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Re: "babble" about cladistics



At 11:40 AM 11/19/97 -0800, Terry Jones wrote:
>The recent discussions about cladistics has been most impressive.  To my
>mind this has been one of the most valuable discussions I have seen from
>this list.  A topic I have not seen covered yet is how characters are
>chosen.  Is there any consensus on what makes a "useful" character?

Good question.  I guess the big point here would be "define utility".  Is a
useful character one which helps support a particular tree?  Is it one which
is easily preserved?  Is it one which can be clearly described and is
verifiable from specimen to specimen and by different workers?  All these
are probably good definitions of "utility", and given character in a given
group of organisms may or may not demonstrate all these properties.

A good criterion for the utility of a character in a given database is
distribution.  Any character found in ALL taxa in a particular study is not
useful for sorting out relationships.  Presence of a bony skeleton or of DNA
will not indicate if tyrannosaurids are closer to ornithomimids than to
dromaeosaurids (or humans to chimps than to gorillas).

Characters found only in a single operational taxonomic unit (OTU) in a
study are also not that useful in sorting out relationships.  (They can,
however, be quite useful in determining how different a particular OTU is
from other taxa in a study, and of course they may be important in other
aspects of biology, such as behavioral or functional morphological studies).
Thus, didactyly in tyrannosaurids does not help sort out if tyrannosaurids
are closer to ornithomimids than to dromaeosaurids, since (of the three
forms) only tyrannosaurids have this derived condition.

Of course, character distribution might change from study to study.  For
example, a large pubic boot seemed to be diagnostic of Tetanurae in
Gauthier's 1986 paper, as it was found in _Allosaurus_ (which he used as the
main form for Carnosauria), Ornithomimidae, Dromaeosauridae,
_Archaeopteryx_, and some other forms.

However, more recent studies (my on-going work, Sereno's work, etc.) has
shown that (given these data bases), pubic boots have a much more
homoplastic distribution.  For example, advanced carnosaurs such as
_Allosaurus_, _Acrocanthosaurus_, and _Giganotosaurus_ have this condition,
but primitive members such as _Sinraptor_, _Yangchuanosaurus_, and
_Monolophosaurus_ have much smaller structures.  Basal tetanurines such as
_Torvosaurus_ and _Baryonyx_ also lack a big pubic boot: they have
relatively small protruberances.

Despite a century of illustration to the contrary, actual specimens of
_Ceratosaurus_ lack a big pubic boot: instead they resemble _Carnotaurus_
and (to a lesser extent) the coelophysoids in having only a small knob.
_Herrerasaurus_ has a large expansion in the distal pubis (rather than an
abrupt change, as in a true pubic boot), and the herrerasaurid
_Staurikosaurus_ has only a small expansion.

So, is the pubic boot a "useful" character, or not?  It is a character with
a variable distribution, and (although it may support several different
subgroups in various parts of a tree) it no longer supports a single
particular major clade within Theropoda (based on the distribution of other
features).  As a skeletal feature, it is easily preserved.  If "pubic boot"
is explicity defined (say by the abruptness (in degrees) of deviation from
the main shaft), then it can be clearly demonstrated in terms of presence or
absence from specimen to specimen and by worker to different worker.

Is "pubic boot" a useful character?  It depends on your 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