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Re: Paradoxically temporal



At 10:04 AM 7/23/98 -0400, Thomas R. Holtz, Jr. wrote:

[snip-a-dee-doo-dah]

>Ah.  I think I see the confusion here.  A species is, to me, a very precise
>(indeed, specific) thing.  It's First Appearance Datum (FAD) and Last
>Appearance Datum (LAD) give the whole of the known range of that species.
>(In _Eoraptor_, as in most dinosaurs, the FAD and the LAD are the same
>thing, as currently known).  _Eoraptor lunensis_ may indeed extend into the
>early Late Triassic, the Middle Triassic, or earlier, but without direct
>fossil evidence, I cannot demonstrate that *this species* was found then.
>
>Having accepted descent with modification, and operating from a given
>phylogenetic scheme or schemes, I can see if the *lineage* containing
>_Eoraptor lunesis_ extends back to a certain point.

[snip-a-dee-ay]

>As in most cases, it is better to err
>on the side of caution, and not assume that specimens diagnosible as _Y
>species_ are any older than the currently known FAD until such time as they
>are found.

[my-oh-my-what-a-snip-a-dee-day]

   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?

   I think I understand this viewpoint and why.  I think I now also better
appreciate Horner's views in _Dinosaur Lives_, although I think he was
dealing more with the "phylogenetic species concept" as, if I remember
correctly, his ceratopian specimens did fall "properly" in the correct
temporal sequence.

   This does, I hate to say, bring to mind an argument I saw from our
bestest buddy "Cal King" before I stopped reading his posts.  He argued
that MRCA (a node on a cladogram) was a flawed concept because there was no
direct fossil evidence of this animal.  However, this "mystery animal" is
accepted by "cladists" (for lack of a better term) as having existed, even
though there's no fossil evidence.  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.


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