Iguanodon is an ornithopod dinosaur genus, which existed in the early Cretaceous Period, approximately between 126 and 122 million years ago. The genus was established by Gideon Mantell in 1825, based on fossils from England, and the species was originally named as I. anglicus. It was the second dinosaur, after Megalosaurus, which was scientifically named and described. Later, in 1878, further, and more complete Iguanodon fossils was discovered in Belgium by Louis Dollo, he described it as a new species and named it as Iguanodon bernissartensis.
Known occurrences of Iguanodon fossils.
After comprehensive investigations, in the eraly 21st century it became apparent that the original fossils described by Mantell as Iguanodon belongs to four different iguanodontian dinosaur genera: Mantellisaurus, Barillium, Hypselospinus, and also included Iguanodon bernissartensis. Therefore, the originally described type species, I. anglicus, is regarded as a nomen dubium, which means that it is not valid. Based on new fossils from the Camarillas Fromation, Spain, in 2015 a second valid species, I. galvensis was described. A recent taxonomic revision have resulted that only these two species of the genus, I. bernissartensis and I. galvensis are considered to be taxonomically valid.
Iguanodon bernissartensis. Source: By I,Steveoc 86, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3529244
Description and anatomy
They were large herbivors with big thumb spikes which might have played an important role as a weapon in the animal's defense against predators. However, it also might have been used to break hard-shelled seeds and fruits . When Iguanodon was explored, it was believed that this spike was a kind of face-horn on the animal's nose. It also had a fifth finger that was long and capable of gripping food or maybe manipulating small objects. These dinosaurs could walk on two legs, but also could change to quadrupedal walk. Iguanodon bernissartensis weighed abou 3.4 tons , and the adults were about 10 meters long on avarage - the largest ones were estimated to have been 13 meters long. Their skull was narrow, but tall, had beaks without tooth on the anterior part of their jaws, and teeth that are similar to modern iguanas in their forms (from here comes the name "Iguanodon") - the upper jaw had at most 29, the lower up to 25 teeth on each side. The distance between the tooth rows of Iguanodon and the outside of the jaws, alongside with further anatomical details, suggest that this dinosaur probably had some kind of cheeks, which might have been useful for retaining more food in the mouth of the animal  . It had relatively long and massive arms, which were about three-fourth of the hindlimbs in their length. The hindlegse were very strong, each with 3 toes, but probably not capable for running. Ossified tendons aided to support the animals backbone and tail - these tendons turned into bone only in older age of the specimens, during their life. There is no evidence for sexual dimorphism in case of Iguanodon .
Iguanodon, fossilized skull. Source: By Ballista at the English-language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2906654
As mentioned above, Iguanodon was a herbivorous dinosaur. Probably it bit off the plant material with the toothless front ends of its jaws, which were probably covered by keratinous material and formed a beak . Due to the movement of jaws, the upper jaw's teeht were rub against the lower jaw's teeth, which resulted the grinding of the food in the mouth of the dinosaur . The diet of Iguanodon is not exactly known, however, due to its size and abundance, this dinosaur was probably a dominant herbivore for its habitat and ecological community. The size of I. bernissartensis made it capable for browse food from the ground level to the lower tree foliage, and its main food source were probably conbifers, cycads and horsetails. It is also suspected that the low-browsing habits contributed to rarefy gymnosperms and therefore allowing more space for weed-like early angyosperms and thereby fostered their long-term advance . There is no evidence regarding the herding behaviour of Iguanodon - though the Bernissart fossils represents more individuals, which probably died in a relatively short time period, bones of juvenile specimens are missing from there, unlike in remains of animals with herding behaviour.
Iguanodon, fossilized skeleton. Source: By Ben2 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3166764
Due to the fact that early fossils of Iguanodon were deficient, it was depicted for the first time as a quadrupedal, varanus or crocodile-like beast with a horn on its nose. After the discovery of the skeletons at Bernissart, it became clear that Iguanodon was a bipedal dinosaur - for a long time Iguanodon was believed to walk in an upright posture, dragging its tail on the ground. However, a subsequent investigation by David Norman in 1980 proved that this kind of tripod posture was unrealistic, due to the ossified tendons that stiffened the tail of the animal. Further anatomical evidences, e.g. the structure of hand, the relatively immobile wirst, and the robust arms and shoulder bones suggest, that Iguanodon could switch between quadrupedal and bipedal postures. Investigations on fossilized bones of juvenile and adult specimens suggest that young animals walked mostly in their hindlegs, and as they grow older and heavier, they used the qadrupedal mode more and more often. While being in quadrupedal pose, Iguanodon walked on the fingers ond toes of its forearms . The animal was probably capable of running with a maximum speed of 24 km/h in bipedal pose, but not being able for gallop on all four legs.
Iguanodon, early depiction in bipedal posture. Source: By Biodiversity Heritage Library - n147_w1150Uploaded by FunkMonk, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=28302884
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