[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
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
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
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
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.
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp