THOUGHTS ON SAUROPOD NECKS

[darren.naish@port.ac.uk]

The debate concerning sauropod necks and their in-life use is good. 
Here are my views, following on from recent comments proferred by 
Brian Franczak. 

Concerning the weight of some of the big guys, Brian said..

> "Three-digit tonnage"? I think you've *seriously* overestimated the
> weight of sauropods. _Argentinosaurus_ is the only known sauropod 
> that *might* fall into that category, but only Don Lessem thinks it 
> weighed in excess of 100 tons.

Well, Dale Russell was quoted in the _Science_ article that first 
reported _Argentinosaurus_ that he felt that this animal may well 
have weighed upwards of 100 tons - it is not just a Lessemism (god 
help us if it was). Of course, Brian is very correct in that 
virtually all other sauropods - including _Mamenchisaurus_ and most
diplodocids - were considerably less weighty. Current estimates put 
these animals between 10 and 35 tons, or thereabouts.

> And don't discount the weight of that 30+ foot neck. Anteriorly-
> tapering cervical vertebrae or no, that neck still weighed a 
> significant amount.

Sauropod vertebrae and sauropod necks would have been incredibly 
light when not fossilised. It is reasonable to think that one could 
actually pick up a very big _Brachiosaurus_ cervical vert with one 
hand. I was thinking two hands, but Per Christiansen (an expert in 
mathematical models of scaling in tetrapods) reckoned otherwise when 
I recently spoke to him at a conference.

As you'll know, there is overwhelming evidence that sauropod 
cervicals were highly pneumaticised, and there was probably more air 
to them than bone. Dino Frey and John Martin now have good evidence 
for a new model of the sauropod neck, based on osteological 
correlates seen in croc dissections, and they find it highly 
pneumatic with a.. well, they will present this at SVPCA (September 
1998). Sauropods do seem to have reduced the mass of their necks 
considerably, and apparently minimised on any structure that might 
have added weight (another argument against trunks or fleshy lips). 
This is also in agreement with models where necks are nearer 
horizontal than vertical.

And somewhat regretfully, I am coming down on the side of those who 
believe that mamenchisaur necks were nearer the horizontal than they 
were the vertical. The reason is the cervical ribs. Where portions of 
cervical ribs overlap, they cannot slide across one another, but are 
bound by ligaments that allow comparatively little craniocaudal 
extension. This system is now pretty well understood in crocodile
necks (thanks to Dino's dissection papers) where the ligaments that 
bind consecutive cervical ribs mean that, when the neck is 
straightened, the ligaments will pull it back to natural curvature 
with no muscular effort. The point is that the overlapping portion of 
any two cervical ribs cannot be extended beyond the portion of 
overlap.

You can see extensively overlapping cervical ribs in some 
theropods (notably in _Coelophysis_, where a rib will extend the 
length of the following five cervicals), and also in sauropods like 
_Mamenchisaurus_ and _Omeisaurus_. Ergo, these are relatively 
inflexible necks that must be held in a position that does not warp 
the rather straight shape of the cervical ribs. If you look at 
skeletal mounts of long-necked sauropods like _Omeisaurus_ where the 
neck has been put in an S-curve, you will see that the cervical ribs 
are not aligned in parallel as they should be (and as they are in the 
articulated necks of fossils and in the _M. hochuanensis_ mount, 
which has a horizontal neck - contra Wilson and Sereno 1998, this 
taxon does exist), but are sticking out away from the reconstructed 
curve of the neck - not good if you imagine the thing as a live 
animal (and yes, I know that live bone would have been more supple, 
but the cervical ribs already have a slight curvature, plus the 
degree of  bending that would have been needed in life to fit the 
ribs to the S-curve neck is just too much). 

Near-vertical sauropod necks mean that, assuming these animals had to 
lower their necks to drink (yes yes, something else that has been 
flogged to death recently), extensive sliding across cervical rib
overlaps has to be allowed. As argued above, this is not possible. 
Basically, reconstructed long-necked sauropods with near-vertical 
necks actually means that their necks would have been *less* flexible 
than if their necks were horizontal, because of these cervical rib 
constraints.

Finally, one thing that really jumps out when you (or at least I) 
examine a sauropod mount is how wide the base of the neck is. These 
are not slim necks, well demarcated from the thorax, but 
mediolaterally *broad* structures that are essentially cranial 
extensions of the body. In camarasaurs and brachiosaurs especially 
(funnily enough, I haven't ever seen a titanosaur mount:)), seen from 
above or below, the neck tapers from the shoulders toward the head, 
and does not have a constriction at the base that is then continued 
cranially at the same width (as per out necks). As you can see in the 
reconstructions in Wilson and Sereno (1998), and as Greg Paul shows 
for his diplodocids and his newly modified _Brachiosaurus brancai_ 
(and as Czerkas and now Tracy Ford say as well), near-horizontal 
necks that extend from the thorax are correct. 

One area that still troubles me is the alleged upward curve at the 
base of some sauropod necks, as figured by Greg Paul in the recent 
_DinoFest_ volume. This curve has been accentuated by post-mortem 
contraction of neck ligaments and, while it may support an upward 
curve at the base of the neck in some sauropods, continuing curvature 
so a near-vertical neck results is probably not correct and 
near-horizontal necks are not perfectly horizontal anyway. Again, 
this is a matter that will be addressed in the near future and I 
cannot give too much away.

On related subjects that I could talk about (but will not, for the 
sake of time).. giraffes have 8, not 7, cervical verts. One of them 
has become the first thoracic - a fact that was recognised in the 
last century. Plesiosaurs necks: another contentious subject. All 
evidence now indicates that plesiosaurs necks were moderately, but 
not overly, flexible.

"Yeah, but you didn't measure cat skins, perform morphometric 
analyses, or do population censuses"
"No, we have a life" - - Downes, 1998.

DARREN NAISH
darren.naish@port.ac.uk