Are the Pectoral Fins of Tiktaalik Really Legs?
Before we get into
Tiktaalik’s “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 legs—just because we call our arms “legs” doesn’t make them legs. The same might be said of the bony fins of
Crossopterygian fish—we may call them “legs” but that doesn’t necessarily make them legs.
Shubin et al. make much of the claim that
Tiktaalik’s bony fins show a reduction in dermal bone and an increase in endochondral bone.
7 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
Tiktaalik’s fins supported its weight out of water, or that it was capable of a true walking motion. (It certainly doesn’t 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, farther 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 limb—with its robust pelvic girdle securely attached to the vertebral column—that 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 forelimbs 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 they’re
not even attached to the axial skeleton (vertebral column) and thus can’t bear weight on land.
While the endochondral bones in the pectoral fins of
Crossopterygians have some similarity to bones in the forelimbs 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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So Is Tiktaalik a Missing Link?
Finally, what about the popular claim that
Tiktaalik is the “missing link” between fish and tetrapods?
In their review article on
Tiktaalik, Ahlberg and Clack tell us that “the concept of ‘missing links’ has a powerful grasp on the imagination: the rare transitional fossils that apparently capture the origins of major groups of organisms are uniquely evocative.” The authors concede that the whole concept of “missing links” has been loaded with “unfounded notions of evolutionary ‘progress’ and with a mistaken emphasis on the single intermediate fossil as the key to understanding evolutionary transition.”
Sadly, “unfounded notions” of this kind continue to be uncritically taught and accepted in the popular media and in our schools. Even more sadly, these unfounded notions have been used to undermine the authority of Holy Scripture.