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

[Mike Taylor <mike@indexdata.com>]

2009/12/15 Tim Williams <tijawi@yahoo.com>:
>> IF they were homologous. Were they? And to what degree -
>> were they present in the LCA, or (like the wings of birds
>> and bats, which are homologous insofar as they are both
>> formed by the forelimbs) was only the underlying genetic
>> framework present but the actual trait realised
>> independently?
>
> Homology is an absolute concept.  There are never 'degrees' of homology.  If 
> the integumentary structures of theropods and heterodontosaurs arose 
> independently, they cannot be homologous.  Although bat wings and bird wings 
> evolved independently, they are both formed by the forelimbs, which were 
> present in their closest common ancestor; therefore the forelimbs (wings) are 
> homologous.

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.