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Re: Tetrapod note (was: Re: [Re: Resting Sauropods])



On Sat, 18 July 1998, Jonathon Woolf wrote:

> Jonathan R. Wagner wrote:
> 
> > >"Recently, Tetrapoda was formally defined as a crown-group (Gauthier et 
> > >al.,
> > >1989).A crown-group is a clade that includes the last common ancestor of 
> > >two or
> > >more extant taxa, and all its descendants. In this case, Tetrapoda was 
> > >defined
> > >as the clade that includes the last common ancestor of lissamphibians and
> > >amniotes, and all its descendants."

My impression is that this definition is viewed much the same way crown
group Aves is viewed -- making it a crown group hacks off a lot of things
that are usually considered tetrapods (from _Ichthyostega_ to
_Seymouria_), much as making Aves a crown group hacks off a lot of things
usually considered avian (from _Archaeopteryx_ to _Ichthyornis_). I'm not
sure what the exact definition is, but I believe I've been told that most
workers include everything down to the most basal four-limbed vertebrates
in Tetrapoda.

Also, I think this whole discussion got started by someone mixing up the
terms "quadruped" and "tetrapod" -- not the same thing. For example, I'm a
tetrapod, but not a quadruped.

--T. Mike Keesey                                   <tkeese1@gl.umbc.edu>
DINOSAUR WEB PAGES -- http://www.gl.umbc.edu/~tkeese1/dinosaur/index.htm