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Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE



>Feduccia is looking at the same evidence and coming to differing 
>interpretations, which is probably the biggest part of science :-)  
>Looking at, and interpreting the evidence differently is how Tarsitano 
>thinks _Megalancosaurus_ is an avian ancestor, how Whetstone and Martin 
>find that crocs are closest to birds, and even how Brian Gardiner finds 
>birds to be most closely related to mammals.

I think one can be more critical of Feduccia than that.  A claim of
convergence is, in itself, a hypothesis explaining similarities among
character states, and as such must be parsimonious and falsifiable like any
other hypothesis.  In other words, there must be some testable reason
behind the claim.  In the pterosaur-bird case, for example, it is perfectly
fine to claim that a similarity is the result of convergence if you can
back up that claim by a reasonable functional interpretation of the
similarity (eg as a necessary part of a wing elevator complex).

What you cannot legitimately do is cite convergence as some sort of null
hypothesis (eg, by saying that any similarity is convergent until proven
otherwise, which is about on a level with saying that if you can't prove a
structure evolved then it must be the result of special creation).  I think
Feduccia falls into this trap.  When he cites convergence to explain
bird-dinosaur similarities he is not doing so because he thinks they lived
similar lifestyles - in fact he is at pains to argue that birds and
maniraptorians led very different sorts of lives.  He seems to be doing so
based on little more than the following:

a. birds cannot be dinosaur descendants
b. therefore any derived characters they share cannot be the result of
common descent
c. therefore these shared characters must be the result of convergence.

This is an entirely false train of logic, because (c) is required to
establish (a) and yet is derived on the assumption that (a) is true.  On
the other hand, he is quick to cite similarities between birds and a host
of, to my eye, far less birdlike creatures without considering that these,
in their turn, may be the result of convergence - an arguably more likely
case as creatures like Longisquama were probably arboreal and perhaps
glided or flew, so may have faced similar selective regimes.

I am not saying Feduccia is not a good scientist - he is, and has added
immeasurably to avian paleontology.  But on this issue I think he has lost
perspective (read the downright vituperative language in The Origin of
Birds when he discusses any alternate point of view), and, I think, is
misusing the concept of convergence to cling to his own ideas despite the
mounting mass of evidence casting them into doubt.
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net