Thursday, 9 February 2012

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

10 comments:

Jaime A. Headden said...

I'm not sure the terms "chewing" and "grinding" are useful terms for these types of jaw actions and/or teeth. In hadrosaurs, especially, the rasplike portion of the teeth is largely obscured behind a dental lamina, and only the first first few ranks are available, which form a singular shearing edge with perturbating ridges on the lingual surfaces of the dentary equippage. This surface doesn't interact with the upper jaw at all, so the action of "grinding" (between two dental surfaces) doesn't seem to work. Grinding should properly be about two broad tooth surfaces moving transversely across one another, or rostrocaudally, while chewing appears to merely be processing food largely interorally, while it seems hadrosaurs may not do much of that, either -- unless they weren't gastrolith consumers.

Mark Wildman said...

Heh – always going to be picked up when using the chew word to describe mastication within hadrosaurs – or any dinosaur for that matter. I understood that the hinge between the maxillaries and the rest of the skull forced the maxillaries outward as food was bit down upon. They flexed along this hinge resulting in the teeth surfaces sliding across each other which ground up the vegetation.

Interesting comments as always, Jaime. But are you suggesting that the hinge arrangement is simply a glorified cutting arrangement and did not actually perform the function described above which is, or appears to be, generally accepted?

Anonymous said...

Certainly in Iguanodon the teeth did interfere, as shown by the matching angled wear surfaces on the maxillary and dentary teeth. Is the same not true for hadrosaurs?

Paul

Jaime A. Headden said...

I would advocate for a model in almost any ornithopod where the lowers and uppers always operate as a sort of complex, if occasionally oblique, set of shearing blades. A good analogy might be a set of pinking shears in which the hinge is adjustable to reset the angle of the blades, but the edges always slide against one another. They will always be shears, but the amount of crenellation along the edge during the shearing action (orthal or translational) varies, thus increasing or decreasing the size of high-pressure gaps between cusps. In hadrosauroids, iguanodontids, and even ceratopsians, this shearing marging forms a continuous serrated edge, again like pinking shears, and its purpose seems to be to create jagged tears in food while the jaw closes or translates. The teeth never seem to actually occlude against, but rather along one another.

Anonymous said...

Jamie

Accepting your very lucid explanation of the formation and functionality of the 'crenellated shearing blades' formed by the wear facetted most erupted teeth, what are your thoughts on the function of the prominent lateral denticles (with mammilae) and overall leaf shaped morphology of the teeth in, for example, I.Bernissartensis?

Paul

Anonymous said...

"there is no hard evidence to support this theory (Wings & Sander 2007)."

I wouldn't say that, given the following quote.

Quoting Sampson ( http://www.amazon.com/Dinosaur-Odyssey-Fossil-Threads-Life/dp/0520269896/ref=ntt_at_ep_dpt_1 ): "Nevertheless, occasional sauropod skeletons do show appropriately sized cobbles within the rib cage, suggesting that rock-filled gizzards may have been part of the digestive solution for at least some of these dinosaurian giants."

-Herman Diaz

Mark Wildman said...

Hi Herman. Yes I saw that but equally Sampson states that there is nothing to suggest that these supposed gastroliths did not simply wash into the remains of these sauropods. And these stones could be highly polished due to the fact that they were water-carried to site. Always going to be hard to prove.

Anonymous said...

@Mark Wildman

I get what you're saying. I just thought your original statement (I.e. The 1 I quoted in my previous comment) was too broad a generalization.

-Herman Diaz

Mark Wildman said...

Fair enough!

Anonymous said...

I (HD) just wanted to share what I’ve learned since my last comment.

I have 2 major problems w/the hypothesis that sauropods lacked an avian-style gastric mill & compensated for this "by greatly increasing food retention time in the digestive system" ( http://rspb.royalsocietypublishing.org/content/274/1610/635.full.pdf ): 1) It's based on the comparison of sauropod gastroliths w/ostrich gastroliths, ignoring moa gastroliths; Like sauropods, moas were herbivorous browsers (See the Shugart quote) while ostriches are omnivorous grazers (See "Species Characteristics": http://informedfarmers.com/rhea-production/ ); It's probably no coincidence, then, that like sauropod gastroliths, moa gastroliths are polished (Again, see the Shugart quote) while ostrich gastroliths are pitted; 2) It fails to explain sauropod digestion for the same reason that gigantothermy fails to explain non-avian dino physiology; To quote Holtz, it "might apply to large dinosaurs, but would not apply to small species or to babies" (See "Gigantothermy" under "Complications": http://www.geol.umd.edu/~tholtz/G104/lectures/104endo.html ).

Quoting Shugart ( http://www.amazon.com/Earthquake-Other-Tales-Unbalanced-Nature/dp/0300122705/ref=la_B001H6PV82_1_3_title_2_pap?ie=UTF8&qid=1357865237&sr=1-3 ): "When the Polynesians arrived in New Zealand, they encountered birds that had been evolving for 80 million years without the presence of mammalian predators. Among the most striking of these animals must have been the moas (Figure 29). These were gigantic wingless birds standing as much as 10 feet (3 m) tall and weighing as much as 550 pounds (250 kg).1 They are known from a diverse array of remains including eggshells, eggs, a few mummified carcasses, vast numbers of bones, and some older fossilized bone. The eleven moa species that are currently recognized occupied ecological niches customarily filled elsewhere by large mammalian browsing herbivores. They may have had relatively low reproductive rates; apparently, they usually laid only one egg at a time.2
Moas ranked in height from the tallest at about 10 feet to smaller species the size of a large domestic turkey (about 3 feet, or 1 m, tall and weighing 45 pounds, or 20 kg). They were unique in having neither wings nor even residual wing bones. As one expects of large birds that feed on vegetable matter, moas had muscular gizzards. They swallowed small stones up to 2 inches (50 mm) in diameter into their gizzards for grinding food before digestion. These polished stones, called gastroliths, often occur in groups along with moa bones.3
Many gastroliths have been found in what are now human-modified grassy habitats, giving the initial impression that moas were grass-eating animals. But the present vegetation at a site may not be its previous vegetation.4 Based on preserved crop contents from mummified specimens, moas fed on leaves, seeds, and green twigs of trees and parts of shrubs.5 Thus, it appears that they were creatures of the forest and shrub- land—more like browsing deer than grazing cows."

Post a Comment