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Re: Dinosaur Beaks



Kris Kripchak (MariusRomanus@aol.com) wrote:

<Jaime... Please show in what post you delivered clear evidence that
conclusively goes against the premaxilla of "Archaeopteryx", and the other
theropods in question, having a single keratinous covering. This will
answer both my question and the one posted by Mike.>

  This tone has been the essential problem of the entire debate: I never
said *Archaeopteryx* lacked a premaxillary beak. I said that based on
extant observation of the rostra of turtles and birds, which lack an
extensive foraminate surface and have very thin, delicate bone, would seem
to offer that this condition is unlikely in *Archaeopteryx*. Since we do
not know whether they had one or not, based on fossil preservation, it is
not possible as yet to say *Archaeopteryx* did or did not have one. That
*Confuciusornis* shows a beak only on _some_ specimens supports this
statement. We can now reasonably assume that _all_ individuals of *C.
sanctus* and *C. dui* had a beak, maybe of different shapes per species or
gender (cannot tell).

<Now, to be perfectly clear, I do not want variations on the surface of
the bone found under rhamphotheca. I want evidence that goes against there
being a continuous covering on the premaxilla.>

  Really? I explained this data both before (my very FIRST reply to David,
to which YOU initially responded) and above. Variations in the
premaxillary surface as in corugations and grooves occur in some beaked
animals and means of finding data in dinosaurs that may corroborate in a
beaked dinosaur the presence of a rhamphotheca. In hadrosaurs, for
instance, this is true. Ridges and crenellations occur frequently in
living beaked forms, but foramina do not except in select regions: the tip
of the rostrum in *Apteryx australis* relates to a softer keratin that is
flexible and is underlain by numerous nerves for substrate probing.

  There are reasons for assuming the extensive external foramina,
continuous with more further back in the dentary, conform to a more
"reptilian" design of flesh and ligamental tissue around the jaw, rather
than a "beak", including comparison to lizards. The crocodilian condition
of extensive external foramina relates to sensory organs in the skin that
are assumed on grounded data (basal taxa lack the facial pits and
foramina, they are arranged as in lizards only close to the oral margin)
to be unique to crocodilians and not likely in theropods. These are
variably arranged and extensive withint theropods, as well as other
dinosaurs. Based on current data, and ongoing research of which Dr. Larry
Witmer is at the forefront of, it is not possible to determine either/or.
I never said the condition was definite either way, as it seems I have
been assumed to have said, and I would like this to be acknowledged.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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