Re: Tiktaalik

[Tim Williams <twilliams_alpha@hotmail.com>]

David Marjanovic wrote:

> I disagree with the whole idea of "macroevolution". The kind of 
> transformation I see involves a lot of rather small changes.

Yes, but over time these small changes accrue into very large changes.  In 
the case of the fish-amphibian or theropod-bird transformation, these 
changes ended up being profound.  The evolutionary pathway from basal 
sarcopterygians to tetrapods involved a huge change - especially in terms of 
breathing and locomotion.  By contrast, the transition from basal 
sarcopterygians to modern lungfish was more conservative.

Similarly, the evolution of birds resulted in the development of a whole new 
locomotor module, involving the wings and tail.

> It's just that some of those changes are rare (loss of gills, opercula, 
> gulars, fin rays, gain of digits...) and photogenic.

True, these guys are photogenic.  :-)  But I don't think the rarity of these 
changes is the critical factor here.  It's the consequences of the changes 
that are important.  In this case, the sarcopterygian can walk on land.

> Archie used to be a good example. It has become a quite bad one. Where's 
> the difference to *Microraptor* and *Graciliraptor* and 
> *Jinfengopteryx*...?

I think _Archaeopteryx_ is important because there is still a chance that it 
(or something very much like it) was directly ancestral to ornithothoracine 
birds.  I don't think we can say that for microraptorans, which were 
probably a dead-end.  Microraptorans may well represent a separate theropod 
experiment in aerial locomotion.  Although birds and microraptorans are 
almost certainly descended from a common ancestor that was adapted for some 
kind of aerial locomotion, it seems likely (to me) that the two built upon 
this template in very different ways.

Cheers

Tim