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Re: Dinosaur beaks



Kris Kripchak (MariusRomanus@aol.com) wrote:

<The feet of a bird and most likely theropods of similar size and build
are covered almost always with a very heavy podotheca that has very little
in the way of (1) Open areas of soft tissue. (2) Internal bacteria like
which is found in the mouth and (3) Items such as mucus, eye balls ect...
that bacteria, hence decay, can build on before burial. I collect bird
bones and it is very common to have a bird that has lost all of its flesh
on its head and is down to the bare bone and yet the legs, usually from
the proximal end of the tarsometatarsus down, are still covered with a
full podotheca... nails and all. Arm chair hypotheses often do not work
out very well in the real world.>

  Hoping to draw attention away from the idea of "arm-chair" hypotheses,
but David was correct on the basic statement that only the claw sheaths
are preserved, suggestive of their condition being limited (heavy, hard
keratinous coverage versus light, as in "podotheca"). In *Confuciusornis*,
rhamphotheca are preserved, as are claw sheaths, but not the pedal scales
as hard traces or carbonized elements on the slab surface, whereas in NGMC
91 (or "Dave") the claw sheaths are present, and impressions rather than
material traces of the foot integument show many small, polygonal scales
as in the variegated pattern seen in falconiforms, called "reticulate."
Fossil preservation does not follow patterns modern birds suggest, and the
difference between hard and soft keratin preservation, as above, would
imply that "hide" as in "podotheca" should preserve, but it doesn't.
Despite this, artists still do, and are welcome to do so, restore the
pedal scales as are not even indicated in fossil crocs from Messel but
would certainly have been there. In this manner, the extant phylogenetic
bracket holds.

  It does not, as the post this replies to does not relate to, keratinized
beaks of any form, which as a hard keratin usually preserves as some trace
along with feathers and hair, and the outlines or sheaths of claws, or
hard scales with an osseous core (some croc scutes, for instance). In
modern animals, most beaks forms have a smooth, unforamenated bone beneath
the sheath, as in birds, often with the posterior edges and the very tip
being highly porous, thin, and very weak in construction, but with a
cancellous internal structure; this is seen in the rostra and mandibular
symphyses. Occassionally, this beak will have numerous vascular grooves,
but this is not a truism for bird beaks, in general, and typically occur
in BIG beaks, such as phororhacids, gastornithids, etc.. Greg Paul has
used the suggestion of a highly foraminated premaxilla and rostral dentary
(1988, _Predatory Dinosaurs of the World_, discussion on *Ornitholestes*
and *Velociraptor*) to support applying a rhamphotheca, but it has been my
own observation and examination of de-keratinized beaks that this
condition does not relate unequivocally or even generally to beak
placement.

  Fire away,

  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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