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Re: dino-to-bird



Caleb Lewis wrote:
>     I think that dinosaurs weren't entirely birds,or entirely reptiles.
        This is highly dependant on how you define "reptiles". Using an
older definition, "all amniotes which are not mammals or birds", and using a
Linnean taxonomic frame of reference, what you say makes *some* sense. The
problem is that, as Eugenio points out, these taxa are our creations, or, if
you will, they claim to be manifestations of a (unevolutionary) Platonic
Form (consciously or otherwise). In either case, you really have two
options, lump dinos into the "bird" or "reptile" class, or reate a new class
for them. Either way the groups you are using are not evolutionary entities,
and statements made about their evolution are not necessarily illustrative
of an evolutionary process or result.
        The central paradigm of modern biology is evolutionary. I'd guess
that the problem you are finding it so difficult to resolve results from
using a non-evolutionary frame of reference.

"Paradigm, what does that mean?
"What"
"What does that mean?"
"Model"
"Well, why didn't you say that?"
"OK, 'model'..."
        -Mabet

>I believe that they were a hybird of both.
        "Hybird", I like that! :)
        Seriously, although individuals may hybridize, and I can imagine
that populations might, I doubt if whole species do (indeed, by definition
they should not), and higher taxa *certainly* cannot hybridize (they are not
breeding units, but groups of breeding units, and in this case, not even
necessarily real groups in an evolutionary context).
        Dinosaurs display a mix of characteristics we recognize in the
modern "Class Reptilia" and "Class Aves". This is not due to some undefined
"hybridization" process, but due to the fact the dinosaurs share share a
more recent common ancestor with modern members of "Class Reptilia" and
"Class Aves" than with any other living group. Indeed, birds share a more
recent common ancestor with some "Reptilia" than with others (ie. the
crocodylians), and share an even more recent common ancestor with "dinosaurs".

Eugenio Spreafico wrote:
>I think we should not forget that taxa are *our* creations, useful to think
>and work about living and past organisms:
        Indeed, Hennig, Gauthier and others have pointed out that Linnean
taxa are *not* very useful for thinking and working with living and extinct
organisms in an evolutionary context. It is more useful to use phylogenetic
taxa, groups defined on the basis of common ancestry and descent. This is
because these groups are real entities, derived by evolutionary processes,
which may be found in nature. Couching one's considerations of phylogeny in
terms of phylogenetic taxa is more likely to lead to a concrete, workable
hypotheses about patterns and consequences of evolution.

>If it's relatively easy to classify into a taxon an organism
>that displays clearly carachters typical of that taxon (because *we* have
>stated that all organisms with those carachters will be grouped in it),
>problems may rise with an organism with carachters that are between two
>taxon:
        This is precisely the reason why Gauthier and de Quiroz have
suggested we abandon character-based (or diagnosis-based, if you will) taxa
and base all of our taxon definitions on the principle of common ancestry
and descent. This moves taxonomy away from a classification and into the
realm of recognition of real evolutionary groups and processes.

>but it (the organism) simply exists (or existed). Saying that e.g.
>Archaeopteryx was not a dinosaur only because it was feathered and so it was
>automatically a bird is a thautology.
        Well, it is certainly a bad idea to define (or diagnose) birds based
on the presence of feathers, as we have no proof that feathers did not
evolve earlier.
        Under a phylogenetic definition of Dinosauria, _Archaeopteryx_ is
dinosaur because it stems from the common ancestor of dinosaurs. Of course,
under such a definition, *all* birds are dinosaurs.
        As if this isn't confusing enough, since the common ancestor of
dinosaurs is defined phylogenetically as "the most recent common ancestor of
birds and _Triceratops_ and all of its descendants", within a phylogenetic
frame of reference birds will *always* be dinosaurs under anyone's
interpretations of the specific relationships of birds and other animals,
even if it means that other "dinosaurs" (say maybe sauropods) might not be
members of the Dinosauria.

>P.S. Only a precisation: birds and reptiles are not orders but classes and
>dinosauria are inscripted to two (some authors say more) orders.
        A large amount of work has been done in the past two or three
decades which has demonstrated that the "orders" Saurischia (when
resconstituted without _Ornithosuchus_ and some other critters, and with
birds thrown in) and Ornithischia, as they traditionally used, form a
monophyletic group outside of other "reptiles".
        Of course, at some level, they always formed a monophyletic group
(all life is decended from a common ancestor), and the idea of a
polyphyletic dinosauria is as much a result of the non-evolutionary
structure of the Linnean system as it is a misjudgement on the part of
scholars past.
        What were the other orders, I've never heard of them?
----------------------------------------------------------------------------
      Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock TX 79409
            Web Page:  http://faraday.clas.virginia.edu/~jrw6f