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Re: Polish find in Nature



Erik Boehm <erikboehm07@yahoo.com> wrote:


> 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.


It is possible that those 'fishapods' with better developed limbs moved out of 
the niche occupied by their ancestors and sister taxa with less developed 
limbs.  _Tiktaalik_ would therefore be a 'relic' taxon for its time.


In a similar fashion, microraptorines existed alongside fully-flighted birds in 
the Early Cretaceous.  There is no reason why flying birds would rapidly 
replace microraptorines - though they might have eventually.  It's not clear 
why microraptorines went extinct.


> I assume that transitioning back and forth was a lot easier
> when the transition to land was first occurring:


I think this kind of 'zig-zagging' is quite common in (what we perceive to be) 
major evolutionary transformations.  We see it in the early evolution of birds 
too.  Across the spectrum of known Early Cretaceous birds, the distribution of 
'advanced' flight characters (i.e., those seen in modern birds, and associated 
with avian flight) is quite uneven.  


In the early evolution of tetrapods, as in the early evolution of birds, the 
development of novel locomotory abilities (terrestrial in tetrapods, aerial in 
birds) didn't proceed as a steady march onward and upward, but was instead 
accompanied by a great deal of experimentation.



Cheers

Tim