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up plant tissues between the teeth to release the nutritionally §

usable 'cell sap' that is enclosed within plant cell walls. Herbivores § eat large quantities of plant food in order to be able to extract sufficient nutrients from such comparatively nutrient-poor material. As a result, herbivores tend to have barrel-shaped bodies that accommodate large and complicated guts, which are necessary to store the large volumes of plants that they have to eat and allow sufficient time for digestion to take place. Herbivores' large guts house dense populations of microbes that live within special chambers or pouches in the gut wall; our appendix is a tiny vestige of such a chamber, and hints at herbivory in our primate ancestry. This symbiosis allows herbivorous animals to provide a warm, sheltered environment and constant supplies of food for the microbes; in their turn, the microbes have the ability to synthesize cellulase, an enzyme that digests cellulose and converts it into sugars that can then be absorbed by the host animal.

By most standards, Iguanodon (11 metres long and weighing about 3-4 tonnes) was a large herbivorous animal, and would have consumed plants in large quantities. Given this background information, questions about precisely how Iguanodon fed and assimilated its food can be explored in detail.

One persistent theory concerning its method of feeding was its suggested use of a long tongue to pull vegetation into the mouth. This began with Gideon Mantell, who described one of the first, nearly complete lower jaws of Iguanodon. The new fossil included some tell-tale teeth, so the ownership could not be doubted, and it had a toothless, spout-shaped front end. Mantell speculated that the spout shape allowed a long tongue to slide in and out of the mouth, rather like a giraffe's does. Mantell could not have known that the tip of the newly discovered lower jaw was incomplete and was capped by a predentary bone that filled in M the 'spout'.

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£ It is very curious to note that in the 1920s Louis Dollo provided further support for Mantell's conjecture. Dollo described a special opening in the predentary at the tip of the lower jaw; this formed a tunnel passing straight through the predentary bone that

Predentary

allowed a long, thin, muscular tongue to be projected outward to grasp vegetation and draw it into the mouth. Large bones (ceratobranchials) that had been found lying between the jaws of Iguanodon were suggested to act as the attachment for the muscles that would have operated this type of tongue. Such a structure fitted neatly with Dollo's concept of Iguanodon as a high arboreal browser with a giraffe-like long, grasping tongue.

Careful re-examination of the lower jaws of a number of Iguanodon skulls from Bernissart failed to reveal Dollo's predentary tunnel. The predentary has a sharp upper edge that supported a turtle-like horny beak. The predentary, and its beak, bit against the similarly toothless beak-covered premaxillae at the tip of the upper jaw, and this arrangement allowed these dinosaurs to very effectively crop the plants upon which they were feeding. The advantage of the horny beak was that it would have grown continuously (unlike |

teeth, which gradually wear away) no matter how tough and ;

abrasive the plants that were being cropped. The ceratobranchial o bones still require some explanation. In this instance, they would §

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