Ungulate-Type Nuchal Ligaments in Hadrosaurs? The Fossils Say No?

["Jonathan R. Wagner" <znc14@TTACS.TTU.EDU>]

        Hello all, long time no hear... I am currently steeped in the throes
of hadrobabble, producing my thesis in nowhere near record time. Again, it
was nice meeting so many of you at SVP lo nearly half a year past now.
        I took a little time out this weekend to ponder something this
weekend, and I thought I'd share...
        Greg Paul is in my mind THE foremost living dinosaur paleoartist,
both for his contributions to the field, his pertinent observations, and his
extremely good reconstructions (I trust GSP's hadrosaur skull reco.s, and he
is the ONLY individual I can say that about*). So, when he started restoring
hadrosaurs with ungulate-like nuchal ligaments spanning the length of the
neck from above the shoudler girdle to the base of the skull, I was forced
to take the idea seriously. That Paul would take this Czerkas Czuggestion
(tm) and run with it (to the point of modifying all of his skeletal
reconstructions) meant there must be some merit to the idea...
        ... And indeed there is. Surely, you'd think anything that could
keep these boys from having to hold up their big fat heads on those skinny
necks would be helpful. And, of course, there are prominant attachment
points for nuchal ligaments on the skull. Now, none of this constitutes
positive evidence of such ligaments, and, AFAIK, no evidence has shown up in
known hadrosaur "mummies". However, the thought is worth entertaining.
        First, I'll go very very briefly into the biomechanics of the
situation. My persumption (I am not a mammalogist) is that the nuchal
ligaments of ungulates extend from approximately the whithers to the axis,
and tension in these ligaments holds the neck up when the animal is at rest.
I assembled an ultra-simple physical model of the condition illustrated by
Paul for hadrosaurs. I will not bother you with the math, it is simple and
can be worked out using an elementary physics book, but would probably glaze
most reader's eyes over instantly. According to the model, nuchal ligaments
serve to balance the head on the neck, and direct the weight of the cranium
into tension in the ligaments (and therefore shear stress on the vertebrae
they attach to) and into compressional force down the centra of the cervical
series.
        Anyway, I found that, given nuchal ligaments supporting the head in
a resting pose as Paul shows, and assuming a feeding posture with the elbows
bent, feeding nearly at ground level, any ungulate-like nuchal ligaments
would have to stretch to nearly twice their length in order to allow such a
range of movement.
        Here I must admit my profound ignorance of the properties of
ligaments. Does this sound reasonable to anyone? It certainly did not to me.
I assume that one reason that ungulate nuchal ligaments are positioned so
close to the base of the neck (rather than halfway back on the dorsal
column, on top of the cranial arch, as shown by Paul) was to compensate for
their relative inflexibility. So, although I am very much open to
counter-arguments, I suggest that perhaps nuchal ligaments may not have been
present in the form illustrated by Paul (presumably after Czerkas... I never
saw the original talk).
        So then I took another look at the skeletons of hadrosaurs. While
some hadrosaurs do seem to have a slight "whithers" hump over the dorsal
series, there does not appear to be a definite point in the column which
could be assigned as anchor for the ligaments. Therefore, there is nothing
to say that nuchal ligaments could not have attached lower/more cranially,
nearer to the base of the neck. This would sacrifice mechanical advantage
for a more reasonable feeding posture. The first or second dorsal usually
has a large neural spine and might be a good candidate. However, it should
be noted that there does not appear to be any thing peculiar or distinct
about the cranial dorsal series which might suggest an attachment for such
ligaments. Indeed, the vertebrae appear to transition smoothly to larger and
more robust structures going backwards, with no obvious breaks until halfway
back in the dorsal series.
        Another possibility is that hadrosaurs did not hold their heads so
upright. This seems less plausable on the grounds that it is best to keep
the head elevated in "travel mode" or for defense. However, we cannot simply
assume that this was necessary... if hadrosaurs did indeed herd there may
have been appropriate alternatives worked out among the group. If the head
were held lower, there would be a longer moment arm for the neck "lever",
and therefore stronger tendons would be needed, but they would have to
stretch less in order to reach the ground.
        And the third possibility holds that the nuchal ligaments were
arranged as in other dinosaurs, and the head was held back during normal
locomotion to direct most of the weight of the head down the cervical
centra, perhaps with strong ligaments at the base of the neck redirecting
the force there. While this would be perhaps less efficient, it would
probably offer a greater range of motion than the above possibilities, and
does not rely on suppositions for which there is no obvious evidence.
        In summary, I know of no positive evidence for the presence of
ungulate-like nuchal ligaments in hadrosaurs. Unless my assumptions
concerning the elasticity of ligaments is wholly wrong, it does not seem
likely that, if these ligaments were present, they were arranged as Greg
Paul illustrates. It is possible that they had their origin cranioventrally
along the cervical series, but there is (again) no positive evidence I know
of to support this. It is also possible that the head was held lower (and
subsequent postmortem distortion of the posture of the cervical series
accounts for the preserved articulation), but, again, there is no positive
evidence, and, AFAIK, the posture illustrated by Paul is concordant with the
orientation of the occipital condyle. Also, there does not seem to be any
particular *need* to posit such structures. Certainly they would be helpful,
if arranged properly, but I doubt that such an animal could not survive
without them.
        So, to conclude, I am not convinced that ungulate-like nuchal
ligaments (yes, I am aware of how poor my grammar is...) were present in
hadrosaurs, much less in the arrangement Paul illustrates. This is
especially welcome news to me, since I always thought Paul's old hadrosaur
illustrations were especially graceful, an element lost in the newer
reconstructions. And we all know how important it is that dinosaurs be pretty.

        Anyone with comments, good refs on nuchal ligaments in mammals, or
anything else of relevance?

        Later,

        Wagner

        P.S. This post is NOT in any way to be construed as an attack on
Greg Paul or his work. I believe I have made it entirely clear that I have
the highest oppinion of Paul, his work, and especially his art.

        P.P.S. Please don't anybody in the art community take offense, I
love y'all too! Well, except... well, that one's not on the List anyway...

        * For non-hadrosaurs, I am fairly pleased with Dr. David Norman's
skull reconstructions. I think he really takes the time to figure out how
the bones fit together in life.

--------------------------------------------------------------------------------
     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi