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RE: Tawa hallae: everything you know about basal saurischians is wrong...

Mike Taylor wrote:

<Well, yes; the forelimbs of birds and bats are homologous; but the character 
"forelimbs modified into wings" is NOT homologous in those taxa. So although 
you're right that a character either is or not a homology with no intermediate 
states, we do need to be a bit careful about exactly what we're saying is the 
homologous thing -- in this case, the existence of the forelimbs, or their 
modification for flight.>

  Ah, an analogous function for nonhomologous structure deriving from an 
homologous system (of bones).

  Not only is the functional anatomy and behavioral use of the forelimb a host 
of derived features, they are overlapping complexes of chaarcters that are hard 
to fully disengage. The entire forelimb, shoulder, sternal complex, gastralia, 
pelvis portions of the hindlimb, dorsosacral, and caudal vertebrae are all 
involved in the avian flight apparatus, and appear to have been from very close 
to the beginning of flight or wing-assisted movement, or the first organ to be 
called a "wing" in dinosaurs. Bats, on the other hand, have a more 
fundamentally involved system as it incorporates more of the hind limb 
initially (apparently) while lacking many of the precursorial qualities that 
birds developed from in regards to things like the vertebrae, the pneumatic 
pelvic-driven gastric and forelimb-driven sternal/furcular pumps, etc.

  Indeed, Gauthier's "forelimbs modifed for flight" feature that defined 
Avialae in the beginning would almost certainly have to be abandoned due to 
such a mess of applicability and comparability, not to mention the hundreds of 
distinct traits involved in birds alone.


Jaime A. Headden

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