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Re: Parrish's neck work ...
Okay. You said:
"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."
Well, first off, the use of zygapophyses to constrain movement is not
a perfect system, but then nothing in vertebrate paleo functional
studies ever is. My point was that other researchers who may disagree
with Parrish and Stevens can go back and repeat what they did.
Another thing to remember is that these motions represent the
theroetical limit to what was possible with neck movements -- adding
ligaments, muscles, etc., would futher CONSTRAIN not enhance the
No vertebrate that I have ever seen or dissected commonly partially
dislocates joints in its body. When a giraffe stretches its neck, it
is definitely placing strains on its zygapophyses, but they do not
come apart! Consider what would happen if you dislocated one of your
cervical vertebrae while stretching. At each junction between
vertebrae there is a foramen. Through this foramen pass the cervical
nerves. Were you to dislocate your cervical vertebrae, not only would
you feel intense pain, but the foramen (which is formed by the two
articulated vertebrae) might constrict, pinching or severing nerves!
And, let's not forget that the spinal cord itself runs through the
neural foramen, underneath the neural spines. Dislocate the
vertebrae, and you may have paralyzed yourself. It is in a
vertebrate's best interest NOT to dislocate vertebrae, even only
partially. I have a friend in a lab where they kill rats for biochem
studies. They kill them by "cervical dislocation."
I hate to use anecdotes, but to drive the point home, one of my
brothers briefly dislocated his elbow joint once while cheering at a
game. His joint popped back together, good as new, except it caught
his ulnar nerve (known as the funny bone). Ouch! The doctor had to
dislocate his arm again to free the pinched nerve. Can you point me
toward the reference where you heard this?
You said: "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."
Okay, but again my point is that models or something quantitative
needs to be done if we are to begin to understand what these animals
were doing to produce occlusal wear facets. Is it due to the tough
plants they were eating, or is it due to tooth-to-tooth contact, or
both? Dentists make molds of pateints' teeth all the time, and see
where and how the teeth are occluding. It is different, of course,
for dinosaurs since their teeth were shed continuously, but a study
like I mention I am unaware of being done.
If you really do have a reference on the joint-disarticulation thing,
I would love to read it, because I have never heard of it in all my
anatomy learning and teaching. Thanks for the reply.
Dept. Biological Sciences
Northern Illinois University