Crown groups

[Tim Williams <twilliams_alpha@hotmail.com>]

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!

>     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.


Tim

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