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Re: Some thoughts on AVES



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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