Iguanodon (meaning "iguana-tooth") is a genus of ornithopod dinosaur that lived roughly halfway between the first of the swift bipedal hypsilophodontids and the ornithopods' culmination in the duck-billed dinosaurs. Many species of Iguanodon have been named, dating from the Kimmeridgian age of the Late Jurassic Period to the Cenomanian age of the Late Cretaceous Period from Asia, Europe, and North America. However, research in the first decade of the 21st century suggests that there is only one well-substantiated species: I. bernissartensis, that lived from the late Barremian to the earliest Aptian ages (Early Cretaceous) in Belgium, between about 126 and 125 million years ago. Iguanodon's most distinctive features were its large thumb spikes, which were possibly used for defence against predators, combined with long prehensile fifth fingers able to forage for food.
Named in 1825 by English geologist Gideon Mantell, Iguanodon was the second dinosaur formally named, after Megalosaurus. Together with Megalosaurus and Hylaeosaurus, it was one of the three genera originally used to define Dinosauria. A large, bulky herbivore, Iguanodon is a member of Iguanodontia, along with the duck-billed hadrosaurs. The taxonomy of this genus continues to be a topic of study as new species are named or long-standing ones reassigned to other genera.
Scientific understanding of Iguanodon has evolved over time as new information has been obtained from the fossils. The numerous specimens of this genus, including nearly complete skeletons from two well-known bonebeds, have allowed researchers to make informed hypotheses regarding many aspects of the living animal, including feeding, movement, and social behaviour. As one of the first scientifically well-known dinosaurs, Iguanodon has occupied a small but notable place in the public's perception of dinosaurs, its artistic representation changing significantly in response to new interpretations of its remains.
Description
Iguanodon was a bulky herbivore that could shift from bipedality to quadrupedality. The best-known species, I. bernissartensis, is estimated to have weighed about 3 tonnes (3.5 tons) on average, and measured about 10 metres long (33 ft) as an adult, with some specimens possibly as long as 13 metres (43 ft). This genus had a large, tall but narrow skull, with a toothless beak probably covered with keratin, and teeth like those of an iguana, but much larger and more closely packed.
The arms were long (up to 75% the length of the legs in I. bernissartensis) and robust,[4] with rather inflexible hands built so that the three central fingers could bear weight. The thumbs were conical spikes that stuck out away from the three main digits. In early restorations, the spike was placed on the animal's nose. Later fossils revealed the true nature of the thumb spikes, although their exact function is still debated. They could have been used for defense, or for foraging for food. The little finger was elongated and dextrous, and could have been used to manipulate objects. The legs were powerful, but not built for running, and each foot had three toes. The backbone and tail were supported and stiffened by ossified tendons, which were tendons that turned to bone during life (these rod-like bones are usually omitted from skeletal mounts and drawings). Overall, in body structure, it was not too dissimilar from its later relatives, the hadrosaurids.
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