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Re: Nemegtosaurus: brach like skull?



In a message dated 96-07-11 13:00:26 EDT, Thomas_R_HOLTZ@umail.umd.edu
(th81) writes:

> Instead of filling in the eroded portion with a flat,
> Diplodocus-type skull, he restored it after Brachiosaurus.  And it
> works.  It looks good.  I flipped back and forth from pictures of
> Dicraeosaurus, Brachiosaurus, Diplodocus, and Nemegtosaurus, and
> there are a lot of derived features similar between Nemegtosaurus
> (and Quaesitosaurus) and Brachiosaurus.

One of the key unsolved problems in sauropod phylogeny involves the lack of
sauropod genera with skull anatomies intermediate between those of
brachiosaurids/camarasaurids/euhelopodids and diplodocids/dicraeosaurids.
Just how was this transition effected? What would the skull of such an
intermediate form have looked like? Like Coria's reconstruction of the skull
of _Nemegtosaurus_?

Another unanswered question concerns the position of the external nares in
sauropods. I've seen no convincing arguments to explain the shift in the
location of the nares from near the tip of the snout (primitive for
dinosaurs, not just sauropods) to the skull roof in between the orbits. This
is accomplished by a considerable elongation of the premaxillaries at the
expense of the nasals and frontals. Every known sauropod shows at least some
displacement of the nares caudally from the primitive position; it's a
sauropod synapomorphy. Why? What purpose did it serve to get the nares away
from the tip of the snout? It must have aided breathing. Did it help the
animal breathe better by eliminating the slight to moderate right-angled bend
the air would have had to make through the skull after passing up that
enormous trachea during exhalation? Or did it allow the animal to breathe
continuously while feeding, something like a secondary palate does? If so,
does this indicate that sauropods >needed< to breathe continuously because
they had a rapid metabolism?