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STEM V. NODE BASED CLADES



Jeff Poling wrote:
<<I did a search of the DML archives, usenet, and the web, and didn't find a
definition for a crown clade.  What is a crown clade?>>

I believe (someone correct me if I'm wrong) that a crown clade is defined as
"all the descendants of all the living members of a clade."  For example, the
crown clade Mammalia is defined as {Monotremes + Marsupials + Placentals};
which means (I think) that some really mammaly things like Multituberculates
aren't mammals per se, unless of course Multis are really closer to
Placentals as some have suggeted.
 
<<One other question:  I carefully reread the definitions for stem and node
based clades (and then did the requisite search of the archives).  It looks
like the only difference between a node and stem based clade is how each is
defined.  There has to be more to it than that ... what IS the difference?>>

It is basically the way the are defined, but there is a difference.  For
example, Dinosauria is a node based clade {_Triceratops_ + _Corvus_}, so some
animals that are close to dinosaurs (_Marasuchus_, _Psuedolagosuchus_,
_Lagerpeton_ etc) but don't fall within the clade Dinosauria, aren't
Dinosaurs.  IF however Dinosauria was defined as a stem based clade {_Corvus_
> _Pterodactylus_}, then animals like _Marasuchus_, _Psuedolagosuchus_, and
_Lagerpeton_ would be included within the clade Dinosauria.  Just as animals
like _Cryolophosaurus_ (probably...) fall within the stem based clade
Carnosauria {_Allosaurus_ > _Corvus_}, but not within the less inclusive node
based Allosauroidea {_Allosaurus_ + _Sinraptor_ + _Carcharodontosaurus_}.
 
<<And one other one other question:  how is it decided when one definition
has precedence over the other?  As part of my example, you mentioned that
under those conditions Dinosauria would become synonomous with Archosauria,
and Archosauria would be sunk.  I thought Archosauria had been named first?>>

It is decided by which name was published first, which is Dinosauria (1841 v.
1869).  A problem comes up when a lot of names are synonymised to mean the
same thing and were published at the same time.  For example, if birds turned
out to be closer to crocodiles than to _Velociraptor_, then Tetanurae
{_Corvus_ > _Ceratosaurus_} and Maniraptora {_Corvus_ > _Ornithomimus_} mean
the same thing.

By the way, I have been using my own cladistic shorthand throughout this
email with node based clades (all the descendants of the most recent common
ancestor of both A and B) as: {A + B}; and stem based clades (all those
organisms that share a more recent common ancestor with A than B) as: {A >
B}.

Peter Buchholz
Tetanurae@aol.com

"Kissing . . . is a goodness," he explained.  "It beats the hell out of card
games."