iguanodon n : massive herbivorous bipedal dinosaur with a long heavy tail; common in Europe and northern Africa; early Cretaceous period

Iguanodon ( or /ɪˈgwænədɒn/, 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 2000s suggests that there is only one well-substantiated species: I. bernissartensis, that lived from the Barremian to the early Aptian (Early Cretaceous) in Europe, between about 130 and 120 million years ago. Iguanodons most distinctive features were its large thumb spikes, which were possibly used for defence against predators.

Discovered in 1822 and described three years later 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 3.08 tonnes (3.5 tons) on average, and measured about 10 metres long (32.8 feet) as an adult, with some specimens possibly as a long as 13 metres (42.6 ft). Other species were not as large; the similarly robust I. dawsoni is estimated at 8 metres long (26.2 ft), and its more lightly-built contemporary I. fittoni at 6 metres (19.7 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. although their exact function is still debated. They could have been used for defense, or for foraging for food. The little finger was elongate and dextrous, and could have been used to manipulate objects. The legs were powerful, but not built for running, and there were three toes on each foot. 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). This family traditionally has been something of a wastebasket taxon, including ornithopods that were neither hypsilophodontids or hadrosaurids. In practice, animals like Callovosaurus, Camptosaurus, Craspedodon, Kangnasaurus, Mochlodon, Muttaburrasaurus, Ouranosaurus, and Probactrosaurus were usually assigned to this family. but his proposal has been rejected. Not everyone agrees that the story is false, though. Regardless of the exact circumstances, he combed the area for more fossils, and consulted the fossil experts of the time as to what sort of animal the bones might belong to. Most of the scientists, such as William Buckland and Georges Cuvier, thought that the teeth were from fish or mammals. However, Samuel Stutchbury, a naturalist from the Royal College of Surgeons, recognized that they resembled those of an iguana, albeit twenty times larger. He neglected to add a species name to form a proper binomial, so one was supplied in 1829 by Friedrich Holl: I. anglicum, which was later amended to I. anglicus.

A better specimen was discovered in a quarry in Maidstone, Kent, in 1834, which Mantell soon acquired. He was able to identify it as an Iguanodon from its distinctive teeth. The Maidstone slab allowed the first skeletal reconstructions and artistic renderings of Iguanodon. As such, he made some mistakes, the most famous of which was the placement of what he thought was a horn on the nose. The discovery of much better specimens in later years revealed that the horn was actually a modified thumb. Still encased in rock, the Maidstone skeleton is currently displayed at the Natural History Museum in London. The borough of Maidstone commemorated this find by adding an Iguanodon as a supporter to their coat of arms in 1949. This specimen has become linked with the name I. mantelli, a species named in 1832 by Christian Erich Hermann von Meyer in place of I. anglicus, but it actually comes from a different formation than the original I. mantelli/I. anglicus material. Shortly before his death in 1852, Mantell realized that Iguanodon was not a heavy, pachyderm-like animal, as Owen was putting forward, but his passing left him unable to participate in the creation of the Crystal Palace dinosaur sculptures, and so Owen's vision of the dinosaurs became that seen by the public for decades.

Bernissart

The largest find of Iguanodon remains to date occurred in 1878 in a coal mine at Bernissart in Belgium, at a depth of 322 m (1056 ft). Many of them went on public display beginning in 1882 and are still present for viewing; 11 are displayed as standing mounts, and 20 as they were (approximately) found. He was not completely correct, but he also had the disadvantage of being faced with some of the first complete dinosaur remains. A problem that was later recognized was the bend he introduced into the tail. This organ was more or less straight, as shown by the skeletons he was excavating, and the presence of ossified tendons. In fact, to get the bend in the tail for a more wallaby or kangaroo-like posture, the tail would have had to be broken. With its correct, straight tail and back, the animal would have walked with its body held horizontal to the ground, arms in place to support the body if needed. During World War I, when the town was occupied by German forces, preparations were made to reopen the mine for palaeontology, and Otto Jaekel was sent from Berlin to supervise. The Allies recaptured Bernissart just as the first fossiliferous layer was about to be uncovered. Further attempts to reopen the mine were hindered by financial problems and were stopped altogether in 1921 when the mine flooded. However, what had been a European genus was now being found worldwide, with material in Africa (teeth from Tunisia and elsewhere in the Sahara Desert), Mongolia (I. orientalis), and the United States in North America (I. ottingeri from Utah). Another North American species, from South Dakota, once assigned to Iguanodon as I. lakotaensis, has since been re-classified as the genus Dakotadon.

Iguanodon was not part of the initial work of the dinosaur renaissance that began with the description of Deinonychus in 1969, but it was not neglected for long. David B. Weishampel's work on ornithopod feeding mechanisms provided a better understanding of how it fed, and David B. Norman's work on numerous aspects of the genus has made it one of the best-known dinosaurs.

