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Crown clades a-frikkin'-gain [was Re: Tetrapod note]
Mike Keesey wrote:
>My impression is that this definition is viewed much the same way crown
>group Aves is viewed -- making it a crown group hacks off a lot of things
>that are usually considered tetrapods ese1/dinosaur/index.htm
That's exactly what it is, a crown group.
Jonathon Wolf wrote:
>Sorry. I thought non-cladistic definitions had been outlawed on this list.
As a matter of fact, I was specifically avoiding the subject of
taxon definitions, and concentrating on definition of the colloquial term in
an attempt to help our non-biologist friend sort things out. :)
>c), as you point out, is the cladistic one, and the cladists can have it
>for all I care. Any definition that says _Seymouria_ isn't a tetrapod has no
>rational basis as far as I can tell..
We've been through "rationality" and taxonomy before. Please check
the archieves under "Arctometarsalia".
The idea behind crown clades (at least in part) was to restrict taxa
based on extant animals to the most recent common ancestor of all extant
froms and all of its descendants. The purpose was to "clean house".
Biologists frequently have made, and still make today, blanket statements
about the characteristics of particular taxa based only on living members of
that taxon. Often these statements cannot be reasonably applied to fossil
forms because we have no evidence for much of the biology and ecology of
extinct animals. If we include fossils in the taxon, we are left with a
dilemma: either we have to go back and put a little asterisk beside every
single statement, with a footnote saying "statement may not apply to some
fossil species included in the taxon", or we can restrict these taxa to the
crown clade, since it is most parsimonious to conclude that features shared
by all members of that taxon are inherited from their most recent common
ancestor, and were likely present in extinct fossil representatives of the
The entire problem apparently stems from the neonto-centric [sic?]
nature of biology and taxonomy. It certainly seems as though many biologists
simply do not care to think about fossil forms when making broad
generalizations (I'm not sure I blame them). Some biologists certainly do
not believe fossils to be of much value, even in areas such as elucidating
phylogeny (a mild example being Colin Patterson). Further, until recently,
there seems to have been a strong urge to try to classify as many fossil
forms as possible into modern taxa (for an extreme take on this, see Colin
Patterson in the Paleo. Society _Major Features of Vertebrate Evolution_
short course). I will not attempt to explore the reasons for this. However,
the end result is an almost instinctive urge to include every flying
dinosaur in Aves, every four limbed animal in Tetrapoda.
Why? Why is this so important, I ask you? Because it makes sense?
Again, I say, consult the various taxon name rationality discussions of
I should also point out that there are indeed some good arguments
against specific applications of the crown clade principle (e.g. Aves).
>Yeah, those turn up a lot when cladists get loose.
Crown clades are not a necessary part of cladistics, as far as I
know. The need for them may have grown out of an application of the
principles of phylogenetic systematics, but you can be a "cladist" and not
approve of crown clades (e.g. Chiappe).
>Funny thing, that -- almost seems to be saying there's something wrong with
>either the methodology or the way it's being applied. Scientific tools and
>methods are supposed to reduce confusion, not increase it.
It is always simpler to attribute your own confusion to a weakness
in the method you are trying to understand. And it is certainly those people
who resist change because of the turmoil inherent in the process who end up
being remembered not as the valiant champions of order and right they
envision themselves to be, but rather as the misguided holdouts of
anacronism and rigidity which they truly are.
Now, for crying out loud, lets get back to dinosaurs!
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"...To fight legends." - Kosh Naranek