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Various taxonomic issues



Well, it looks like the taxonomic thread is dampening down.  Just a few
comments to throw in there, which have more to do with sociological and
historical than nomenclatural issues:

Ken Kinman wrote:
>Well, I suspect this is the first time on this list that there has been
a serious discussion of whether Aves should be given Class status (correct
me if I am wrong).  If so, I think that is productive.<

YIKES!!!!  Well, there are only a handful of people here who are old-timers
like me, and remember the days before Mickey Rowe was running things (i.e.,
sometime in the Mesoproterozoic).  In any case, this subject has come up a
LOT: check out the dinosaur mailing list archives for some historical
reviews.  I have to say, though, that the discussions here (the latest
included) were never as acrimonious as the equivalent on
sci.bio.paleontology, nesting ground of California Kingsnakes, rampaging
mathematicians, and other such net.entities.

"Archosaur" writes:

>As for above, traditionally creatures have always been defined up to their
class level (e.g. Humans = _Homo_ = Hominid = Primata = Mammalia). Birds
already had a class before their dinosaurian relationships were worked out.
That means that they had enough differences to warrant a huge separate
category. Differences that stayed around throughout most of dinosaur science
and have only recently been taken down because a select few theropods show a
lot of these traits. For those theropods, I can understand the use of the
term
"non-avian", but for the rest I don't see a point to it. And since the nine
thousands living avialans share more in common with each other than with
"Mesozoic dinosaurs" I don't understand why we have to remove the term Aves
in
favour of Dinosauria; which seems to be the case.<

Working backwards in terms of the paragraph above:
1) No one is suggesting removal of the taxon name "Aves": it is a very
useful word.  Just as use of the words Eutheria or Archonta do not mean we
stop using Primates (incidentally, this is the proper Latin form of that
taxon name) or Hominidae or _Homo_, so to use of the words Dinosauria or
Coelurosauria or Maniraptoriformes do not preculde the use of Aves.
2) The use of the phrase "non-avian" is in part to force the recognition of
the 'unnaturalness' of "Dinosauria" as traditionally conceived.  An analogy
would be discussion of the "non-chiropteran mammals": bats, after all, are
separated by an anatomical gap from the majority of non-flying mammals, just
as modern birds are separated by an anatomical gap from the majority of
non-flying dinosaurs.
3) Actually, the number of character complexes that separate birds from
other LIVING vertebrates are quite a lot.  However, starting with the work
of T.H. Huxley in the 1860s and working through to Ostrom in the 1970s,
Gauthier and Paul and others in the 1980s, and numerous other
paleontologists in the 1990s, it has been shown that these features do not
appear as a sudden jump in the lineage leading to birds.  Instead, the
series of transformations from basal archosaurs to birds included the
evolution of anatomical novelties at every step.  Birds are NOT separated
from other dinosaurs by any big jump of anatomical changes: instead, there
is the steady accumulation of changes from basal archosaur to basal
ornithodiran to basal dinosaur to basal saurischian to basal theropod to
basal neotheropod to basal tetanurine to basal avetheropod to basal
coelurosaur to basal maniraptoriform to basal paravian/eumaniraptoran
THROUGH basal avian to basal pygostylian to basal ornithothoracine to basal
ornithurine to basal carinate to basal neornithine (to list some of the
better known groups...).

So, the *separation* between Aves and other dinosaurs is not a gulf of any
sort.  Indeed, one could argue that the separation of Ornithischia from
other dinosaurs or Ceratopsia from other dinosaurs is even more profound, as
these both include the appearance of entirely new bones.

And therein lies one of the fundamental flaws with the Ashlock criterion for
the basis of rank concepts: big anatomical gaps are not evolutionary events,
but rather statements of ignorance on our part as observers.  The reason big
anatomical jumps appear in the record stem from our non-recovery of the
pertinent fossils.

Note that this is also a flaw with over-emphasizing such cladistic tree
descriptors as Bremer decay indices: a BIG Bremer support value may very
well simply indicate that we haven't found the once-existing members of that
branch which show the accumulation of some but not all of the evolutionary
novelties of the known members of that clade.

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): 301-314-7843