Re: Sauropod Necks

["Matthew Bonnan" <mbonnan@hotmail.com>]

Jeffrey Martz said:


>     When Ken Carpenter mounted the Diplodocus at DMNH (1494), he got to 
> play
> with the actual vertebrae rather then a computer simulation,

Okay, but the computer model was based on careful digitizing of the actual 
zygapophyses in two very well known and complete diplodocids: Apatosaurus 
louisae CM 3018 and Diplodocus carnegii CM 84/94.  Here is the reference for 
those interested in reading the paper:

Stevens, K. A., and Parrish, J. M. 1999. Neck posture and feeding habits of 
two Jurassic sauropod dinosaurs. Science, 284:798-800.

While I applaud the efforts of anyone who has ever taken on the task of 
putting up a sauropod skeleton mount, it is often times difficult to 
manipulate more than one section of a limb or neck at a time, because the 
material is both heavy and fragile.  I have no doubts that Ken Carpenter was 
extremely careful and judicious in his mounting of Diplodocus hayi at 
Denver.  However, it would have been impossible for him or anyone to have 
taken the entire series of cervical vertebrae and moved them as a whole into 
various poses.

That's where Stevens and Parrish come into the picture.  They wanted to see 
what would happen if they could string the verts together and moved them as 
a whole interconnecting unit rather than one vert at a time.  Keep in mind 
that the model did not take into account cartilage, ligaments, and other 
soft structures that would have further RESTRICTED the motions they 
reported.  Therefore, the upper ranges of neck flexibility they reported are 
maxima based on where and when the zygapophyses contact each other.  For 
those who may not be aware, sauropod neck verts are not the same as the ones 
you see in a typical mammal or us.  In many mammals, the centrum (the body 
of vertebra) is flat on both ends, and fibrocartilage discs between the 
verts allow a sliding motion to occur, which, along with relatively open 
cervical zygapophyses, allows a lot of sliding and sometimes twisting 
motions.  In a sauropod, the centrum has a ball one end and a socket on the 
other.  This ball and socket arrangement alone greatly restricts sliding and 
twisting motions.  The zygapophyses further constrain these motions.

To quote from the paper:

"The neutral pose and flexibility among cervical vertebrae was constrained 
by the placement, size, and 3D shape of their pre- and postzygapophyses.  
The movement of adjacent vertebrae, relative to the ball-and-socket 
articulations of the centra, induces rotation and translation of the 
articulated pre- and postzygapophyses.  This movement places tension on the 
synovial capsule surrounding each zygapophyseal pair.  Our manipulation of 
muscle and ligament preparations of extant bird necks indicated that 
synovial capsules constrain movement such that paired pre- and 
postzygapophyses could only be displaced to the point where the margin of 
one facet reaches roughly the midpoint of the other facet, at which point 
the capsule is stretched taut.  In other words, one facet could slip upon 
the other until their overlap was reduced to about 50%.  In vivo, muscles, 
liagments, and fascia may have further limited movement; thus, the digital 
manipulations reported here represent a 'best case' scenario for neck 
mobility."

Because of the size and fragility of the sauropod bones, making a computer 
model of the neck is about the best option we have now to investigate 
sauropod neck flexibility.  Perhaps at a future date, we will have access to 
full, three-dimensional, perfect scans of the cervical verts of every 
sauropod.  For now, we have a new way to investigate the functional 
morphology of the entire neck series of two diplodocid sauropods that both 
spares possible damage to the bones and allows us to investigate numerous 
hypotheses on neck posture that were simply not possible before.

Matt Bonnan
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