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Re: Sauropod Necks

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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