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Re: Cladistic taxonomy (was Dietary factors)
Ken Kinman wrote:
I'm afraid that there is a stalemate between cladists (who say birds
ARE dinosaurs) and the rest of us (who say birds are dinosaur descendants).
The distinction is purely semantic. Don't let Ken scare you. :-) It's all
much ado about nothing. A squall in a teacup.
When discussing tyrannosaurs or ceratosaurs with evolutionary biologists I
do not say "non-avian theropods". And I support the idea that birds evolved
from small theropods.
Nor do I call camels, pigs, hippos and ruminants "non-cetacean
cetartiodactyls" in general conversation (including with other scientists).
Never have, never will. I'm open to the idea that whales evolved from
within the Artiodactyla, and so are certain people I have spoken to; and
they all know what I mean when I say "artiodactyl".
In written scientific correspondence, it helps to be explicit when
discussing one's ideas. Hence the utility of such long-winded (and
admittedly cacophonic) terms as "non-avian dinosaur" (=Dinosauria of
traditional usage) or "non-cetacean cetartiodactyls" (=Artiodactyla of
These terms facilitate the communication of ideas and concepts, leaving
little room for ambiguity. When someone writes "non-avian dinosaur" he or
she is letting it be known that a certain subset of a monophyletic group is
being excluded (such as for convenience). The reader does not have to pause
and wonder, "Does the writer mean _all_ dinosaurs, or is the writer
excluding _Archaeopteryx_ and other birds?".
This is especially important for birds, since the delineation between birds
and non-avian dinosaurs is entirely arbitrary. In effect, if a genus is
more closely related to modern birds than _Archaeopteryx_, then it is
considered a bird *in the scientific sense*. We (as humans) draw the line
under _Archaeopteryx_. But Nature makes no such clear distinctions. The
transition from "reptile" to "bird" (or from non-avian theropod to avian)
essentially means the acquisition of one or two extra characters.
Classifications are an attempt on the part of humankind to impose a system
on nature. We stick species in boxes, and label those boxes. Traditional
Linnaean classifications have tended to emphasise a gulf between Reptilia
and Aves. Follow both lineages back in time, and (as the Spice Girls say)
"two become one". The difference between _Archaeopteryx_ and a crow (or a
rooster, penguin, eagle etc) is far more than between _Archaeopteryx_ and
_Deinonychus_ (or _Velociraptor_, _Sinornithosaurus_ etc). Phylogenies
should reflect this, and so should discussions based on these phylogenies.
Back in the days of Linnaeus, it was quite clear what was a bird and what
wasn't. Now, we have a plethora of fossils that show a mixture of bird and
non-bird traits, _Archaeopteryx_ and feathered dromaeosaurs among them. As
scientists, we have to adjust our definition of "bird" accordingly; we have
new information, and must incorporate it. As everyday people living
everyday lives, we can stick to the traditional definition of "bird", since
150-million-year-old transitional forms are not germaine to most everyday
Ken quoted Peter Dodson:
"For example, the word dinosaur was not previously problematic--it was
universally understood. Within cladistics it has > now been redefined to
include birds (holophyly), and then a new and cumbersome phrase, non-avian
dinosaur, has been substituted. This is > not progress; this is semantic
obfuscation not enlightened communication."
I think "non-avian dinosaur" would qualify as obfuscation if it comes up in
chit-chat by the water-cooler. In a scientific paper on _Archaeopteryx_ or
_Rahonavis_, the term is actually quite useful. The aim of any scientific
paper is the clear communication of ideas, not poetry.
It all comes down to a critical difference between _vernacular _ and
_scientific_ communication. When I look up at a _Tyrannosaurus rex_
skeleton in a museum, I see a dinosaur. When I carve up a turkey on
Thanksgiving, I see a bird. But if I was writing a paper on the evolution
of _Meleagris gallopavo_ from _Archaeopteryx lithographica_, I may have to
be more careful with my definitions of "bird" and "dinosaur".
Timothy J. Williams
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
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