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RE: SCIENCE AND CLASSIFICATION



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Ken Kinman
>
>       Anyway, I have been watching the bird-dinosaur debate for
> many, many
> years, and have immersed myself more deeply into it for the past
> couple of
> years.  Even among the vast majority, who now agree that birds
> are dinosaur
> descendants, there have already been endless debates over whether
> mononykines are birds, or if Rahonavis is a bird, or Caudipteryx,
> Protarchaeopteryx, Bambiraptor, Unenlagia ("half-bird"), Microraptor,
> oviraptors, and so on (and then there's the Protoavis mess which
> is a story
> unto itself).

Indeed.   All of these questions, though, are ultimately phylogenetic: where
do these various theropods fit relative to the Archaeopteryx-modern bird
clade.  People who find these taxa fall outside that clade are happy to say
that they aren't birds; people who find them within that clade are happy to
call them birds.

> But we
> now have sufficient data to define Aves in a more scientifically rigorous
> fashion (osteologically), just as was done with Mammalia decades
> ago.

Do we indeed?  You can't use the semilunate carpal block, as the homologies
and variablity of that element EVEN IN A SINGLE INDIVIDUAL means that you
could have one arm of animal a bird and the other one not...  So what
precisely are you going to use?


>  The
> presence of hair and lactation roughly corresponds to the scientific
> definition,

We do not know that.  We do not know if Cynognathus, or Thrinaxodon, or
Morganucodon, or Diarthrognathus, or any of these Permian and Triassic
derived synapsids had hair and/or gave milk.

Furthermore, at what point is lactation lactation?  Like other tissues,
mammary glands did not appear suddenly out of whole cloth?  There would
almost certainly have been several steps in the development from a
run-of-the-mill sweat gland (or whatever was the direct precursor) to the
milk-leaking element of monotremes.

> so there have been no big problems reconciling the two
> (communication went smoothly, at least until cladistic
> splintering muddled
> it up with Mammaliamorpha and the like).

Communication went smoothly, as long as you didn't look in the technical
literature.  Were tricondonts always considered mammals?  How about
morganucodonts?  Or monotremes?  Take a look.

> All those more primitive
> "protofeathers" on
> Sinosauropteryx,

Which may be more complex than originally thought (see the recent Canadian
Journal of Earth Sciences paper)

> the newly-discovered pterosaur,

So now you are relying on poorly translated newspaper copy as evidence?

> and even that
> bristly-tailed psittacosaur, are apparently just that (PROTOfeather
> homologs).  Whether the structures of Beipiaosaurus are best regarded as
> advanced protofeathers or primitive vaned feathers remains to be
> seen.  In
> any case, a precise osteological definition should take precedence over
> feathers, eggshells, origins of flight, and other characters that
> fossilize
> poorly (and that obviously includes bird excrement as well).  :-)

Well, one would hope so.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796