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Re: Cladistic taxonomy (was Dietary factors)




Philosoraptor wrote:

Convenience and ease of communication are important, but that doesn't mean that the points at issue are entirely semantic. Another (many would say more important) issue has to do with the real similarities and differences among the organisms in question.

and

O.k., but that doesn't mean that the distinction between birds and non-avian dino.s is "entirely arbitrary". A lack of *clear* boundaries doesn't mean that there are no real similarities and differences...though these may be difficult to represent

As I said previously (and "Philidor" also pointed out) at a certain point in the *known* fossil record the differences between the most primitive birds (such as _Archaeopteryx_) and certain bird-like dinosaurs shrinks to almost nothing. That "almost nothing" is certainly something, but certainly insufficient to demarcate a *clear boundary* between birds and non-avian dinosaurs.


The boundary of the Aves (or Avialae) is *arbitrarily* set at _Archaeopteryx_. A change in the polarity of one or two character states (which may happen with the discovery and description of new theropods) may place oviraptorids (for example) above _Archaeopteryx_ and therefore closer to modern birds.

The point I'm trying to make is that is that there is not a great deal anatomically that separates _Archaeopteryx_ from certain dromaeosaurids. This is the case whether you express the differences as an intuititively-based "phenetic gap" or as some quantitative estimate of disparity, such as their relative position in dentoskeletal "morphospace".


Bas VanFraassen sez: you don't need clear boundaries to have a legitimate distinction; all you need is a clear case of the one kinda thing and a clear case of the other kinda thing.

In a vernacular context, you're right. A python is clearly a kinda reptile and a sparrow is clearly a kinda bird. A five-year-old can distinguish a bird from a reptile at the zoo. But, go back 120-150 million years, and _Archaeopteryx_ is very kinda like _Sinornithosaurus_. The boundaries are less clear, and the criteria for making the distinction between bird and non-bird are totally subjective and arbitrary.


Somebody very politely pointed out to me (off-list) that this discussion does not relate to the word "bird", since "bird" (as opposed to "avian" or "avialan") is just a vernacular term to which no scientific meaning has ever been attached. Historically, I would disagree. For example:

Chiappe, et al. (1998). The skull of a relative of the stem-group bird _Mononykus_. Nature 392: 275 - 278.


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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