Re: Sauropod ONP

["Kent A. Stevens" <kent@cs.uoregon.edu>]

Jaime, all:

< The issue, I think, is that some restorations and arguments would  
imply that sauropods were engaging in neck elevating postures as a  
matter of course, rather than constantly held horizontal.>

Yes, and moreover, some silhouette drawings suggest that those  
elevated postures come naturally, with little or no deflection away  
from the ONP (due to inadequate detail in the drawing of the  
articular surfaces, particularly the zygapophyses, or what is worse,  
a subtle-but-cumulative distortion in how the vertebrae are drawn).

It is important to "lower the boom" on these exaggerations of ONP.   
What one might then care to speculate about the fraction of the time  
sauropods held their necks above (or below) the ONP is just that,  
speculation.

My colleague Mike Parrish and I speculate that the ONP was indeed the  
default resting head height and it already reflects some built-in  
droop, particularly cranially.  It has the advantage of holding the  
head pointing downward, for what we term "downward feeding" and BV  
(for "Browse by Ventriflexion"), which is the most common form of  
feeding among large extant terrestrial herbivores including giraffes  
(!).  The latter story might surprise some; enjoy this site and  
forget what you might have been told earlier about giraffe feeding  
(but save some of Kipling's "just so" stories for night time):

http://natureinstitute.org/pub/ic/ic10/giraffe.htm

The ONP for the giraffe neck is NOT the maximum of dorsiflexion, as I  
have shown by modeling then articulating a virtual giraffe neck in  
DinoMorph (using CT scan data of actual giraffe cervical vertebrae).   
But the ONP appears to be close to the "average" elevation for  
standing and walking giraffes.  Sometimes they hold their heads  
nearly vertically while vigilant, but overall the ONP appears to be  
roughly predictive of the resting position, for adult giraffes.

In our published work and SVP and SVPCA presentations, Mike and I  
have only attempted to establish the true ONP for sauropod necks.   
Only recently we've been asked, and have asked ourselves, what ONP  
might mean.  As a result of the very helpful review process for the  
Curry Rogers and Wilson book chapter we began to examine what is know  
about the relationship between ONP and behavioral stances in extant  
tetrapods.  It turns out virtually no formal work has been done on  
this important issue.   Observations of captive herbivores in zoos do  
not necessarily transfer to the natural setting, of course, and we  
have found little published literature on the relationship between  
browse height and ONP.  There's a lot of behavioral/comparative  
osteological work to be done before generalizations/predictions  
regarding the relationship between ONP and, say, browse height, will  
firm up.

< ... So it seems rather more romantic and "aesthetic" to raise the  
neck and argue that contradictory theories would be wrong.>

Yes.  And in reconstructing an accurate ONP for each sufficiently- 
complete sauropod taxon, aesthetics and romantic notions simply  
should not enter into it.  Getting the ONP correct remains an inexact  
process, as there will be a question of how much "wiggle room" to  
provide in the intervertebral separations, which of course affects  
the cumulative curvature of the neck.  That's where examining  
articulated series in situ, and extant necks with opisthocoely,  
becomes important.  We find, quite unambiguously, that strongly  
opisthocoelous vertebrae (and for that matter strongly procoelous  
cervicals in some other forms) assume an osteological neutral pose,  
ONP, when three factors are simultaneously satisified: aligned  
zygapophyses + parallel external lateral margins of condyle/cotyle  
pairs + very little separation deep within the ball-and-socket.  So  
we don't in fact find much wiggle room in the reconstruction of the  
overall ONP.

<  We should also consider straightening the spines of sauropods  
BEHIND the shoulders as well >

YES!  And that tends to raise the head, particularly when one couples  
a straighter dorsal column with a more ventral placement of the  
pectoral girdles as various studies are suggesting.  Now, the latter  
topic might bring on additional rants from some quarters, but let's  
please spawn a separate thread for shoulder placement, distinct from  
axial ONP matters.

< ... since ventrally-wedged dorsals seem rare to my knowledge. >

Yes.  But there is clear-but-subtle osteological evidence for  
differing degrees of arch to the back within the sauropoda (as Scott  
and others are also finding).

<  ... but when providing a skeleton that is largely static, there is  
no reason to diminish the effectiveness of the restoration by  
exaggerating features for "effect". >

I agree.

Kent