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