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RE: Combined answer Archaeopteryx running...



Jim Cunningham wrote:

> Are you sure that the pygostylian body plan didn't limit them from some
> of the ecological niches available to the Archaeopteryx and Rahonavis 
> body plans?  I'm not saying that it did, only that the possibility 
> might be there, and should be considered.  

Oh, for sure.  For one thing, Archaeopteryx had a fully mobile and grasping
manus.  That must have been a wonderful instrument - for clasping prey,
grasping branches, scritching an itch...  I do believe that continued use of
the manus in grasping is entirely reconcilable with the early development of
the wing.  However, at a certain point (probably early in the Pygostylia) it
became advantageous to dedicate the manus and carpus to flight, so both were
absorbed into the wing.  (Though some early Tertiary birds retain a manual
claw on the wing...)


> My real point in all this is
> that it doesn't pay to arbitrarily close off which options and 
> possibilities should be considered.

Here's what I meant.  I think we both would agree that, on the whole, birds
such as _Archaeopteryx_, _Rahonavis_ and the North Korean bird ("Proornis")
had inferior flight capabilities compared to pygostylians.  Sure, there were
exceptions.  However, the improved flight abilities allowed pygostylians to
fly over longer distances.  This allowed them to achieve a worldwide
distribution very quickly.

Also, it is clear from the Cretaceous fossil record that pygostylians do
indeed outnumber non-pygostylian avialans by a fairly comfortable ratio.  I
think this says something about their relative adaptability.  I also believe
that it's reasonable to infer that the radiation of the Pygostylia was due
(at least in part) to the overhaul of the avian flight apparatus in the
divergence between "primitive" "_Archaeopteryx_-grade" birds and Pygostylia.

I'm not claiming that _Archaeopteryx_ and its morphologically conservative
ilk were inferior to pygostylians - any more than lungfish are inferior to
ray-finned fishes.  Sure, Archie had a very conservative anatomy compared to
modern birds; but the evidence (as I see it) points to Archie being a
specialized and versatile hunter in its own right.

Nevertheless - and excuse the "just-so"-ing on my part - one could build a
case that the advancements in the avian flight apparatus spurred on the
evolution of the pygostylian birds.  By contrast, the cumbersome flight
abilities of _Archaeopteryx_ (compared to pygostylians) precluded these
birds from forging certain novel niches - e.g. anything that involved aerial
hovering, marathon flights, or stationary take-offs.  


Tim


------------------------------------------------------------ 

Timothy J. Williams 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 3163