This remarkable system I named pleurokinesis ('side movement'). On the one hand, the system can be seen as a means of avoiding catastrophic failure of the skull during normal biting. However, the pleurokinetic mechanism allows a grinding motion between opposing sets of teeth. This mimics the grinding motion achieved by herbivorous mammals in a totally different way.

This new chewing system could be linked to another important observation concerning dinosaurs such as Iguanodon. Its teeth are recessed (set inwards) from the side of the face. This creates a depression that might have been covered over by a fleshy cheek -another most un-reptilian feature. Given that the upper teeth slid past the lowers to cut up their food, it seems logical to expect that every time they bit through food in the mouth, at least half of it would be lost from the sides of the mouth ... unless, of course, it was caught and recycled in the mouth by some sort of fleshy cheek. So these dinosaurs appeared to be not only capable of chewing their food in a surprisingly sophisticated way, but they also had mammal-like cheeks, and of course to aid the positioning of the food between the teeth before chews, they would have needed a big, muscular tongue (and strong ceratobranchials - the tongue-muscle bones).

Once this new chewing system had been identified, I was able to recognize that the pleurokinesis was not a 'one-off invention associated with Iguanodon. It was actually widespread among the general group of dinosaurs known as ornithopods, to which Iguanodon belonged. Tracing the general evolutionary history of ornithopods across the Mesozoic Era, it became clear that these types of dinosaur became increasingly diverse and abundant in time. The ornithopods reached their greatest expression in the M ecosystems of the latest Cretaceous period, at which time they are | often reported as the most numerous of all land-animal fossils £ recovered. In some parts of the world, ornithopod dinosaurs, represented at this time by duck-billed or hadrosaurian dinosaurs, were exceedingly abundant and diverse: some discoveries in North America hint at herds of hadrosaurs numbering many tens of thousands of individuals. Hadrosaurs had the most sophisticated dental grinders (each of which had as many as 1,000 teeth in them at any one time) and a well-developed pleurokinetic system.

It seems plausible that these dinosaurs became abundant and diverse in large measure because they were very efficient at eating plant food, using the pleurokinetic system. Their evolutionary success was probably a result of their inheritance of the novel chewing mechanism first identified in Iguanodon.

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