Re: Cladistic taxonomy (was Dietary factors)

["Tim Williams" <twilliams_alpha@hotmail.com>]

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 
traditional usage").

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 
conversations.

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".


Tim



------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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