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Re: Cladistic taxonomy (was Dietary factors)
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.
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
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.
Timothy J. Williams
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
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