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

Mike Taylor <mike@indexdata.com> wrote:

> Well, yes; the forelimbs of birds and bats are homologous;
> but the character "forelimbs modified into wings" is NOT homologous
> in thos 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.

Yes; sorry if my language came across as imprecise.  The wings of birds are 
bats are homologous, but only because they are forelimbs that share a common 
origin.  This is true of all vertebrate forelimbs.  However, the modification 
of the forelimbs for flight is certainly not homologous for birds and bats.  
Similarly, the forelimbs of whales and seals are homologous (by dint of being 
vertebrate forelimbs), but their modification for swimming is not.

The original context for this was the origin of integumentary structures, such 
as the feathers of theropods and the filaments of _Tianyulong_.  Unless both 
structures have a common origin from the same ancestral integumentary 
structure, they cannot be homologous.  This holds even if similar developmental 
genetic processes were involved in the formation of both structures.