Sauropod Neck Mechanics (Was: Gallery and Commentary for Copenhagen Mamenchisaurus)

[Mike Taylor <mike@indexdata.com>]

> Date: Mon, 21 Jun 2004 23:03:29 -0400 (EDT)
> From: Danvarner@aol.com
> 
>        Another fascinating paper (not freely available, I'm afraid)
> on the subject is, "The ligament system in the neck of Rhea
> americana and its implication for the bifurcated neural spines of
> sauropod dinosaurs" by Takanobu Tsuihiji, Journal of Vertebrate
> Paleontology, 24(1), March 2004. Dr. Holtz mentioned this one a
> while back, but I haven't heard much discussion about it.

I have issues with that paper.  It makes a big deal about how it's
using Witmer's extant phylogenetic bracket method, but as the paper
says itself (p. 165, right column, last paragraph), "The bifurcated
neural spines are associated with a ligament system that is unique to
_Rhea_ among modern Aves".  The only way you can apply EBP to derive
information about dinosaurs is if the feature is _primitive_ in birds
_and_ is also found in crocs, so the whole argument EPB falls down on
two counts.

What's left is an argument that reduces essentially to "_Camarasaurus_
cervicals are morphologically similar to _Rhea_ cervicals, so they
might have had similar ligaments".  But in fact the cervicals are not
very similar at all, in part because _Rhea_ has nothing that a
sauropodophile would recognise as neural spines (compare Figure 2C
with Figure 5).

> It certainly makes Camarasaurus a new beast, somewhat reminiscent of
> one of Christman's reconstructions.

Yes indeed.  Figure 4 of the paper shows pretty much the entire
presacral sequence, but because it's rotated anticlockwise the
weirdness of the animal is minimised -- it sort of looks like a long
neck.  But turn the page fifteen degrees clockwise, so the dorsal
sequence is horizontal, then imagine how the neck would look if this
were indeed how the ligament attached.  Weird indeed.  It's also not
clear from the paper where Tsuihiji thinks the epaxial muscles go on
such a beast, but I don't know enough about extant bird necks to
comment on that.

Finally, can anyone explain the last paragraph of this paper?

        Functionally, the tensional force of this postulated
        ligament would work most effectively when the neck
        tilts downward and the insertions of the ligament are
        below the level of its origin.  Stevens and Parrish
        (1999) reconstructed the neutral posture of the neck
        in both Apatosaurus and Diplodocus as extending almost
        straight forward with a downward slope.  As they
        suggested, a ligament like the one I propose would
        have been taut in this posture and would have exerted
        a stronger tensional force than it would in a more
        erect neck posture.  Furthermore, its insertion on
        scars at the bases of the cleft between the
        metapophyses, rather than on the top of the neural
        spine, would have increased the leverage of the
        tensional force of the ligament by lowering the
        positions of its insertion, thereby making this
        ligament a more effective bracing device.

The last sentence seems exactly wrong to me.  Surely there is more
mechanical advantage to be gained by anchoring on the metapophyses
than in the interspinal trough?

 _/|_    _______________________________________________________________
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