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Re: Polish find in Nature

How reliable is the trackway dating?
How reliable is the dating of tiktaalik?

It seems odd to me that tiktaalik would exist in the presence of fishapods with 
better developed limbs, but that may just be my inability to conceive of a 
niche that would favor the development of stubby leg like fins over "true" 
limbs or "true" fins.

Or is it possible that Tiktaalik is a "degenerate" tetrapod, in the process of 
returning to a more aquatic life style from more terrestrial ancestors?
I assume that transitioning back and forth was a lot easier when the transition 
to land was first occurring:
No special blood chemistry like whales have for oxygen storage, no special skin 
that they breath through (like amphibians - particularly a species of "lungless 
frog"): they'd still have gills, their limbs would still be very fin like, and 
their bodies would still be hydrodynamic.
Likewise the relative vacancy of land, and their muscular stubby fin-legs would 
allow them to go on land and (evolutionarily speaking) not be too far behind 
other terrestrial vertebrates, and of course they already had basic lungs.

The evidence for a tidal marine origin over a swampy freshwater origin is 
interesting - although I think that fits in well with extant tetrapods 
producing Urea (or Uric acid), whereas extant freshwater fish have lost that 
ability (or am I mistaken on this point?).
Wasn't part of Urea's original function to bring internal salt concentration 
closer to that of the sea?
Freshwater fish can simply let ammonia/nitrogen waste diffuse through their 
I had previously thought it was merely fortuitous that Sarcopterygians made it 
onto land where Urea production was beneficial, before loosing the ability to 
make urea in favor of the simpler system of diffusion of Ammonia waste through 
the gills in freshwater fish.

--- On Wed, 1/6/10, Dawid Mazurek <dawidmazurek@wp.pl> wrote:

> From: Dawid Mazurek <dawidmazurek@wp.pl>
> Subject: Polish find in Nature
> To: dinosaur@usc.edu
> Date: Wednesday, January 6, 2010, 11:51 AM
> Check out th
latest issue cover.
> There's of course a paper inside. Plus 
> an accompanying movie: 
> http://www.nature.com/nature/videoarchive/tetrapods/index.html
> DM
> ----------------------------------------------------
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