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Feeding

One of the first details noted about Iguanodon was that it had the teeth of a herbivorous reptile, although there has not always been consensus on how it ate. As Mantell noted, the remains he was working with were unlike any modern reptile, especially in the toothless, scoop-shaped form of the lower jaw symphysis, which he found best compared to that of the two-toed sloth and the extinct ground sloth Mylodon. He also suggested that Iguanodon had a prehensile tongue which could be used to gather food, like a giraffe. More complete remains have shown this to be an error; for example, the hyoid bones that supported the tongue are heavily built, implying a muscular, non-prehensile tongue used for moving food around in the mouth. The giraffe-tongue idea has also been incorrectly attributed to Dollo via a broken lower jaw.

The skull was structured in such a way that as it closed, the bones holding the teeth in the upper jaw would bow out. This would cause the lower surfaces of the upper jaw teeth to rub against the upper surface of the lower jaw's teeth, grinding anything caught in between and providing an action that is the rough equivalent of mammalian chewing. Because the teeth were always replaced, the animal could have used this mechanism throughout its life, and could eat tough plant material. Additionally, the front ends of the animal's jaws were toothless and tipped with bony nodes, both upper and lower, providing a rough margin that was likely covered and lengthened by a keratinous material to form a cropping beak for biting off twigs and shoots. Its food gathering would have been aided by its flexible little finger, which could have been used to manipulate objects, unlike the other fingers.

Exactly what Iguanodon ate with its well-developed jaws is not known. The size of the larger species, such as I. bernissartensis, would have allowed them access to food from ground level to tree foliage at 4–5 metres (13–16 ft) high. A diet of horsetails, cycads, and conifers was suggested by David Norman, although iguanodonts in general have been tied to the advance of angiosperm plants in the Cretaceous due to the dinosaurs' inferred low-browsing habits. Angiosperm growth, according to this hypothesis, would have been encouraged by iguanodont feeding because gymnosperms would be removed, allowing more space for the weed -like early angiosperms to grow. The evidence is not conclusive, though. Whatever its exact diet, due to its size and abundance, Iguanodon is regarded as a dominant medium to large herbivore for its ecological communities. In England, this included the small predator Aristosuchus, larger predators Eotyrannus, Baryonyx, and Neovenator, low-feeding herbivores Hypsilophodon and Valdosaurus, fellow "iguanodontid" Mantellisaurus, the armoured herbivore Polacanthus, and sauropods like Pelorosaurus.