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Re: Parrish's neck work ...



I wrote:

<<This would be easily explained if the muscles of the
neck were brought into question. Sauropods like the
"euhelopodids" and neosauropodans (brachios, titanos,
diplos, apatos) had enormously sized scapulae,
probably for the express purpose of supporting the
trapezoideus and related neck muscles, to support the
neck. Now, granted, the biggest scapulae to back
muscles belong to sauropods like *Brachiosaurus* and
*Euhelopus,* exceptional with upright or semi-upright
necks, but this accentuates my point.>>

Matthew Bonnan wrote:
 
<Yes, I pounded that response out quickly, and didn't
mention the neck musculature per se. However, a few
things to note. One is that our understanding of
sauropod back musculature is pretty poor. While one
can do a rough qualitative assessment of back muscle
"size," determining how much certain muscles
contributed to neck and back elevation is still pretty
much in the range of speculation (although I wish I
had a sauropod to dissect so we could know for sure).>

[snip]
 
<Using zygapophyses to determine neck constraints,
while it does not take ligaments, muscles, and tendons
into account, is a solid, concrete way of examining
neck movement. It is a testable, repeatable,
falsifiable, and predictive model. Until we find a
good way of repeatedly interpreting neck muscle
origins and insertions, using a model like Parrish and
Stevens, or others who may do similar bony things, are
right now our best bet...>

  The use of zygapophyses to determine max or min
range of movement is not entirely an ideal model; such
processes do not absolutely limit the movement of the
cervicals, as there can be a limited amount of
dislocation a pair of bones can deal with without
causing undue pain or discomfort. We experience this
ourselves. The animals can, and oft-times will,
stretch their bones to their limits, and sometimes
beyond, to do something they _really_ want to do. What
exactly that is is up to the animal. Usually, as in
giraffes, us, etc. it's for scoping out or feeding.

  This was expounded on previouisly on the list, last
year I think, when Nathan Myhrvold's work was first
announced on the "super sonic tail" of diplos, and the
original _Discover_ article was out on both
Myhrvold's, and Parrish's and Stevens' work.

<<*Antarctosaurus wichmanianus* had a very squared off
jaw, and perhaps perfectly exapted to graze with, as
is seen in grazing animals like the black rhinoceros
as opposed to the white rhinoceros, whose snout is
more pointed. *Diplodocus* has a more rounded front
end of the snout, and the tooth pattern looks like a
cookie cutter formed of an arc of a third of a circle,
with a single, flawless, corrugated edge; the lower
set occlude inside the upper set, perfect for chomping
with. *Apatosaurus* was more pointed, and narrower
relatively, but formed a semi-circle...>>

<...we must be careful when comparing sauropods, or
any dinosaurs, to living mammals. While it is probably
safe to assume that certain convergences in gross
feeding styles have reoccurred (squared-off muzzles
with grazing, rounded muzzles with browsing), black
rhino do not have pin-heads on the ends of long necks.
 =)  Furthermore, beyond studies on tooth occlusal
wear, I am unaware of any serious modeling studies on
jaw mechanics in sauropods, where the researchers
looked at tooth occlusion, jaw movement, etc., in a
variety of sauropods. Probably this has not happened
yet because sauropod skulls are rare and not very
available.>

  It is, however, readily available in the fossils
themselves. For instance, at least in two
*Camarasaurus* skulls and one *Diplodocus* skull (the
first two are the famous "baby Camara" and the skull
in the Dinosaur National Monument wall, the second
refered to as "D. sp" in Glut's 1997 book [sorry, no
specimen number at hand]). The first two show that the
teeth of *Camarasaurus* (at least) alternate and fit
together like interlocking fingers, or the points
anyway, and posteriorly the dentary teeth are medial
to the maxillary set. In *Diplodocus,* the entire set
of lower teeth are behind the upperset, and the
specimen is of a closed-jawed fossil.

  Whether these features are taphonomical I haven't
read on, but the high occurance of medial or posterior
offsetting to produce an occlusive action would
suggest either posterior muscular contractions for the
latter, or crushing for both (not evident in the
former), or natural articulation for all.

  As for tooth occlusal wear, I do believe brachiosaur
and camarasaur teeth have occlusive facets, especially
as noted in *Pleurocoelus* (= *Astrodon*?), and
similar tooth taxa. Diplodocid teeth are rather
inappropriately refered to as "pencil-like" because
that suggests they were pointed, and they're not.
Individual diplo teeth (in Apato and Diplo) have
chisel-shaped heads, with a slightly concave rear
surface near the tip. Perfect tools for shearing.

  However, I do agree with you in that sauropod
feeding mechanisms, aside from Bakker's 1979(?) work,
should be apporached more avidly, given the paucity of
data on them, and especially on their teeth.

===
- Greek proverb: "Knowledge is Inherent;
  Stupidity is Learned." -

Jaime A. Headden

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