The skull of Nigersaurus, I maintain, is remarkably thin, fully taking into account the fact that it would not be processing plant matter primarily orally as in a mammalian ungulate. An ostrich is not a close functional parallel, in that this particular diplodocoid has a terminal tooth battery with a top rate of tooth wear.

The better analogy is to other dinosaurs of comparable size that have evolved very active wear facets and tooth batteries (self-supporting dentition), namely the hadrosaurids and ceratopsids. In both cases, of course, these are laterally positioned tooth batteries—which is a lot more managable biomechanically than a terminal row. They parallel mammalian ungulates in a number of regards (robustness, coronoid process, closure of all fenestrae, reduction of post-dentary bones, etc.). Nigersuarus does none of this, has paper thin jaw bones, and actually has extra fenestrae.

Then we must admit the probability that it was browsing close to the ground, cropping. Thus, in basic feeding function it is closer to a grazer than a narrow-mouthed browser. Again, in mammals that kind of feeding usually adds bulk to the jaws and skull. Of course, it would not have been eating grass, yet other plant possibilities are not that soft. It was terrestrial and common; it was not an aquatic feeder. it was wearing its teeth down at about one per month, which is extraordinary. Of course, like most dinosaurs, it was not mouth processing food other than to clip it. Yet, it fed a body some 35 feet long, and we are finding out that these guys are reasonably fast growers.

Nigersaurus obviously lived, and quite successfully at that, so it was able to function well. Its extraordinarily light skull and axial construction, however, is quite an anomaly, even among diplodocoids, none of which have developed comparable tooth batteries. So we are looking at a truly bizarre dinosaur, rather than an adaptation that led to an entire clade. Again, this highlights the extraordinary nature of the design of this skull; perhaps it was a design near the structural limits of bone support.

It awaits a palatal reconstruction prior to finite element analysis. sorry for the delay in my response.

Best regards,

Paul


**********Dr. John Hutchinson***************************
I agree with Dr. Seth. Although Nigersaurus was large, its thin skull is not
terribly surprising as the robust bones needed in large animals are
typically specialized to support body weight or large bite/chewing forces.
Unless Nigersaurus was standing on its head or biting trees in half I do not
see a need for a robust skull, either.

I would clarify that bone tends to develop as a response to compression, or
be removed under tension. If Nigersaurus was plucking/shearing vegetation it
might have been generating more tensile forces. Ostriches engage in similar
behavior and have very lightly built skulls. I suggest a study is made of
living animals with similar behaviors (analogous to those proposed for
Nigersaurus) and anatomies, like ostriches. Finite element models would
provide some insights, but to me it seems a foregone conclusion that might
require only simple quantitative analysis of bone geometry in extant
animals.

Cheers,
John

============================================
John R. Hutchinson
Structure & Motion Lab
Royal Veterinary College, Univ. London
Hawkshead Lane, Herts AL9 7TA, UK
phone (+44) (0)1707-666-313
fax (+44) (0)1707-666-371 or 652-090
mobile (+44) (0)7843-629-162
web http://www.rvc.ac.uk/sml and
http://www.rvc.ac.uk/Staf...
============================================


******** Dr. Ajay Seth************************
> -----Original Message-----
> From: Ajay Seth [mailto:aseth@stanford.edu]
> Sent: 16 November 2007 23:43
> To: s-koppes@uchicago.edu
> Cc: jrhutch@rvc.ac.uk; Clay Anderson
> Subject: A response to Prof. Sereno's call to biomechanists
> to explain the skull/jaw structure of Nigersaurus
>
> Dear Mr. Koppes:
>
> I have a simple, perhaps obvious, theory to explain the
> unusually light
> structure of Nigersaurus's skull and mandible in response to
> Professor
> Sereno's call to biomechanists in NPR's Health and Science article
> entitled "'Mesozoic Cow' Rises from the Sahara Desert".
>
> One possible explanation is that unlike grazing mammals the
> Nigersaurus
> did not "chew its cud" and did not
> rely on mechanical forces of the jaw during rumination to
> break down the
> plant cellulose (like grass). Maybe it was sufficient to just cut the
> vegetation into small pieces (thus its shearing like teeth) and it
> could extract sufficient nutrition to survive. One could argue in an
> environment of abundant vegetation Nigersaurus could get away with
> inefficient nutrient extraction system as long as it had a
> large enough
> supply of vegetation. Also, Nigersaurus may have had other
> enzymatic or
> digestive structures such as a gizzard, which could masticate
> vegetation
> with muscles closer to its stomach and thus did not require
> strong jaw
> muscles.
>
> We know mammals were not so lucky with dramatic climate changes that
> made vegetation more scarce and so maximizing nutrient
> extraction became
> the name of the game. In the mammal case then, mashing and ruminating
> became much more advantageous. I think we could agree that sheering
> takes less muscle force (as long as the teeth are sharp) over
> a shorter
> duration than masticating which requires higher forces and longer
> duration. Having several rows of teeth (like sharks) seems to
> indicate
> that having a set of sharp cutters was of great importance to
> Nigersaurus' survival.
>
> We could build a computer model to show that Nigersaurus could only
> really shear vegetation but that might already be obvious
> (i.e. if there
> are no molars for grinding). If all one needs to do is cut grass into
> pieces and not grind it to a pulp orally then one doesn't
> need massive
> jaw muscles nor need to develop the dense bones to
> accommodate the high
> loads of mastication. From a developmental point of view we know that
> bone density and shape is in great part a response to
> loading, and so we
> can only conclude that Nigersaurus did not generate the high
> sustained
> jaw loads experienced by the large grazing mammals of today.
>
> Thank you for this opportunity to respond to the article. I
> am looking
> forward to Professor Sereno's reply.
>
> Ajay
>
> Ajay Seth, PhD.
> Postdoctoral Research Fellow - Simbios
> Neuromuscular Biomechanics Laboratory
> Bioengineering Department
> Stanford University
>
> tel: 650-725-9486
> email: aseth@stanford.edu
> http://www.stanford.edu/g...