Re: Crown groups

[Tim Williams <twilliams_alpha@hotmail.com>]

Christopher Taylor wrote:

>     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).

Not true.  A crown group is only a crown group because (by definition) it 
has members that survived into the Recent.  By contrast, a paraphyletic 
group is always a paraphyletic group.  The "Thecodontia" is one of the more 
infamous paraphyletic groups.  If I went back to the Jurassic period and 
constructed a cladogram based only on taxa that had existed up to that 
point, I would still find the Thecodontia to be paraphyletic.  The 
Thecodontia is paraphyletic because dinosaurs (including birds), 
crocodylians and pterosaurs were arbitrarily excluded from this group.  All 
of these groups had appeared by the end of the Triassic.



Tim

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