Re: Crown groups

["Jonathan R. Wagner" <jonathan.r.wagner@mail.utexas.edu>]

Tim Williams wrote:
> The very concept of a "crown group" is anthropocentric: it is anchored in
> those taxa that happen to have survived into the Holocene.  Thus, these
are
> the critters that we can actually clap eyes on because they are the
> survivors.

    While I am usually the first person to bandy accusations of
anthropocentrism, I feel that the claim is not justified in this case. The
purpose behind preferring a crown group definition  has nothing to do with a
preference for those species that haven't had the decency to return their
constituent parts to the rock cycle.


> But if an alien taxonomist landed on Earth 66 MYA, the
> crown group would be Dinosauria (or Xzylbtttfggzzt in his language). [...]

    If aliens landed on Earth 66 MYA, I imagine they would find different
groups would be useful for communication than we do today. One of the
philosophical advances of the last three decades has been the idea that taxa
(as species/populations and clades) exist in nature regardless of whether we
choose to baptize them. We pick taxa for baptism based on criteria we find
USEFUL for communication. That's why we suffer through the formality of
nomenclature, to have names we can use to talk about taxa that are useful to
talk about. Obviously, crown clades ARE useful to talk about them, because
they are usually what neontologists ARE talking about. But, ultimately, the
choice of which taxa to name is *arbitrary*.



Christopher Taylor wrote:
>     Just for the record, I'm not convinced by the case for crown-clade
> restriction - neontologists aren't really assuming such statements are
> applicable to extinct stem taxa, they're merely ignoring them as the
> impossibility of knowing about them makes them irrelevant to their
studies.

    You are quite correct that they often ignore extinct taxa rather than
deliberately using crown clade definitions. However, they do associate taxon
names with particular attributes on a REGULAR basis, e.g., "Mammalia is
diagnosed by hair," "the crocodilians have a semi-erect gait," "Aves is the
sister-group to Crocodilia [sic]."  The point is, do you want taxonomy to
agree with the statements of 99+ percent of natural scientists, or will
paleontology always be the whiney little know-it-all who has to correct the
teacher every time ("but some members of Aves had teeth...")?

    I will readily admit that, as a justification for an action, preferring
crown clade definitions is ultimately quite weak. But can you find a more
objective place on the great Bulletin Board of Life to thumbtack those
names? I'll save you the trouble of answering: you can't. The problem is
that, due to the wonders of what Chris Brochu once called the Linnean
"floating point definition," it is often difficult to establish
unequivocally EITHER what is the original intent OR the "current use" of a
taxon name, and it is usually impossible to reconcile BOTH such that
everyone will be satisfied with the application of the name. By opting to
recognize crown clades, we apply  the only available, remotely objective
criterion for separating things that "look like X but aren't" from things
that "are X."

    Sure, the line that criterion dictates is a product of historical
contingency, and is almost always NOT what your average neontologist
associates with the term. I've seen some very savvy phylogeneticists slip on
this sort of thing quite regularly, including a frog systematist who
insisted that we should use Amphibia instead of Lissamphibia (I agree), and
apply it to the crown clade, then turned around two lectures later and
called Eryops an "amphibian." I think that paleontologists, by applying the
name outside the crown clade, have confused the neontologists, and they seem
to be just trying to keep up.

     So, yeah, crown clades are a *little* more objective, but please no
one. However, they do have the added benefit that they correspond to what
neontologists WRITE (as opposed to what they think they mean, if they have
even considered the question at all). Crown clades DO provide an external
reference, albeit an arbitrary one, for communication across disciplines. If
Tim Rowe says he has a mammal, I can make certain predictions about the
animal: it will have limbs, it will have glandular skin, etc. In fact, I can
pick up any mammalogy textbook and tell you a whole bunch about it. However,
if Spencer Lucas says he has a mammal, the most I know is that he has a
non-pelycosaurian-grade synapsid ("panmammal"), and that is only if I know
him and his work! A neontologist might very well assume he means a
crown-clade mammal and start mentally applying their characteristics to the
animal in question. Is this communication?

   Ultimately, neontologists study extant things, and they often make
sweeping pronouncements about the characteristics of groups, *effectively*
applying a crown-clade definition (because they fail to check with
paleontologists first, those fools! ;). And neontologists outnumber
paleontologists by a substantial margin. It makes sense to adopt a strategy
that maximizes the number of people *already* using your system correctly,
minimizing the need for constant, irritating corrections. The ones who
suffer are the paleontologists, who, in the end, are probably the only ones
sensitive to the issue in the first place. I, for one, am willing to "take
on for the team."


Wagner