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Re: Slidey Zygs



Mike,


>So can I take it from the silence that Scott Hartman and I are the
>only people on the list interested in zgyphyseal disarticulation?  :-)


By no means!  I'm just up to my ears in other stuff at the moment, and
sinking fast...

Have you looked at Slipper's classic on this?  (E.J. Slipper, Comparative
biologic-anatomical investigations the vertebral column and spinal
musculature of mammals, 1946.  My copy is in its own binding (done by the
library here - it's and ex library book), but there's a label at the front
listing the the source as Verh. K. ned. Akad. Wet. Amsterdam (2) V.42(5),
pp1-128).

Since you seem to be developing a fascination with vertebral functional
morphology (can't think why ;-) i think this is a must have for you, and
I've been meaning to copy it and send it over as soon as my reprint of the
Brodie paper arrives (you'll like that too).

Slipper gives most of his attention to the vertebral bodies, the spines, and
the musculature (also of interest to you, judging by your post on
flamingoes - and I think you'll adjust your concepts of the factors
affecting neural spine morphology when you go through it).  Usefully, he
illustrates the maximum range of movement for a number of common mammals
(dogs, cats, horses).  If you were able to get hold of an vertbral column
form any of these animals (does the BMNH have anything in it anymore?) and
flex it to the limits illustrated, you'd be able to see for yourself the
degree of zygapophyseal sliding involved.  I've been meaning to do this for
a while with some sheep and kangaroo bones I've picked up, but I haven't had
the time.  (I'll probably have to start teaching this before I get around to
it....)

He doesn't talk about the zygapophyses much, but about limits to vertebral
mobility he does say;

"Very little is know about the factors that limit the mobility of some parts
of the vertebral column.  As follows from the data of table 4, a general
rule about this question up to the present moment cannot be given.  Neither
the position of the zygapophysial articular surfaces, nor the absence or
presence of anapophyses gives a satisfactory explanation.  The anapophyses
indeed are able to limit the mobility to a very high degree (especially,
however, in a lateral direction; fig2, 107), but there are also mammals that
show a comparatively large mobility in the region provided with these
apophyses (fig. 56).  In the first place, the anapophyses serve as muscular
attachments." [p. 59]

I make no claims of understanding the functional morphology of vertebrae -
I'm still pretty confused about many aspects of this - but I've wondered for
a while about some of the standard assumptions about this subject that seem
to get handed down in vertebrate palaeontology (particularly from those of
us who haven't done anatomy in any depth), and the old story about the
function of the zygapophyses being to limit movement seems to be one of
these.  I can see how the zyg's can effectively prevent rotation of the
vertebrae about the cranial-caudal axis, and how this can be a good thing
(rotation is a good way to disrupt the spinal chord).  I can also see how
the zyg's can be useful in helping to brace shear stresses across the column
which might otherwise dislocate the vertebrae (and also disrupt the chord),
but they can't do this on their own and the large and complex array of
spinal ligaments and muscles, together with the shape of the articular faces
of the centra, are possibly more important than the zygapophyses in
preventing dislocation as a result of shear.  But in the vertebral columns
that I've played with, I can't get the zygapophyses to restrict lateral
bending or dorso-ventral bending at anything other than extreme angles.  I
wonder how much the spinal ligaments and muscles play a more important role
in this.  Consider the difference in the flexiblilty of your spine compared
with that of a contortionist - the difference is in the soft tissues, not
the bones.

Don't know how useful any of this is, other than to confuse you still
further.  Perhaps you should post this thread across to the vrtpaleo list -
there are some very competant anatomists on that list who I don't think
subscribe to this one.  They may be able to clear this up for a little.  But
if there is anyone here who can tell us the real story (Scott Hartman?  Matt
Bonnan?) then I'd love to hear it as well.

 >My question: does anyone know of published work on the extent to which
>zygophyses can slide across each other, ideally in a variety of taxa?
>Or is Stevens & Parrish's "unpublished data" the state of the art?


As I recall from Steven's presentation of DinoMorph at the AMNH SVP (1996),
they didn't consider cervical ribs in the model.  I don't know if they've
incorporated them since, but I don't understand how you can say anything
meaningful about neck mobility without considering them - look at how they
restrict neck flexibility in crocs.

>(Subsidiary question: does anyone know where I can get a wide
>selection of necks on the cheap?  :-)


I've recently discovered that butchers and abbatoirs are no good, because
health regulations here require the carcass to be processes in a prescribed
manner that is of limited use for investigating this question (except as an
excuse for the red wine essential in initial hypothesis formualtion....).  I
imagine the same is true in most OECD countries.  Knackerers can be better,
because they are subject to less stringent regulations, although you'll need
to find one sympathetic to your cause.  Best source of necks here is
roadkill - you can take your pick of just about any species of small,
medium, or large mammal, bird, or reptile.  Good enough excuse for a trip
over?

Cheers
Colin

 P.S. I heard there was a great game of rugby at the weekend.....I think we
had a Guinness on that result, didn't we?