Tetrapoda (and the new *tetrapod* genus _Tiktaalik_)
kinman2 at YAHOO.COM
Sun Apr 9 22:00:24 CDT 2006
I have been thinking about this all weekend, and the transition from lobe-finned fish to tetrapods is a relatively gradual development in many areas of the skull and also the forelimbs. Some of these changes could be quantified to draw a line between them, but I think the primary division should be something which occurred relatively quickly (in evolutionary terms). Frankly, it is no surprise that those who described Tiktaalik refer to it as a fish or a "fishapod", but I have to disagree with this characterization. We need a clear definitional (synapomorphic) line between fish and tetrapods, and waffling on the issue (with a "fishapod" label) does not inspire confidence or stability.
I propose that the loss of the bony gill cover is just such a (relatively) quickly evolved characteristic, AND the fact that it is functionally associated with the origins of a mobile neck is just icing on the cake!!! Therefore, Panderichthys would remain in the sarcopterygian Order Osteolepiformes, and Tiktaalik (which has lost the bony gill cover) would be classified as a primitive tetrapod. I am still not sure if we know for sure whether Elpistostege had a bony gill cover or not. It certainly resembles Tiktaalik in the expansion of bones in the snout, but are they really sister groups? Therefore, Elpistostege is the one genus which I am not sure which side of the line to put it.
Everything else can be easily placed by possession or loss of the bony gill cover. Other associated (but gradual) characteristics can be quantified to show their more gradual evolution before and after this event. This event apparently occurred well before tetrapods could spend any appreciable time on land, and walking along shallow water bottoms was the main activity of the earliest tetrapods (including Acanthostega). It is becoming increasingly clear that the origin of Tetrapoda, as well as the slightly later tetrapods with real independent digits, preceded the true invasion of land. Hopefully the inclusion of Tikaalik in Tetrapoda will make it even more abundantly clear to everyone that tetrapody came first, and that the complete weight-bearing adaptations for land invasion followed later.
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