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Re: (no subject)
> Date: Sat, 20 Dec 2003 16:15:33 EST
> From: MarkSabercat@aol.com
> [Lots of good stuff snipped.]
> I think that Martin et al are correct in assessing the abilities of
> the two major neck bracing systems of sauropods as effective
> adaptations to supporting the neck horizontally, but I disagree that
> this would not be effective in holding the neck in a subvertical
> position as well for feeding.
For those who've not come across it, Mark's referring to
MARTIN, J., MARTIN-ROLLAND, V. and FREY, E. 1998. Not cranes
or masts, but beams: the biomechanics of sauropod necks.
_Oryctos_, 1, 113-120
which is neatly summarised in an article in the Currie & Padian
encyclopedia. What the basically says is that there seem to be two
separate forms of bracing in sauropods, corresponding to what's seen
in made-made cantilever structures. The one we're probably most
familiar with is dorsal bracing with a tension member (e.g. the
putative nuchal ligament analogue that Alexander has speculated
about). In this arrangement, the centra of the cevical vertebrae are
in compression, and something above them, attached to the neural
spine, is in tension. That might be a big elastin ligament as
Alexander suggests, or lots of small ligaments joining adjacent
vertebrae, or muscles, or of course a combination.
So far, so good. The second model that Martin et al. suggest is of
ventral bracing with a compression member. In this scenario, the
cervical centra are in tension, and something below them -- most
likely the cervical ribs -- are in compression.
(The third model is a combination of dorsal and ventral bracing.)
The problems with the ventral bracing model are two-fold. First, it
requires that the centra and the intervertebral cartilaginous discs
are strong in tension, which does not seem obviously true. And
second, it requires the cervical ribs to be strong in compresson,
which seems obviously false. (I can expand on "obviously" if
necessary, but for now, just consider the cross-sectional area of the
cervical ribs, and consider how great a compressional stress they
would be under. It ain't gonna work, folks.) Crucially, the Martin
et al paper contains no figures at all: there's no attempt to work
through the forces involved.
I happened to find myself sitting next to John Martin at the recent
British Dinosaurs meeting on the Isle of Wight, so I asked him about
this between sessions. His answer was basically that he also
considers ventral bracing pretty unlikely, but thought it was worth
publishing on because it was important that it got into print.
But here's the thing. What the heck _was_ _Mamenchisaurus_ doing for
bracing? Because it's got this monster-long 13m neck, but the most
feeble little neural spine, which would have given any dorsal tension
members very little leverage. So how did it hold its neck up? The
most parsimonious explanation at present is probably Magical Rays From
Outer Space, but I welcome any alternative suggestions. Comparision
with Alexander's worked examples of nuchal ligaments and muscles for
_Diplodocus_ necks strongly suggests that neither elastin ligaments
nor muscle would have done it for _Mamenchisaurus_. So, I dunno,
collagen ligaments? But would they give enough flexibility?
Which brings me to another question. Does anyone know a good book I
can get where I can look up basic stuff like the Young's modulus and
yield point of collagen, elastin, muscle and bone? Or is this going
to be another case of "It's sprinkled through the literature, you have
to read several dozen papers, and there's no way to find out which
/o ) \/ Mike Taylor <email@example.com> http://www.miketaylor.org.uk
)_v__/\ "You're a bright lad: we could do with someone like you to
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