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RE: Definitions



Firstly, I want to point out that stem- and node-based taxon definitions are
part of phylogenetic taxonomic nomenclature (in other words, a particular
scheme for labelling parts of the tree of life).  This is not exactly the
same thing as cladistics, or phylogenetic systematics.  The latter is a
methodology for estimating patterns of shared common ancestry (that is,
attempting to reconstruct the shape of the tree of life).

As for stem- and node-based taxon definitions: you could check out the Holtz
& Brett-Surman taxonomy and systematics chapter in The Complete Dinosaur
(and the references therein).

In breif, though, neither stem- or node-based are "better" than the other,
since they are meant to capture different things.  Node-based taxon
definitions (of the form "the most recent common ancestor of Taxon X and
Taxon Y and all of that ancestors descendants") have the advantage of being
diagnosible: we can be relatively certain that the common ancestor of that
clade shared a certain suite of derived features found in the known members
of the clade.  However, node-based taxa do not "capture" the primitive
relatives of that group.

Stem-based taxon definitions (of the form "Taxon X and all taxa sharing a
more recent common ancestor with Taxon X than with Taxon Y") are designed
precisely to capture all the primitive relatives of Taxon X.  These
primitive relatives will have some, but not all, the derived features of the
derived node-based taxa within it.  However, you can not diagnose stem-based
taxa, because ultimately the earliest members of this clade would have been
morphologically identical to the most primitive members of the sister
stem-based taxon (i.e., "Taxon Y and all taxa sharing a more recent common
ancestor with Taxon Y than with Taxon X").  The ancestral population which
was ultimately split into ancestors of X on one hand and Y on the other
would only have accumulated the synapmorphies of their respective lineages
as time went by: in their very beginnings, they would have been essentially
identical.

Note: it IS possible to diagnose a node near the base of the stem.  For
example, say that the tree topology for eumaniraptorans is ((Dromaeosaurus +
Velociraptorinae)+ Sinornithosaurus) + (Archaeopteryx + (Rahonavis +
Pygostylia)), with Deinonychosauria being everything closer to the
velociraptorine Deinonychus than to modern birds, and Avialae being
everything closer to pygostylian modern birds than to Deinonychus.  We
technically cannot give a diagnosis for either Deinonychosauria or Avialae,
but we can diagnose the basalmost node in each (Sinornithosaurus +
dromaeosaurids sensu stricto and Archaeopteryx + later birds, respectively).
There would have been earlier and more primitive members of each lineage,
however, which would have had some but not all the derived features of each
lineage.

Hope this helps.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796


> -----Original Message-----
> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Jean-michel BENOIT
> Sent: Friday, August 17, 2001 5:43 AM
> To: dinosaur@usc.edu
> Subject: Definitions
>
>
> Hello dear listmembers
> thanks to some of you HP's, I start to have a faint idea of cladistics (
> I'm going through slowly, slowly . .).  Just one point : I've read one
> could define a clade node-based or stem-based. What's the difference? Is
> there one better than the other ( I hope my question doesn't sound silly
> ).
> Thanks for your answers
> Jean-Michel
>