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Re: cladistics question



In a message dated 7/21/98 7:52:54 PM Eastern Daylight Time,
jdaniel@aristotle.net writes:

<< One question that I have had with cladistics for a long time was the
 time factor.  As an example, the ceratosaurs at least used to be
 considered "basal" or "primitive" theropods, right?  Yet they didn't
 appear until tens of millions of years after the earlier more derived
 theropods, a huge expanse of time in which a great deal of evolution
 could and probably did occur.  So how can they be considered primitive
 or basal.  >>

I'm not familiar with the details of the above example--so I can't speak to it
specifically--but the general point as to how cladistics deals with temporal
issues is not clear to me either.  Incomplete information about the relative
age of specimens could lead to misleading cladistic analyses--couldn't it? 

As an example: say you have (all else being equal) two taxa of the same age
with long, pointed ears and one taxa (again, all else being equal) of
*unknown* age with long, non-pointed ears.  Couldn't one interpretation of
this be that long non-pointed ears are the pleisiomorphic state and that
pointed ears are a synapomorphic character?  But, in the absence of relative
ages, couldn't another interpretation be that the long pointed ears were a
pleisiomorphic character of an unknown common ancestor and that the *absence
of pointedness* was the derived condition expressed only in a single taxa?

How do you deal with this in cladistic analysis?  Forgive me my ignorance in
this area, but there appear to be at least two of us that would benefit from a
little discussion of this. 

Thanks.