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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 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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