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Ancestors... (was Re: Stratigraphy, biogeography & cladograms)



At 01:59 PM 1/19/99 -0800, Betty Cunningham wrote:

>>In cladistics one doesn't search for ancestors, but for sister taxa.
><snip>
>> HOWEVER, under phylogenetic taxonomy, "modern birds" (Gauthier's Aves, most
>> people's Neornithes) has been defined as all descendants of the most recent
>> common ancestors of paleognaths and neognaths
>
>If you can define descendants, aren't you be default define ancestors?

Please reread the first sentence in the quote.  "Search" is the key word.

Recognizing that the chance of recovery of a true ancestor (i.e. a member of
that population which gave rise to the later group) is *vanishingly* small,
it is more productive to search for those forms which are most likely to
have shared a more recent common ancestry with the group in question.

As noted in the part you snipped out, true ancestors COULD be found, and
COULD be recognized.  However, because the fossil record is so spotty
(particularly over the time lengths we are dealing with), that recognition
of a direct ancestor is problematic.  One could argue, for example, that
_Eobrontosaurus yahnipin_ is indeed the primitive sister taxon to
_Apatosaurus ajax_ and _Apatosaurus exclesus_.  That is something we could
discover from a phylogenetic analysis.

Demonstration that it is the direct ancestor of these taxa, however,
requires additional information, as previously noted (earlier stratigraphic
position, sharing some derived conditions with the later two but lacking its
own, etc.).  However, it remains problematic as to whether _Eobrontosaurus_
is indeed the direct ancestor, or whether it had already diverged from
another lineage which contained the true ancestral population of the
_Apatosaurus_ species.

Additionally, clade names should be defined (these are labels, after all).
One copuld then search for an ancestor for a particular clade, but they
should be discovered, not defined.

>> BTW, who is "Seino"?  The compsognathid _Sinosauropteryx_?  The
>> enantiornithine _Sinornis_?
>
>Seino is Sino spelled very late at night, and while distracted.
>
That still didn't answer the question: I'll presume it is _Sinosauropteryx_.

DinoGeorge added:
>
>Not only that, but the ancestors are paraphyletic taxa, since they don't
>include any of their descendant species (let alone all of them).
>
Yep. Darn straight.  Ancestors are paraphyletic.  Everyone agrees on that.

Jack Horner has a lot to say about ancestors these days, some of which is
discussed in "Dinosaur Lives".  He may have even convinced me to drop my
(hitherto still morphologically- and specific mate recognition system-based)
species concept altogether and like him recognize that a lot of taxonomy is
bass-ackwards: that lineages are the real thing, and species are the human
constructs.

(Which, interestingly, combines certain elements of Simpsonian and Hennigian
phylogenetics.)

Back to your regularly scheduled listserve.

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