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Re: Paradoxically temporal



At 05:21 PM 7/23/98 -0400, Jeff Poling wrote:

>   In other words, you can hypothesize a lineage back to a MRCA between two
>species, but cannot hypothesize that B evolved from A, if both have
>concurrent FDA, because there's no guarantee that either A or B, or both,
>hasn't also changed in the interim?  And even if A appears before B the
>ancester/descendant hypothesis is still problematic for exactly the same
>reason?

Actually, if A is the purported ancestor, and does occur prior to B, it
might very well be the direct ancestor (and could be accepted,
provisionally, as such).

>Yet "cladists" are not willing to
>accept the hypothesis that one species begat enother because of a lack of
>fossil evidence.  I'm not sure I see the distinction.

I've got an example I hope will help.

Say (for point of argument) that Archaeopteryx and other pre-Late Cretaceous
birds weren't known, but that we established that birds and dromaeosaurids
were sister taxa.  Dromaeosaurids are too specialized to be direct ancestors
of birds.

We could (and should) speculate that the bird line extends back to the FAD
of dromaeosaurids.  However, we would be overstepping the evidence by saying
that _Ichthyornis dispar_ (the most primitive bird in this hypothetical
scenario) went that far back in time.  Without direct fossil evidence, we
cannot in fairness tell where in geologic time the bird line acquired the
species-distinctive characteristics of _Icthyornis dispar_, even if we can
be confident that Avialae did indeed extend that far back.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661