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Crown Clades etc. (was: Re: Quick cladistics question)



At 02:09 PM 3/31/97 -0500, 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?
        Quick Definition: The restriction of a well known taxon containing
living organisms to the most recent common ancestor of all living memebers
of the origional taxon and all of its decendants. There are good reasons for
such a restriction, and they are reviewed in one of the many de Quiroz and
Gauthier articles you will find referenced in Holtz 1996 (sorry, I don't
have the refs handy, but I can provide if needed).
        Example: In order to make the synthesis of biology and paleontology
more firm through the application of the principles of phylogenetic
taxonomy, Gauthier defined Aves as the crown clade of modern birds. This (at
least in today's interpretation) excludes _Archaeopteryx_, but allows a
greater likelihood that generaliztions previously made by biologists about
Aves based on examination of living forms are more likely to hold true for
fossil animals.

        Note: Most "crown clades" (Aves, Mammalia, etc.) do not seem to be
defined as "the most recent common ancestor of all >whatevers< and all of
its decendants", and thus do not *necessarily* represent the true crown
clade of that group of organisms. They are frequently defined so that they
will include the common ancestor of all living forms and all of its
decendants IF the tree topology assumed by their definor is not
significantly altered.

        A related concept is the Total Group, a stem based clade which is
defined as "all animals more closely related to one crown clade than to
another". Ornithosuchia is the total group containing Aves, and
Crocodylotarsi (= Pseudosuchia) is the total group of Crocodylia, both of
which are the two sister groups (as Gauthier points out, only stem based
clades can truly be sister groups) within Archosauria, the crown clade
defined by the most recent common ancestor of all crocodylians and birds and
all of its' decendants.
        
>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?
        There really isn't *that* big a difference between the two. As noted
above, only stem based taxa may be sister groups. Node based clades
emphasise the common heritage of two or more organisms, stem based clades
emphasize the distinctiveness of one group of decendants with respect to
others. Node based clades include a node, stem based clades begin after a
node. Node based taxa are especially useful for exploring and elucidating
the closeness of diverse organisms, and stem based taxa are useful for
"picking up the pieces", providing a taxon for those animals which are of
uncertain affinities within a group and filling out the tree of life with
names when nodes are just too specific. I could go on for hours.
        The point is that both are necessarily monphyletic (when defined
carefully), both are based on shared ancestry and common decent. You just
pick the right tool for the right job (and to you Peter Buckholzes out
there, the right tool is not always a node based clade :) .

>   And one other one other question:  how is it decided when one definition
>has precedence over the other?
        When they describe the same common ancestor, they are synonymous,
and the older name has priority.
        Example:
        In the some current theropod tree topologies (Norell, in that
Audubon article), Maniraptora includes Troodontidae, Oviraptoridae,
Dromaeosauridae, and Aviale. Given:
        Bullatosauria (Holtz 1994) = { + _Troodon_ , + Ornithomimus }
        Maniraptoriformes (Holtz 1996) = { + Aves, + _Ornithomimus }
        These two taxa define the same common ancestor and the same
decendants, and are thus synonymous, and the newer, seemingly more
appropriate Maniraptorformes is sunk into the less appropriate but spiffier
Bullatosauria.
        No one seems to have decided what happens if you specifically define
an old taxon to sink a newer one.
        I'm gonna duck now, I remember last time I peed in Dr. Holtz's pond! :)
----------------------------------------------------------------------------
      Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
            Web Page:  http://faraday.clas.virginia.edu/~jrw6f