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Re: Parrish's neck work ...



Jaime:

You said, in relation to my response on the nuchal ligament:
"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."

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).

Yet another problem is the origins and insertions of muscles.  The
trapezius in living archosaurs originates on the more posterior
cervical vertebrae and first several dorsal verts and inserts onto a
portion of the blade of the scapula.  A muscle's insertion is where
the action takes place, and this would mean the trapezius would have
had an affect on the shoulder, not on the neck.  Plus, the trapezius's
action is dependent on other scapular and cervical muscles -- while it
can assist with neck extension, it is only to a small degree.  It's
major action in a quadrupedal vertebrate is assisting the various
shoulder, pectoral, and upper limb muscles with swinging the arm
forward and elevating the scapula.  And, just to note, the trapezius
does not leave any good origin and insertion marks.

The scapula is the location of the rotator cuff muscles, the deltoid,
and many origins and insertions of shoulder and arm muscles, but it
does not "support neck muscles."  While some neck muscles originate on
the scapula, many originate on the dorsal and cervical vertebrae
themselves.  While your observations of the differing sizes of
scapulae may indeed have a functional siginficance (in fact, one I had
not off-hand considered), I don't think the scapulae will tell you
much about neck musculature.

As far as neck musculature of birds and crocs, it is very complicated
and we have very few good muscle landmarks.  The musculature in bird
necks is described in some texts, but I am unaware of anyone who has
done a detailed, comprehensive anaylsis of muscle origin, insertion,
action, and innervation on bird necks.  This makes modeling muscle
contributions to sauropod neck action again rather speculative -- and
even if you draw on muscles based after birds or crocs, these pictures
do little to tell you what the range of motion was.  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 (although they can always be
falsifed!).  Man, I wish I had that sauropod dissection manual ... =)

You said:
" *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. Adaptations in
specific feeding styles, and food, probably, as Krause
et al. have proposed for the Mahajanga titanosaurs,
and as is seen in other titanosaurs, the later
compared to the earlier."

While your observations are estute, 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.  Perhaps in a few
more years, good scale models based of MRIs or CT scans of the skulls
can be used to do the trick.  Does anyone on this list know of
sauropod jaw modeling studies?

Good comments.  There is a lot of un-trodden sauropod territory out
there with respect to functional studies.  Many folks assume the work
has been done, but you'd be surprised at what has actually been
studied and what still needs to be investigated.  We will not run out
of interesting questions and scenarios for these lovely beasts.

Matt Bonnan
Dept. Biological Sciences
Northern Illinois University