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More on classification.



Since I have been asked to expand upon my comments about dinosaur
taxonomy, I am sending this note.

A few days ago Dr. Brett-Surman published his classification
of the dinosaurs.  That was an example of a *cladistic*
classification.

In a cladistic classification, the only aspect of evolution
that is represented is the *branching* *sequence*.  That is,
after an evolutionary tree, called a cladogram, is derived,
the classification is produced directly from this tree by
simply naming most nodes.  The result is a classification in
which ALL descendents of a given common ancestor are included
in its taxon.  Also, pairs of sub-taxa abound.

This results in unbalanced classifications, with a few large,
morphologically heterogenous groups, and many small, slightly
differentiated groups.  Examples of such large, heterogenous
groups in Dr. Brett-Surman's classification include: Tetanura,
Coelurosauria (which includes Dromaeosaurs and Segnosaurs!!),
Genasauria, and Cerapoda.  Small, poorly differentiated groups
include Lesothosaurs, Fabrosaurs, Dryptosaurids and so on.
[Also, seperating the Hypsilophodonts at a higher level than
the remaining subgroups of the Ornithopoda is sort of both
together].

So, I presented my alternative classification.  It is
more efficient as a filing system, since it has fewer levels,
and on average 3-5 subgroups for each taxon, instead of the
2-3 of the cladistic classification.  (Cognitively, 3-5
subgroups is more 'natural' for the human mind than two).

I believe my taxa are also more internally cohesive.  That is,
the animals included in each are more similar in some sense.
This also increases the utility of the classification.

In fact, the actual descent hypotheses that my classification
is based on are nearly the same as those used for the cladistic
one.  [I follow Dr. Paul in placing Ornitholestes closer to
Allosaurus than to the Compsognathids - that is the main
difference].

However, I was deliberate ambiguous in some places. For instance,
there is some controversy as to whether the crested and non-crested
hadrosaurs ("duck-billed" dinosaurs) share a common origin from
the Iguanodontids or not.  If they do not, they *must* be placed
in seperate families, it they do, then, in a non-cladistic
classification, they can be treated *either* way, according
to other criteria.  By choosing to place them in seperate families,
I leave open the possibility of seperate origins, without
actually commiting myself one way or the other.  [Note, in
a cladistic classification, this option is not available,
I am *forced* to take one position or the other, and must
change my classification if I change my mind].

There are also some other areas where I am somewhat hestitant
about my choices.  For instance, the seperation of the
Scutellosaurids as a seperate family is essentially a
compromise.  The problem here is that they are part of
a series of intermediate forms between the basal ornithischians
and the Thyreophora.  Because the transition series is so
gradual, almost any subdivision of it into seperate taxa
will split similar forms.  In this case, the scutellosaurs
have similarites to both the fabrosaurs and the scelidosaurs.
This makes it difficult to know where to place them.  (I have
seen classifications go both ways).


swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.