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Re: Stratigraphy, biogeography & cladograms



At 11:54 AM 1/19/99 -0500, Thomas R. Holtz, Jr. wrote:
>Note that some sister taxa in an analysis might potentially be true
>ancestors (i.e., if they occur earlier in time than all of the sister taxa;
>that they share some derived features with the sister taxon; that they don't
>have autapomorphies (shared derived features of their own).

I think this last criterion is too strong.  Given the prevalence of
reversals in accepted phylogenies, a limited number of autapomorphies is
not really sufficient to rule out true ancestry.

[For instance, I am fairly certain that some population cluster within the
broad group often included within _Homo erectus_ is ancestral to humanity -
whether this ancestor is within  H. erectus sensu stricto, or within H.
rudolphensis or H. ergaster is a subtle, complex issue].

>  However, one
>cannot demonstrate that a fossil IS ancestral (that is, it is part of the
>population that gave rise to the later grou) rather than a primitive sister
>taxon.

This, however, is quite true.  The best one can do is say something is a
potential ancestor.

>  On the other hand, one can demonstrate that the fossil is NOT
>ancestral if you find a member of the more advanced clade at the same
>stratigraphic level or below.

This assumes that an ancestral species cannot survive past speciation.  It
is quite possible for an ancestor to be found next to a descendent unless
one uses a species concept that insists *both* sides of a speciation event
are new species.

--------------
May the peace of God be with you.         sarima@ix.netcom.com