Tiktaalik to the rescue?
In the April 2006, issue of Nature, Daeschler, et al. reported the discovery of several fossilized specimens of a Crossopterygian fish named Tiktaalik roseae. These well preserved specimens were found in sedimentary layers of siltstonecross-bedded with sandstonesin Artic Canada.4
Like the other lobe-fin fish, Tiktaalik was declared to be late Devonian (between 385-359 million years old) by means of a dating method known as palynomorph biostratigraphy. This method presumes to date sedimentary rock layers on the basis of the assumed evolutionary age of pollen and spores contained in the rock. Most importantly, the discoverers of Tiktaalik claim that it represents an intermediate between fish with fins and tetrapods with limbs.
Tiktaalik is a fish
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.)
An interesting propaganda piece that is heavy on opinion and light on evidence.
No one contests that Tiktaalik was fish. What is important is that it has characteristics that are similar to that tetrapods.