Re: Crown groups

[Christopher Taylor <ck.taylor@auckland.ac.nz>]

On 3/6/04 10:29 am, "Tim Williams" <twilliams_alpha@hotmail.com> wrote:

> 
> Jaime Headden wrote:
> 
>>   De Queiroz et al. will be offering multiple pan-stems for the various
>> possible placements of some living turtles to one another. Now imagine
>> doing that with snakes ... or birds.
> 
> 
> Or insects.  Egad!

Especially when the name "Panorthoptera" is already in use for a clade
including not just the total stem group of Orthoptera, but also Phasmidea
(Hmmm.... Panpanorthoptera?). Also, try saying 'Panmantodea' three times
fast. On the upside, it allows great scope for entomological doggerel -
'Panisoptera' rhymes with 'Anisoptera' :).
> 
>>     The MAJOR problem I have with pan-stems is that these are very
>> important historical points in the history of things DYING and SURVIVING
>> (we honor what lives today with names, and ignore the various fossils
>> species outside these relatively few nodes -- they are usually IN the stem,
>> or in another sister or more inclusive stem, under this philosophy, as is
>> generally applied in the node-stem triplet application to cladistics). They
>> have _little_ utility in recognizing diversification, new features or
>> populations, or essentially record any information about the speciation and
>> the arrangement of species or populations save those that live today.
> 
> I think I know what you're driving at here, and it's an excellent point.
> 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.
> 
> There is no fundamental phylogenetic principle behind the "crown group"
> concept; it is wholly taxonomic.  For example, we are all familiar with the
> crown group Aves.  But if an alien taxonomist landed on Earth 66 MYA, the
> crown group would be Dinosauria (or Xzylbtttfggzzt in his language).
> Similarly, Neosauropoda would be a crown group prior to the K/T boundary,
> but not the long-extinct Prosauropoda.
> 
    Despite not overmuch liking the PhyloCode, I find this argument somewhat
spurious (It has also been used to justify the use of paraphyletic taxa -
they were holophyletic once, and it's only from the perspective of the
Holocene that they look paraphyletic). True, there is no ultimate reason for
taking our time as more significant than any other, but it happens to be the
one we're in, and all our studies are influenced by that fact.
    Also, crown groups are significant in that characters that cannot be
established from fossils (behaviour, biochemistry,...) can only be
definitely assigned to crown groups. This is, after all, the main argument
behind restricting familiar terms to crown groups (it avoids the inherent
possibility for error in common statements such as 'all mammals produce
milk').
    Just for the record, I'm not convinced by the case for crown-clade
restriction - neontologists aren't really assuming such statements are
applicalble to extinct stem taxa, they're merely ignoring them as the
impossiblity of knowing about them makes them irrelevant to their studies.

    Cheers,

        Christopher Taylor