Re: Croc classification (was Re: Sarcosuchus and Dumbing things down)

[chris brochu <cbrochu@blue.weeg.uiowa.edu>]

> I understand most of cladistics, but can someone briefly explain "crown
> groups"?  Preferably a cladist; having an anti-cladist describe cladism is
> like having a flat-Earther explain gravity, or a creationist explain natural
> selection (not discussing either of those!!!!, just an example...).



Basically, a crown clade is a node-based group in which living taxa 
are the specifiers; hence, Crocodylia is now defined as the last 
common ancestor of Gavialis, Alligator, and Crocodylus, and all of 
its descendents.  "Crocodilia" sensu lato included a bunch of extinct 
things we now call crocodyliforms, but not crocodylians.

Crown groups have a number of benefits, one of which is stability - 
and here is where I really think Steve Salisbury is missing the point 
of stability.  We can never achieve stability of content, which is (I 
think) what he meant by "Crocodilia in the old sense had stability" - 
everyone agreed on the content.  I disagree with that (witness his 
agreement with me that the lower bound of "Crocodilia" was poorly 
defined), but more importantly disagree on whether everyone agreed on 
the underlying  *meaning* of Crocodilia.  Ask a neontologist to 
describe a "crocodilian," and you'll get a fairly different opinion 
than if you'd  asked someone like Al Romer. They may agree on paper 
on what the contents are, but the actual meaning of the name as used 
by these groups will be different.

A good example is Crocodylus - the "genus" including modern "true" 
crocodiles (Australian Salties, Nile crocs, etc.).  There was a 
modest "debate" in the literature over the age of Crocodylus - was it 
a fairly young clade (middle  Miocene at the oldest), as molecular 
data sets argued, or was it some sort of ancient lineage going back 
to the Eocene, or even the Cretaceous, as paleontologists argued?  In 
fact, these groups of people were talking about entirely different 
entities - molecular data sets addressed "crown genus" Crocodylus, 
and paleontologists were talking about a form taxon.  In fact, the 
data sets were in broad agreement, once people realized that they 
were talking past each other because there was no stability of 
meaning.

The truly important point is that phylogenetically-defined names have 
stability in meaning; stability in content, in diagnosis, and in time 
of origin can never be stable (under ANY taxonomic system), as we 
will always be making new discoveries.  Arguments about whether 
particular names should be restricted to crown groups are a 
completely separate matter, though I think in some  cases (including 
Crocodylia) it is justified.



chris

--
------------------------
Christopher A. Brochu
Assistant Professor
Department of Geoscience
University of Iowa
Iowa City, IA 52242

christopher-brochu@uiowa.edu
319-353-1808 phone
319-335-1821 fax

www.geology.uiowa.edu/faculty/brochu