. It is not known how many radials lie distal to the first, second and fourth in the proximal series. Note the dorsal expansion of the distal articular facets on the ulnare and third distal radial/mesomere. The dorsal expansion of these facets would have facilitated extension of the distal fin.
a, Stereo pair of left pectoral fin of NUFV 108 in dorsal view showing disparity in size and position of anterior (alp) and posterior (plp) unjointed lepidotrichia and the relative position of dermal girdle elements. b, Right pectoral fin of NUFV 110 in anterior view showing preservation of anterior lepidotrichia (alp), clavicle (cl), scales (sc) and endochondral bones in articulation (H, humerus; U, ulna; u, ulnare; r, radials). The anterior lepidotrichia terminate at the elbow, thus allowing a full range of flexion at that joint. c, Right pectoral fin of NUFV 110 in ventral view showing positions of coracoid (co) and endochondral and dermal fin elements. an, anocleithrum; cb, ceratobranchial; clth, cleithrum; int, intermedium; ri, rib; suc, supracleithrum. Both images from Shubin et al. 2006.
Now, recall the story I posted a few days ago. Well, this new material is the result of that expedition that went up to the arctic looking for this type of stuff. Earlier, I posted this image which was from a book published in 1997, documenting the gap in the fossil record between fishes and tetrapods.
The authors put the fish into the context of the following phylogeny, or 'evolutionary tree', using a computer-based analysis.
Tiktaalik is the sister group of Acanthostega + Ichthyostega in one of the two most parsimonious trees, and clades with Elpistostege as sister to the tetrapods in the other. Tree length = 149, consistency index = 0.8389, consistency index excluding uninformative characters = 0.7966, retention index = 0.8140, and rescaled consistency index = 0.6828. The characters list and data matrix are available as Supplementary Information. From Daeschler et al. 2006As you can see, Tiktaalik fits squarely in the gap between Panderichthys and other tetrapods -- a precise confirmation of the predictions made by the existing theory and geological knowledge.
The important point here is to ask why this animal fits there. Isn't this somewhat arbitrary? Well, no. We already know a lot about what sets tetrapods and fishes apart. Moreover, we've got some really important details from Tiktaalik that allows the authors to place it in the tree where it is.
The skull is remarkably tetrapod like. Compare here and here. The resemblances are striking. Most notably, is the lengthened snout of Tiktaalik, where even in very tetrapod-like fishes such as Panderichthys, the snout is relatively short and the skull roof behind the eyes is longer. The conditions are opposite in Tiktaalik and early tetrapods.
Tetrapods all lack a bony gill cover, while primitive ones (and modern amphibians) retain gills but no bony gill cover. The result is a moveable neck. Unlike even the most tetrapod-like fishes known previously, Tiktaalik has a neck and no bony gill cover. The propotions and shape of the head are a lot more like Acanthostega or other early tetrapod than like any known fish, even (in my opinion), Panderichthys.
Another interesting aspect of both Tiktaalik and Panderichthys is their shoulder girdle. What's great about Tiktaalik however, is that we have more, better, and more accessible specimens. The shoulders bone where the limbs attach, is rather puny in most fishes. On the other hand, tetrapods have a rather beefy affair that really helps support the weight of the animal. Tiktaalik shows something much more like the tetrapod condition, with a very beefy shoulder bone.
The fin, as illustrated above, is quite interesting. Not only is it fragmented into a bunch of little bones called radials (which are strikingly similar to digits), but there are apparently moveable joints between the units of the fins. When looked at closely, there are evident "roller surfaces" on the bones that permit the neighboring elements to flex, sort of like a primitive wrist.
The ear region shows a marked advance on the condition myself and Per Ahlberg described for a similar fish, Panderichthys. Unlike this latter form, however, Tiktaalik has a much broader spiracle and an even shorter hyomandibular bone -- the "gill arch" bone that eventually becomes the "stirrup" bone of the middle ear. In the earliest tetrapods, this bone looks neither like a fish nor like that of modern tetrapods. It's a short nubbin of bone that pokes out from the side of the braincase. The hyomandibula of Tiktaalik, if we can call it that, was a stubby little nubbin, too. However, it was still oriented in the spiracle like a hyomandibular bone.
More news will certainly come about this interesting animal. The publications in Nature are generally only preliminary reports and a much more complete description awaits. It is usually during this process that the really interesting comparisons come out and we find out how similar/different Tiktaalik is when compared with tetrapods and other lobe-finned fishes.
Tiktaalik presents a sort of classic 'transitional form', y'know the kind that creationists say doesn't exist. I can't wait to see how they're going to spin this. I wonder if it will be "just a fish" or if it will attack the integrity of the authors. We'll see. In the meantime, science marches on.
EDIT: Pharyngula has it too, with more pics.
Ahlberg, P.E. and Clack, J.A. 2006. Palaeontology: A firm step from water to land. Nature 440: 747-749.<link>
Daeschler, E.B. et al. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440: 757-763. <link>
Shubin, N.H. et al. 2006. The pectoral fin of Tiktaalik rosae and the origin of the tetrapod limb. Nature 440:764-771. <link>
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A good friend of mine from back home in Ottawa sends me the story.
I'm about to go ballistic!
The Social Sciences and Humanities Research Council (SSHRC) denied funding McGill professor Brian Alters on the following grounds:
"The committee found that the candidates were qualified. However, it judged the proposal did not adequately substantiate the premise that the popularizing of Intelligent Design Theory had detrimental effects on Canadian students, teachers, parents and policymakers. Nor did the committee consider that there was adequate justification for the assumption in the proposal that the theory of Evolution, and not Intelligent Design theory, was correct. It was not convinced, therefore, that research based on these assumptions would yield objective results. In addition, the committee found that the research plans were insufficiently elaborated to allow for an informed evaluation of their merit. In view of its reservations the committee recommended that no award be made." [my emphasis]
It appears there are other grounds for the decision, but the very fact that the emphasized claim was made makes the whole thing stink of "creationist agenda". Never does a researcher have to justify an "assumption" that is, in fact, a recapitulation of the prevailing scientific opinion. Let's even give them the benefit of the doubt: I'll go so far as to pretend that evolution is wrong. It is still the established scientific principle and Prof. Alters doesn't have to justify it any more than we have to cite "Darwin (1859)" every time we want to talk about evolution!
The article quotes Janet Halliwell (a chemist, and SSHRC's executive vice-president) who justified the decision:
Ms. Halliwell added there are phenomena that "may not be easily explained by current theories of evolution," and the scientific world's understanding of life "is not static. There's an evolution in the theory of evolution."Sounds like we have our classic "closet creationist" who uses the fact that science progresses as evidence that current science may be bogus. How did this person ever get a chemistry degree? Should we pan applications that make presumptive appeals to atomic theory simply because we 'don't know everything about the structure of an atom' and that 'there's an evolution in the theory of the atom'? Of course not, because we don't think atoms conflict with our previously conceived viewpoint.
This is a disgrace to academia in Canada. It's ironic that we pronounce the acronym "SSHRC" as "shirk", isn't it?
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Tuesday, April 04, 2006
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