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Neutral pose



One last thing on the sauropod neck posture discussion, which I missed
($#!@& non-paleo fieldwork!).

In addition to doing range-of-motion estimates, Stevens and Parrish also
presented reconstructions of Apatosaurus and Diplodocus with their necks
in neutral pose, i.e. with the neck neither extended nor flexed, with
zygapophyses overlapping maximally.  These are the reconstructions that
show the diplodocids' necks angling gently downward to put the head near
the ground.  If I recall correctly, they also asserted that the neutral
pose probably represented the pose in which the neck was normally
carried, as any elevated pose would require 'continuous firing of
cervical dorsiflexors.'

In their 1985 study "The mechanics of ligamentum nuchae in some
artiodactyls," Dimery et al. found that the only artidactyl whose nuchal
ligament was strong enough to support the neck and head without help
from the epaxial musculature was the camel, and (again, IIRC) that even
then the nuchal ligament alone could only support the neck in a
horizontal posture.  So if a camel holds its head and neck in any
posture other than horizontal, its cervical dorsiflexors must be firing
continuously.  Same goes for the other artiodactyls in any elevated
posture.

Now, extrapolating from artiodactyls to sauropods is pretty risky
because the ligaments and musculature of the neck are quite different in
the two groups, at least based on osteological evidence and comparative
anatomical studies of extant saurischians.  But without referencing
specific muscles, it seems to me that if artiodactyls exert continuous
muscular force to hold the head and neck above neutral pose, then
sauropods may have also.  At the very least, Stevens and Parrish assumed
that 'continuous firing of the cervical dorsiflexors' was something that
happened only rarely, and I am curious about the basis for that
assumption.  In a lot of animals, certain muscle groups that have to
exert some level of tension almost continuously during normal standing
and walking.  Is it reasonable to assume that sauropod cervical
dorsiflexors did not? (I am asking out of genuine ignorance here)

None of this should be taken as a dig at the work of Stevens and
Parrish.  I know them both, I like them both, and I have tremendous
respect for them for actually developing a way to test their hypotheses
about sauropod biomechanics, instead of just spouting unfounded
speculation like many others, myself included.  I just want to know how
reasonable that particular assumption is.

Thanks in advance for any replies.

Matt Wedel

P.S.  By the by, interested parties should check out R. McNeill
Alexanders' "Dynamics of Dinosaurs and Other Extinct Giants" and Chris
McGowan's "Dinosaurs, Spitfires, and Sea Dragons."  Both books tackle
the hot topics in sauropod paleobiology, and even better, both books
reference a ton of cool papers on biomechanics, which is how I gleaned
what little knowledge I have on the topic.