To Chew or not to Chew?

In my recent post about Daspletosaurus I made reference, in my imaginary scenario, that tyrannosaurs were almost certain to swallow their food whole once they had torn away the flesh and bone from a carcass. Tyrannosaurs obviously did not chew and so this is a perfectly logical assumption but hadrosaurs, on the other hand, did indeed utilise a form of chewing with their unique jaw hinge arrangement and their impressive array of grinding teeth that lined the jaws.

Sauropods, of course, present another conundrum. I mentioned in my previous post that sauropods were almost certainly perpetual eaters and it has always fascinated how so small a head, mounted on a long neck, was capable of consuming enough food to maintain their giant bodies. Well, like tyrannosaurs, it appears that sauropods also swallowed without chewing and, since so much fodder would have been required, then this simple action would have been a necessity.

This actually makes a lot of sense since, simply put, large bodies demand large amounts of food and it seems a reasonable assumption that, since sauropods survived until the very end of the Cretaceous, they had obviously developed a very efficient feeding and digestive system.

Hadrosaurs were also big animals and some, such as Shantungosaurus, grew to around fifty feet in length but these animals employed a much more complex chewing and swallowing technique. They were able to process vegetation into much smaller bits of matter; indeed the process of cutting the plants with the horny beak began the process before the dental battery more or less ground the fodder to pulp. This processed mulch was then broken down in the gut much quicker and easier than if it had been swallowed whole.

To generate the force needed to operate the hadrosaur jaw mechanism required significant power and the skull was quite robust and muscled accordingly.  Sauropod skulls, on the other hand, were small and kinetically weak.  Sauropods were so big that they did not have the time to utilise such a process like that used by the hadrosaurs and this is the primary reason, I suspect, why hadrosaurs had virtually reached their size limit with animals like Shantungosaurus.  

What sauropods did have, however, was a remarkably long neck and this appears to have enabled the animals to consume vast amounts of fodder in various positions and angles without actually having to move. Physiologically this all makes good sense. If you need to consume copious amounts of vegetable matter, you are better off with a very simple head that swallows matter whole, utilising minimal effort with only the neck moving whilst the body remained virtually motionless. Perfect.

Also of interest is the fact that a lot of Mesozoic flora actually appears to be very nutritious. You may recall during my SVP recap that a recent reappraisal of the Jurassic flora  found in the Morrison Formation and a nutritional analysis of their equivalent extant varieties reveal that many species of plant were very nutritious and ideal for the sauropods (Gee 2011). Horsetails were one of these likely to have been consumed and today’s examples are very tough and fibrous but if you are swallowing your food whole then this does not represent a problem (Sander et al 2011). Plants such as horsetails would also have been tough on teeth, and recovered sauropod teeth are often heavily worn, but they would have been replaced frequently so this would not have been an issue. 

Of course, large amounts of fodder require a large stomach for digestion and the vat of gastric juices, comprising of billions of bacterial microbes needed to break down such tough fibrous plants, would have been very large indeed. It was thought that this may have been supplemented by a stone filled gizzard to help the digestive process, something akin to birds that utilise grit in the same way, but, contrary to popular belief, there is no hard evidence to support this theory (Wings & Sander 2007). In any event the process of digestion would have taken a long time but this would have not caused a problem for sauropods since the sheer size of the gut alone would have virtually ensured a continual release of energy to the animal.

Also of use to sauropods were their unique skeletal pneumaticity and an extremely sophisticated avian-like respiratory system. The cervical vertebrae were full of air sacs that ran in tandem with what was obviously a sophisticated series of valves that controlled and regulated both blood flow and air exchange. So not only was the neck lightened considerably, which aided the feeding process, but they were also able to breathe much more efficiently and this also made the task of supporting their entire body structure much easier. In fact this has also just been alluded to in a very recently published paper (Sookias 2012).

There are obviously various feeding techniques in dinosaurs that are yet to be fully determined and perhaps oviraptorosaurs, ornithomimids and therizinosaurs represent those that are amongst the most fascinating and challenging to describe fully.

References

Gee, C. 2011. Sauropod Herbivory During Late Jurassic Times: New Evidence for Conifer-Dominated Vegetation in the Morrison Formation in the Western Interior of North America. Journal of Vertebrate Paleontology, SVP Program and Abstracts Book, 2011, pp115.

Sander, P. M., Christian, A., Clauss, M., Fechner, R., Gee, C. T., Griebeler, E.-M., Gunga, H.-C., Hummel, J., Mallison, H., Perry, S. F., Preuschoft, H., Rauhut, O. W. M., Remes, K., Tütken, T., Wings, O. and Witzel, U. (2011), Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews, 86: 117–155. doi: 10.1111/j.1469-185X.2010.00137.x

Sookias, R.B., Richard J. Butler, and Roger B. J. Benson  2012 Rise of dinosaurs reveals major body-size transitions are driven by passive processes of trait evolution Proc R Soc B 2012 : rspb.2011.2441v1-rspb20112441.

 Wings, O & P. Martin Sander 2007. No gastric mill in sauropod dinosaurs: new evidence from analysis of gastrolith mass and function in ostriches Proc Biol Sci. 2007 March 7; 274(1610): 635–640. Published online 2006 December 19. doi: 10.1098/rspb.2006.3763