Species

Because Iguanodon is one of the first dinosaur genera to have been named, numerous species have been assigned to it. While never becoming the wastebasket taxon several other early genera of dinosaurs became (such as Megalosaurus and Pelorosaurus), Iguanodon has had a complicated history, and its taxonomy continues to undergo revisions.

Species currently accepted as valid

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. Additionally, the front ends of the animal's jaws were toothless and tipped with bony nodes, both upper and lower, The evidence is not conclusive, though.

Posture and movement

Early fossil remains were fragmentary, which led to much speculation on the posture and nature of Iguanodon. As discussed, Iguanodon was initially portrayed as a quadrupedal horn-nosed beast. However as more bones were discovered, Mantell observed that the forelimbs were much smaller than the hindlimbs. His rival Owen was of the opinion it was a stumpy creature with four pillar-like legs. The job of overseeing the first lifesize reconstruction of dinosaurs was initially offered to Mantell, who declined due to poor health, and Owen's vision subsequently formed the basis on which the sculptures took shape. Its bipedal nature was revealed with the discovery of the Bernissart skeletons. However, it was depicted in an upright posture, with the tail dragging along the ground, acting as the third leg of a tripod. During his re-examination of Iguanodon, David Norman was able to show that this posture was unlikely, due to the presence of a long tail stiffened with ossified tendons. The three toed pes (foot) of Iguanodon was relatively long, and when walking, both the hand and the foot would have been used in a digitigrade fashion (on the fingers and toes). which would have been as a biped; it would not have been able to gallop as a quadruped. and Samuel Beckles noted in 1854 that they looked like bird tracks, but might have come from dinosaurs. The identity of the trackmakers was greatly clarified upon the discovery in 1857 of the hind leg of a young Iguanodon, with distinctly three-toed feet, showing that such dinosaurs could have made the tracks. Despite the lack of direct evidence, these tracks are often attributed to Iguanodon.

Thumb spike

The thumb spike is one of the most well-known features of Iguanodon. Although it was originally placed on the animal's nose by Mantell, the complete Bernissart specimens allowed Dollo to correctly place it on the hand, as a modified thumb. (This would not be the last time a dinosaur's modified thumb claw would be misinterpreted; Noasaurus, Baryonyx, and Megaraptor are examples since the 1980s where an enlarged thumb claw was first put on the foot, as in dromaeosaurids.)

This thumb is typically interpreted as a close-quarters stiletto-like weapon against predators, but this has not been accepted, as the spike was not hollow,

Possible social behaviour

Although sometimes interpreted as the result of a single catastrophe, the Bernissart finds instead are now interpreted as recording multiple events. According to this interpretation, at least three occasions of mortality are recorded, and though numerous individuals would have died in a geologically short time span (?10–100 years), However, this is not supported today. Their thumb spikes were mistaken for horns, and they were depicted as elephant-like quadrupeds, yet this was the first time an attempt was made at constructing full-size dinosaur models. Several motion pictures have featured Iguanodon. In the Disney film Dinosaur, an Iguanodon named Aladar served as the protagonist with three other iguanodonts as other main characters; a loosely-related ride of the same name at Disney's Animal Kingdom is based around bringing an Iguanodon back to the present. Iguanodon is one of the three dinosaur genera that inspired Godzilla; the other two were Tyrannosaurus and Stegosaurus. Iguanodon has also made appearances in some of the many Land Before Time films, as well as episodes of the television series.

Aside from appearances on the silver screen, Iguanodon has also been featured on the television documentary miniseries Walking with Dinosaurs (1999) produced by the BBC, and played a starring role in Sir Arthur Conan Doyle's book, The Lost World. It also was present in Bob Bakker's Raptor Red (1995), as a Utahraptor prey item. A main belt asteroid, , has been named 9941 Iguanodon in honour of the genus.

Because it is both one of the first dinosaurs described and one of the best-known dinosaurs, Iguanodon has been well-placed as a barometer of changing public and scientific perceptions on dinosaurs. Its reconstructions have gone through three stages: the elephantine quadrupedal horn-snouted reptile of the Victorians; a bipedal but still fundamentally reptilian animal using its tail to prop itself up; and finally, its current, more agile and dynamic representation, able to shift from two legs to all fours. The second representation dominated the twentieth century, but was slowly overturned during the 1960s.

References

External links

• Paper Dinosaurs, 1824–1969: 2. Mantell's Iguanodon Teeth, 1825, from Linda Hall Library.
• The Bernissart Iguanodons (Iguanodon herd found in Belgium).
• Iguanodon DinoData.

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