Discovery
The first bones of Iguanodon ("iguana tooth") were recovered in the early 1820s, making it the first dinosaur ever discovered in Western civilization! There is some evidence that the Chinese had already recovered some bones of their own before this, but seem to have assumed them to be the dragons of Chinese mythology. Indeed, an Iguanodon shin bone was found in 1809 but went unnamed and was forgotten until the 1970s. However, while Iguanodon was discovered first, Parkinson named Megalosaurus in 1822 and it was not until that 1825 that the great Dr Gideon Mantell published his description of Iguanodon. Mantell had found the bones in Cuckfield, West Sussex while walking with his wife Mary, but there was not a great deal of them and so his initial description was rather inaccurate, placing Iguanodon on four legs and with a single spike on its snout. It is this model that is seen at the famous dinosaur models of Crystal Palace, London.
The great breakthrough came in 1878. Thirty-nine skeletons, many of them near-complete, were found in a mine in Bernisaart, Belgium. This was a discovery of quite astounding size, and it fell to one Louis Dollo to publish the revised description of Iguanodon from it. The "Dollo reconstruction", as it is now known, made Iguanodon bipedal and moved the spike from the nose to the thumb. This explanation was accepted for over a century. Such a large number of Iguanodons in one place also suggested that they were herd animals, or it could simply be that many skeletons were washed down-river and deposited together.
More discoveries have since been made, in southern England, Belgium, North Africa, north-west America, Mongolia and even Spitzbergen, inside the Arctic Circle. Such a wide spread of specimens shows that Iguanodon was a very successful species. The discovery at Nehden, Germany in 1980 prompted a furthur study of Iguanodon by Dr David Norman at Brasenose College, Oxford. Norman restored Iguanodon to Mantell's original four-footed stance but noted that it could have reared up on its hind legs to reach low branches.
Description
Iguanodon was a herbivore from the early Cretaceous, 135 to 125 million years ago. It was around nine metres in length, five metres tall and around 2.7 metres to the hips. Its weight has been estimated to be about four and a half tonnes. The distinctive spike on the thumb could have been used for self-defence, gathering food or even mating and ranged in size from two to six inches. The three central fingers could be splayed apart like a foot and also had hooves. The fifth finger was small but flexible, and was probably used for grasping objects and held clear of the ground when walking.
The two-inch long flattened teeth in the side of the jaw were suitable for grinding, and were used along with the cheeks - Iguanodon would have chewed its food like a modern horse or cattle. This reduced the load on the digestive system, which is typically very intensive in herbivores. Cheeks were actually a relatively recent development in dinosaurs, and were a significant advancement for grazers such as Iguanodon, since food could be chewed repeatedly before being swallowed - important since Mesozoic flora like cycads was much coarser at ground-level than the higher trees eaten by Sauropods. Coupled with the hinged jaw that could move side-to-side, the food was thoroughly ground before swallowing. Iguanodon also had a distinctive beak for clipping tough plants, rather than the incisors seen in earlier dinosaurs. Mantell noticed that the teeth he had recovered were like a giant version of those from a modern iguana, and it is from this that Iguanodon gets it name.
As an ornithischian, Iguanodon had a backward-pointing pubis bone, providing more room for the gut between the hips. This the feature that allowed it to rear up on two legs where necessary, to survey the landscape or eat some of the lower trees, although four would certainly have been more convenient for moving around.
Paleontologists have investigated the morphology of Iguanodon, and by comparing with fossilized tracks, have estimated that a mature Iguanodon could have reached 15-20km per hour (D. Fastovsky and D. Weishampel , The Evolution and Extinction of the Dinosaurs). When up on its hind legs, an Iguanodon could move fast enough to evade most of its contempory predators, namely theropods such as Megalosaurus.
Classification
Iguanodon was related to the Hadrasaurs but is not counted among them since it had no head decoration to speak of. Instead, it has its own family, the Iguanodontid, including other dinosaurus such as Ouranosaurus and Probactrosaurus.
Mantell's original species, which he named I. mantelli is now known as I. anglicus ("English iguana tooth"). A number of other dinosaurs have been misclassified as species of Iguanodon, and since reclassified, but there are still many recognized species: I. atherfieldensis, I. bernissartensis, I. dawsoni, I. fittoni, I. hoggi, I. lakotaensis, and I. ottingeri. Species variation includes to the shape of the snout, the maximum size of the adults, and the overall build which varies from graceful to rather robust. The type species is now I. bernissartensis since this is far better known than the original I. anglicus, but this creates some confusion between older and newer sources.