is tiktaalik a transitional form or just a fish?
tiktaalik is a fish according to answers in genesis...
Whatever else we might say about Tiktaalik, it is a fish. In a review article on Tiktaalik (appearing in the same issue of the scientific journal Nature that reported the discovery of Tiktaalik), fish evolution experts, Ahlberg and Clack concede that in some respects Tiktaalik and Panderichthys are straightforward fishes: they have small pelvic fins, retain fin rays in their paired appendages and have well-developed gill arches, suggesting that both animals remained mostly aquatic. 5
In other respects, however, Ahlberg and Clack argue that Tiktaalik is more tetrapod-like than Panderichthys because the bony gill cover has disappeared, and the skull has a longer snout. The authors weakly suggest that the significance of all this is that a longer snout suggests a shift from sucking towards snapping up prey, whereas the loss of gill cover bones probably correlates with reduced water flow through the gill chamber. The ribs also seem larger in Tiktaalik, which may mean it was better able to support its body out of water.
Without the authors evolutionary bias, of course, there is no reason to assume that Tiktaalik was anything other than exclusively aquatic. And how do we know that Tiktaalik lost its gill cover as opposed to never having one? The longer snout and lack of bony gill covers (found in many other exclusively-aquatic living fish) are interpreted as indicating a reduced flow of water through the gills, which, in turn, is declared to be suggestive of partial air-breathingbut this is quite a stretch. Finally, what does any of this have to do with fish evolving into land dwelling tetrapods?
Are the pectoral fins of Tiktaalik really legs?
Before we get into Tiktaaliks legs, it might be instructive to consider an old trick question. If we call our arms legs, then how many legs would we have? The answer, of course, is two legsjust because we call our arms legs doesnt make them legs. The same might be said of the bony fins of Crossopterygian fishwe may call them legs but that doesnt necessarily make them legs.
Shubin et al. make much of the claim that Tiktaaliks bony fins show a reduction in dermal bone and an increase in endochondral bone.6 This is important to them because the limb bones of tetrapods are entirely endochondral. They further claim that the cleithrum (a dermal bone to which the pectoral fin is attached in fish) is detached from the skull, resembling the position of the scapula (shoulder blade) of a tetrapod. They also claim that the endochondral bones of the fin are more similar to those of a tetrapod in terms of structure and range of motion. However, none of this, if true, proves that Tiktaaliks fins supported its weight out of water, or that it was capable of a true walking motion. (It certainly doesnt prove that these fish evolved into tetrapods.)
The limbs of tetrapods
The limbs of tetrapods share similar characteristic features. These unique features meet the special demands of walking on land. In the case of the forelimbs there is one bone nearest the body (proximal) called the humerus that articulates (flexibly joins) with two bones, the radius and ulna, further away from the body (distal). These in turn articulate with multiple wrist bones, which finally articulate with typically five digits. The hind limbs similarly consist of one proximal bone, the femur, which articulates with two distal bones, the tibia and fibula, which in turn articulate with ankle bones; and finally with typically five digits. In order to support the weight of the body on land, and permit walking, the most proximal bones of the limbs must be securely attached to the rest of the body. The humerus of the forelimb articulates with the pectoral girdle which includes the scapula (shoulder blade) and the clavicle (collar bone). The only bony attachment of the pectoral girdle to the body is the clavicle.
The femur of the hind limb articulates with the pelvic girdle, which consists of fused bones collectively called the pelvis (hip bone). It is this hind limbwith its robust pelvic girdle securely attached to the vertebral columnthat differs radically from that of any fish. (The tetrapod arrangement is important for bearing the weight of the animal on land.)
All tetrapod limb bones and their attachment girdles are endochondral bones. In the case of all fish, including Tiktaalik, the cleithrum and fin rays are dermal bones.
It is significant that the earliest true tetrapods recognized by evolutionists (such as Acanthostega and Ichthyostega) have all of the distinguishing features of tetrapod limbs (and their attachment girdles) and were clearly capable of walking and breathing on land. The structural differences between the tetrapod leg and the fish fin is easily understood when we realize that the buoyant density of water is about a thousand times greater than that of air. A fish has no need to support much of its weight in water where it is essentially weightless.
The fins of fish (including Tiktaalik)
Essentially all fish (including Tiktaalik) have small pelvic fins relative to their pectoral fins. The legs of tetrapods are just the opposite: the hind limbs attached to the pelvic girdle are almost always more robust than the fore limbs attached to the pectoral girdle. (This is particularly obvious in animals such as kangaroos and theropod dinosaurs.) Not only are the pelvic fins of all fish small, but theyre not even attached to the axial skeleton (vertebral column) and thus cant bear weight on land.
While the endochondral bones in the pectoral fins of Crossopterygians have some similarity to bones in the fore limbs of tetrapods, there are significant differences. For example, there is nothing even remotely comparable to the digits in any fish. The bony rays of fish fins are dermal bones that are not related in any way to digits in their structure, function or mode of development. Clearly, fin rays are relatively fragile and unsuitable for actual walking and weight bearing.
Even the smaller endochondral bones in the distal fin of Tiktaalik are not related to digits. Ahlberg and Clack point out that 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 rearranging.
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