The Iguanodontidae evolved in the Middle Jurassic period, about 170 mya, and spread throughout the world, reaching a peak in diversity and abundance by the end of the Lower Cretaceous. From then on, they declined in number and ecological importance and finally disappeared at the end of the period.

Whereas most of the hypsilophondonts were adapted for speed, the iguanodontids were bulky and heavily built with thigh bones longer than their shins. The tail was used to counterbalance the body, and the animals probably adopted a quadrupedal gait for much of the time, because three of the five fingers of their hands had hoof-like nails. The 'thumbs' were developed into prominent spikes which would have been used for defence against contemporary predators such as Megalosaurus (Sect. 11.4.2). They could also have been used for sexual display or to tear down foliage. Iguanodon (Fig. 97) was only the second dinosaur genus to be discovered. It was first described by Gideon Mantell in 1825, but not until 1877, when the skeletons of more than 30 specimens were discovered in a coal mine at Bernissart in Belgium (Colbert 1968), did the true nature of this impressive dinosaur become apparent. Large herds roamed about, browsing on ferns, horsetails, cycads, conifers and other tough foliage. The head of Iguanodon had a prominent snout and powerful jaws with cheek teeth that provided a strong and grinding action, because the bones of the upper jaw moved apart when the lower jaw was raised between them (Norman 1985; Palmer 1999).

Other well-known genera include Camptosaurus (Upper Jurassic), Ourano-saurus (Fig. 101),Muttaburrasaurus and Probactrosaurus (Lower Cretaceous). The demise of the iguanodontids could well have been linked with the rise of the hadrosaurs (Sect. 10.5.4), although they were not affected by the hypsilo-phodontids (Sect. 10.5.2). The reason for this difference may be that they did not compete with the latter family although it was contemporaneous. Hypsilo-phodontids were very much smaller than iguanodontids, and consequently could have fed upon quite different types of plants. Moreover, their jaws and teeth were rather different, as they were adapted to their own special foods.

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