More phylogenetic taxonom (was Re: Def. of Ornithischia)

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 09:25 PM 6/11/97 -0400, Jeff Poling wrote:
>>"Triceratops and all taxa sharing a more recent common
>>ancestor with Triceratops than with birds": Padian and May, 1993.
>...
>>While on this subject, the node joining Pisanosaurus and Genasauria (or
>>Lesothosaurus and Genasauria) has not been named.  "Predentata" might be
>>used as a node-based taxon using either of those definitions.
>
>   One question about phylogenetic naming conventions and priority.  Stem
>based definitions exclude the most recent common ancestor of the taxa in
>question.  Since clades are supposed, if I understand correctly, to be the
>monophyletic representation of taxa and their ancestor, shouldn't node based
>definitions therefore be the ones used, and have the priority?

Both node-based and stem-based defined taxa are clades: they represent an
ancestor and all of its descendants.

>   In other words, why wouldn't a node based Predentata of the most recent
>common ancestor of, say, Pisanosaurus and Triceratops take precedence since
>node based taxa seem to be the underlying principle of cladistics?
>(Oops, that would be the unnamed node Dr. Holtz spoke of.  I meant of
>course whatever animal plus Triceratops would make up a node based
>definition of Ornithischia.)

Both stem-based and node-based taxon definitions are fully compatable with
cladistic methodology.  I think what is confusing people here is the
(incorrect) idea that one must use ONLY stem-based or ONLY node-based
defintions, when in fact phylogenetic taxonomy uses both at the same time.

Perhaps this will work best by example:
Reptilia has a node-based taxon definition: all descendants of the most
recent common ancestor of turtles, lepiodsaurs, and archosaurs.
Sauropsida has a stem-based taxon definition: all taxa sharing a more recent
common ancestor with Reptilia than with Mammalia.

These are DIFFERENT taxa.  The one (Reptilia) is contained in the other
(Sauropsida).  It is possible to be, at the same time, a sauropsid but not a
reptile (mesosaurs, for example, may be non-reptilian sauropsids); but it is
impossible to be a reptile but not a sauropsid.

However, both are clades (an ancestor and all of its descendants).

A second example: Mammalia has a node-based taxon definition: all
descendants of monotremes, marsupials, and placentals.
Synapsida has a stem-based taxon definition: all taxa sharing a more recent
common ancestor with Mammalia than with Reptilia.

(Incidentally, the node joining Sauropsida and Synapsida is Amniota.  The
(as yet unknown or unrecognized) most immediate common ancestor of reptiles
and mammals was an amniote (by definition) but neither a sauropsid nor a
synapsid (also by definition).)

Hope this helps.


Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661