[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: The Unbearable Consequences of Undiscovered Phylogeny [was: Re: "The X Digit"...a Pteroid bone??]

At 01:22 PM 1/25/99 -0500, T. Mike Keesey wrote:
>> Unfortunately, we have no means currentlyu available to alter an
>> established definition.
>There should be.
        Best bet, prove to everyone using PT in your discipline that it
should be changed (good example, definition of Aves...).

>If only the ICZN would expand to cover phylogenetic
>taxa... appeals could be made to refine definitions, much as they are made
>to appoint new type specimens.
        The problem is, there are, as I see it, only two reasons to refine a
        1) The definition results in a potential logical contraditiction
        2) The definition potentially interferes with access to the literature
IMHO, you only get to use #2 once. If you don't get it right the second
time, you just #*(&^$.
        I wouldn't mind you being able to revise to change an suprageneric
anchor taxon to a genus-level anchor (why genus level? Because it's the
highest rank not covered by PT), but unless that falls under #1 (as in
stem-based taxa), there is no need for it. Unfortunately, most people are
too locked into their set idea of the tree topology to admit that, for
stem-based taxa, there is always the possibility of #1 with multiple
inclusive anchors.
        However, I'm sure lots of people would like a #3:
        3) I don't like this definition, because for some reason it doesn't
fit my preconcieved notion of what the taxon is.
        Tough beans. That ain't what PT is about.
        So, anyway, we might almost be better with a strict priority system,
only with clear guidlines on when a definition is invalid (e.g. reason 1 and
for a little while reason two.

>All too often a definition rests on the
>assumption that the cladogram accompanying it is the correct one, and when
>that turns out to be false... ugh. The person defining the clade should
>take every (reasonably) possible phylogeny into account.
        Absolutely agreed. PT folks on this list, please take note of the
above sentence. Someone write this down and send it to a certain big-name
school paleontologist who assume the best anchor taxon is the one "at the
end of the spine" on the cladogram. Please!

>Another thing: I know it is against the "rules" (not that there is a
>formal list of rules for phylogenetic taxa, TMK) to define a stem-based
>taxon with multiple included taxa {A, B > C},
        Actually, AFAIK, it isn't "against the rules." This is just
something I soapbox about regularly. Most people wouldn't do it explicitly
(some seem to think multiple anchors are illegal, but see next point), but
they end up doing it by using a suprageneric taxon as an anchor. 

>but mightn't this style of definition be a good thing?
        Not to put too fine a point on it.

>[...] Ornithodira would become an invalid clade. What's
>wrong with that?
        What's wrong with that is that you want to have your cake and eat it
too. You're using logical contradictions to force PT to behave like a
typological taxonomy. This isn't the point. The idea is that taxa should
remain valid, and we accept plasticity (or fluidity, even) in their content
as a necessary appreciation of how little we know of the history of life.
You must abandon your typological notions and accept the (frustrating) fact
that things change. Or don't, and abandon PT.

>If, instead, "prosauropods" are a parphyletic
>assemblage ancestral to sauropods, "Prosauropoda" becomes invalid. I don't
>see a downside.
        Again, you are still trapped in the old (much easier to deal with,
IMHO) mindset of typological taxa. You must unlearn what you have learned. :)

        And now, back to dinosaurs...
     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