bird-mammal common ancestor

mel turner mturner at acpub.duke.edu
Mon Sep 12 18:00:36 EST 1994


In article <mwspitze-110994201418 at phar2.medsurge.hsis.uci.edu> mwspitze at uci.edu (matt spitzer) writes:

>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.  

Well, many fossil skulls give endocasts that do say something useful about 
brain structure.   I think quite a bit is known about the brains of several 
fossil groups.

 A better answer is to use the related-outgroup comparison method:  
crocodilians are the closest living relatives of birds,  what are their brains 
like?  Lizards, snakes, and tuataras are the next branch down; what are their 
brains like?  What about turtles?  Common patterns among these can be used 
to extrapolate the conditions that were present in the common ancestor of that 
whole big branch of the amniotes.    They can be compared also with amphibian 
brains to get a handle on primitive characteristics of the amniotes.  (the 
basic  logic: those characteristics found in  some members of a variable group 
that are also found in  related outgroups  are the primitive conditions for 
the variable group).   Of course one always has to decide whether the 
similarities  among groups are likely to be homologous-- whether they are 
really similar in detail.

mdt



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