Re: Some thoughts on AVES

["Tim Williams" <tijawi@hotmail.com>]

Ken Kinman wrote:

>     I whole-heartedly agree.  A movement toward more
> apomorphy(character)-based cladistic definitions is a cladisto-eclectic 
> compromise well worth pursuing.
>      Now we just need to do the same thing for Aves.  The question is, 
> just where among theropods is there a serial character transformation which 
> even approaches the clarity of the mammal distinction?

Nick Pharis wrote:

> Wrong.  Defining Mammalia based on auditory ossicles is no more arbitrary 
> than
> defining Aves based on _Archaeopteryx_ and _Passer_.  Why not use
> _Confuciusornis_?  Why not _Velociraptor_?

I have to agree with Nick here - and (once again) respectfully disagree with 
Ken.

One of the pitfalls of character-based definitions is incipient 
manifestations of such characters.  Even supposedly "clear-cut" characters 
do not pop up out of nowhere - like Barbara Eden in _I Dream of Jeanie_.  
The three-bone complex in the mammalian middle ear arose at the end of a 
gradual transition.  This is clear in (what are arbitrarily defined as) 
non-mammalian therapsids.  _Probainognathus_, for example, shows a double 
articulation between skull and jaw, with a tiny articular and quadrate, and 
with the latter already hooked up to the stapes in the middle ear.

It should be noted that this transition from jawbones --> auditory ossicles 
was probably as much morphological than functional - since in most modern 
reptiles these lower jaw bones actually transmit sound waves to the inner 
ear.

If this is what happened in the evolution of mammals, then it is a beautiful 
example of exaptation: the posterior jaw components had dual functions 
before becoming devoted to hearing.  I think an analogous situation occured 
during the evolution of avian flight - with the primitive function of 
predation retained alongside aerial flapping.



Tim



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