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



> > 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.  

If we assume the integumentary structures (dinofuzz), arose as modification of 
scales, then they were homologous.
The question is, is the modification (of the homologous structure) homologous?

Bird wings and bat wings are homologous, in that they both come from an 
ancestral amniote forelimb.
The modification of that forelimb into a wing happened independently, so I 
would say the "modification" is not homologous, but the structures are.
After all, if we say bat and bird wings are homologous, then we must also say 
bird and ptero wings are homologous, and we must be very clear about what we 
mean, lest we imply that common ancestor of dinos and pteros was capable of 
flight/had wings.

So: 
Was the "fuzz" on a pterosaur derived from a structure homologues to the 
structure that dino-fuzz was derived from?
Is a different question in my mind to:
"Is pterosaur fuzz homologous to dino-fuzz"

In a similar manner, I once asked this list to what extent Hair and 
Feathers/Scales are homologous.
The keratin that makes them up is homologous, likely genes active in the region 
of the epidermis where hair/feathers originate are homologous, the question is, 
is the structure itself homologous?
Did amniotes have an integumentary structure that synapsids made into fur, and 
diapsids into feathers?
Or did these struc
logous proteins/genes?