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Re: Arbitrary Paleontology and Monophyly

At 06:26 PM 6/11/99 -0400, you wrote:
>Jonathan Wagner wrote:
>>The odds that all dinosaurs had a common ancestor are probably
>>astronomically huge.

>The thing that puzzles me still is, if the odds were so loaded in favor of a
>common (dinosaur) ancestor, why was this not evident for such a long time,
>to Harry Seeley in 1887, or to Colbert and Ostrom later on? What changed in
>dinosaur science to make something seem so certain (in a field that seems to
>have its fair share of uncertainty)?  Was it the discovery of new evidence?
>Or a new way of looking at the evidence?  Tom Mitchell
        Let me start by saying that I am no expert on the history of
evolutionary thought. What we have here is a situation where, in trying to
be clever and scientifically accurate, I have needlessly complicated your
understanding of what is actually a fairly simple issue. So let me try my
best to clarify this. It appears that, in the past, scientists spoke of
groups "not having a common ancestor, by which they meant that they lacked a
common ancestor which was a member of the proposed taxon. Alternately, the
suggestion of common ancestry was used in justification or, in its converse
the refutation of, the grouping of two taxa at an higher taxonomic level.
Since these were Linnean taxa, part of a typological classification based on
phenetic similarity, it was possible to create polyphyletic taxa as well as
paraphyletic and monophyletic taxa. Recall that paraphyletic and
monophyletic (or "holophyletic") taxa, which were together (I believe,
everyone feel free to correct me) termed monophyletic in the old days, both
consist of a common ancestor and some or all of its descendants. Thus, in
rightly eschewing polyphyletic taxa, a phrasing developed which explictly
invoked common ancestry, but which did not rigourously address the
relationships amongst taxa. This should come as no surprize, since, for some
reason, the Linnean heirarchy was taken to be a more than adequate model for
these relationships.
        So, when the Old Masters referred to two taxa as "not sharing a
common ancestor", it appears to have meant that the two were not members of
a higher taxon exclusive of other taxa. As you can see, especially if you
try writing all this down once or twice, this is a very imprecise way of
dealing with evolutionary issues. I'm sure Colbert, Ostrom, Romer, Cope,
Marsh, and all the rest (with certain notable exceptions), if confronted
with the stream of logic I followed in my initial response, would readily
agree that dinosaurs shared a common ancestor. And, like so many people
today, they would probably chide me for pedantry and for evading the "real
issue" by hiding behind meaningless semantic babble.
        However, my contention is that dealing with the issue in this way is
a necessary and fundamental shift in one's pesonal understanding of
evolution. We must divorce ourselves from the notion that a Linnean taxon
has any true evolutionary meaning. The units of evolution are individuals,
and by extension populations, species and clades. Had the Old Masters fully
realized the import of Darwin (as Hennig appears to have done), I have
little doubt most of them would have embraced such explicit language for the
exploration and descrition of the pattern of evolution.
        So, to answer your question, the way I phrased it, I'm sure
"everybody knew" that dinosaurs shared common ancestors, at least if they
were forced to think about it. The issue they were exploring, and the real
question you want answered, involves whether or not the dinosaurs could be
considered separate offshoots of the "thecodonts". If this question is
rephrased in a modern context, it should appear clear, at least to any
cladist, that it is a non-issue. "Thecodonts" are not a real group. Being
defined as "those archosaurs that are not birds, crocs, pterosaurs or
dinosaurs", then the question of whether dinosaurs shared a common ancestor
above the thecodont grade seems rather arbitrary, doesn't it? It really does
seem to depend on where you draw you lines.
        Now, these days, we would draw up our cladogram, and try to see if
saurischians and ornithischians shared more recent common ancestors with
other archosaur clades than with each other. However, in the old days, for
lack of a better methodology, one tried to find similarities shared among
dinosaurs which were not present in thecodonts (potentially circular,
depending again upon where you draw the line between the two). Sounds oddly
familiar, doesn't it? Of course, without the current emphasis on derived
characters, this could be somewhat more difficult, and I suspect that much
of the saurichian/ornithischian problem is the result of the fact that, in
the terminology of the day, ornithischians were so much more derived than
saurischians. I believe that the decisive (and non-cladistic) dinosaur
monophyly argument was made by Bakker and Galton in the '70s.
        Of course, phylogenetic taxonomy allows one to use the fact that all
life (certainly all vertebrate life) shares a common ancestor to construct
our taxonomy in such a way that all members of a taxon share a common
ancestor (which is included in the taxon), which is not shared with any
other taxon. While this is, IMHO, a much better system than the old one,
since the scientific community as a whole has not yet converted to this new
system, and you don't learn this system in high school, there will be for
some time a lot of confusion. So, sometimes when you ask a question such as
yours, the response is "of course dinosaurs have a common ancestor, it is
part of the definition." this, of course, strikes you as cicular, and you
quite naturally balk at it.
        Confused yet? I am. How about you read through this and ask some
questions. If I can't tackle them, maybe Drs. Brochu and Holtz or others in
the know can chime in. Maybe we can go from there.

     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
 "Only those whose life is short can truly believe that love is forever"-Lorien