bird-mammal common ancestor

[matt spitzer]

In article <mturner.276.00145F21 at acpub.duke.edu>, mturner at acpub.duke.edu
(mel turner) wrote:
> it helps to keep in mind that all of these branching, 
> diverging lineages have been evolving along their own paths  for exactly the 
> same amount of time and it is unlikely that any one of them would  remain 
> primitive in all respects.  ("primitive" =  the characteristics of  the  msot 
> recent common ancestor of a group).   

OK, with this in mind, how does one determine whether a similar brain
structure in animals as distantly related as mammals and birds is
evolutionarily homologous vs. an independently evolved analog?  Fossils are
of no use in this case.