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Iguanodon

Iguanodon, meaning “Iguana tooth,” is a genus of ornithopod dinosaur known from the Kimmeridgian age of the Late Jurassic Period to the Cenomanian age of the Late Cretaceous Period. It lived in Asia, Europe and North America. Research in the early 2000s suggests however that only one species, I. bernissartensis, is well-substantiated, and lived during the Early Cretaceous Period in Europe.

It was first discovered in 1822 and described three years later by English geologist Gideon Mantell. Iguanodon was the second dinosaur to be formally named, after Megalosaurus. Iguanodon was one of the three genera, along with Megalosaurus and Hylaeosaurus, originally used to define Dinosauria.

Legend has it that Mantell’s wife, Mary Ann, discovered the first teeth of an Iguanodon in the strata of Tilgate Forest in Whitemans Green, Cuckfield, Sussex, England in 1822 while her husband was visiting a patient. However, there is no evidence that Mantell took his wife with him while seeing patients. In 1851, he admitted that it was him who had found the teeth.

Studies of Mantell’s notebooks show that he first acquired large fossil bones from a quarry at Whitemans Green in 1820. He at first believed the findings were those of a giant crocodile. He mentioned in 1821 that he found herbivorous teeth and began to consider the possibility that a large herbivorous reptile was present in the strata.

However, in his 1822 publication Fossils of the South Downs he declared that he did not dare suggest a connection between the teeth and his incomplete skeleton, presuming that his finds presented two large forms, one carnivorous and one herbivorous.

Mantell first presented the teeth to the Geological Society of London in May 1822. But the members there dismissed them as fish teeth or incisors of a rhinoceros from a Tertiary stratum. Further analysis on 23 June 1823 by Georges Cuvier also suggested the teeth were those of a rhinoceros, but retracted that theory the next day. After gaining Cuvier’s acceptance, Mantell tried to corroborate his theory by finding a modern-day parallel among extant reptiles.

Mantell visited the Royal College of Surgeons in 1824 with the found teeth. Assistant curator Samuel Stutchbury believed the teeth were those of an iguana he had recently prepared, albeit twenty times longer. Mantell did not describe his findings in detail until 10 February 1825, when he presented a paper on the remains to the RGS of London.

In recognition of the resemblance of the teeth to those of the iguana, Mantell named his new genus Iguanodon. Based on isometric scaling, he estimated that the creature bearing the teeth may have been 60 feet long.

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. Many future discoveries provided even better specimens. During this time, tension developed between Mantell and scientist Richard Owen, whose ideas in creationism were better funded and had more connections in politics and science. Owen opposed early versions of evolutionary science, scaling down giant dinosaurs and determining they were not simply giant lizards, but were more advanced and mammal-like, characteristics given to them by God; according to the understanding of the time, they could not have been “transmuted” from reptiles to mammal-like creatures.

Mantell in 1849 realized that Iguanodon was not a heavy, pachyderm-like animal, but it had slender forelimbs. Mantell’s death in 1952 left him unable to participate in the creation of the Crystal Palace dinosaur sculptures, leaving Owen’s vision of the dinosaurs accepted by the general public for decades.

Moving ahead, the largest Iguanodon discovery to date came in 1878 in a coal mine at Bernissart in Belgium, at a depth of 1056 feet. At least 38 Iguanodon individuals were unearthed at the site, most of which were adults. Many of the specimens went on public display in 1882 and are still present for viewing today. Nine are displayed as standing mounts, and 19 are still in the Museum’s storage. A replica of one of the finds is on display at the Oxford University Museum of Natural History and another at the Sedgwick Museum in Cambridge.  Most of the new finds were referred to a new species, I. bernissartensis, the only recognized species today.

The Iguanodon skeletons are some of the first complete dinosaur skeletons known. The science of conserving fossil remains was in its infancy, and was ill-equipped to deal with what soon became known as “pyrite disease.” Pyrite in the bones was changing to iron sulphate, damaging the remains by causing them to crack and crumble. When in the ground, the bones were exposed to moisture that prevented this from happening, but when removed into the drier open air, the natural chemical conversion began to occur. <br>

Not knowing the true cause, and thinking it was an actual infection, early fossil curators attempted to treat the problem with a combination of alcohol, arsenic, and shellac. This combination was intended to simultaneously penetrate (alcohol), kill any biological agent (arsenic), and harden (shellac) the fossils. This treatment had the unintended effect of sealing in moisture and extending the period of damage. Modern treatments instead involve either monitoring the humidity of fossil storage, or, for fresh specimens, preparing a special coating of polyethylene glycol that is then heated in a vacuum pump, so moisture is immediately removed and pore space is infiltrated with polyethylene glycol to seal and strengthen the fossil.

Excavations at the quarry were halted in 1881, although it was not exhausted of fossils, as recent drilling operations have shown. The mine was prepared to be reopened for paleontological purposes during World War I, when the region was occupied by German forces. However, the allies recaptured Bernissart just as the first fossils were 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. <br>

Because Iguanodon was the one of the first dinosaur genera to have been named, numerous species have been assigned to it. While never becoming a waste basket taxon that several other early genera of dinosaurs had become, Iguanodon has had a complicated history, and its taxonomy continues to undergo revisions to this day. While Iguanodon currently has several named species, I. bernissartensis is the only truly recognized species. This species is best known for the many skeletons discovered in Bernissart, but is also known from remains across Europe. David Norman suggested that it includes the dubious Mongolian I. orientalis, but this has not been followed by other researchers. <br>

One of the first details noted about Iguanodon was it herbivorous teeth. As Mantell noted, the remains were unlike any modern reptile, especially in the toothless, scoop-shaped form of the lower jaw union, which he found best compared to that of the two-toed sloth. He also suggested that Iguanodon had a prehensile tongue which was used to gather food, like that of a giraffe. Although more complete fossils have shown this feature to be an error. Hyoid bones that supported the tongue are heavily built, implying a muscular, non-prehensile tongue used more for moving the food around in the mouth.

Iguanodon teeth are much like that of a modern iguana, like its name suggests. Unlike hadrosaurids, which had columns of replacement teeth, Iguanodon only had one replacement tooth at a time for each position. The upper jaw held 29 teeth per side, with none in the front of the jaw. The lower jaw contained 25 teeth. Teeth in the lower jaw are broader than those in the upper. It is believed Iguanodon had some sort of cheek-like structure, muscular or non-muscular, to retain food in the mouth. <br>

One of Iguanodon’s most distinctive features were its large thumb spikes, which were possibly used for defense against predators and also for foraging for food. Iguanodon could shift from a quadrupedal to bipedal stance, aiding in foraging at a higher level. Iguanodon is estimated to have weighed 3.5 tons on average, and measured up to 33 feet long as an adult. Some specimens may have reached lengths up to 43 feet.

The skull was tall and narrow and 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 jaw teeth to grind anything caught in between and provide an action that is roughly equivalent to that of mammalian chewing. Because teeth were continually replaced, Iguanodon could have used this mechanism throughout its life, and could eat tough plant matter. Additionally, the front ends of the animal’s jaws were toothless and tipped with bony nodes, both upper and lower, providing a rough margin that was likely covered and lengthened by a keratinous material to form a cropping beak for biting off twigs and shoots.

The exact diet of Iguanodon is not known. But the size of the largest specimen would have allowed them access to food from ground level to tree foliage upwards of 16.5 feet high. Whatever its diet may have been, Iguanodon is regarded as a dominant medium to large herbivore during its reign on earth.

Iguanodon was initially portrayed as a quadrupedal horn-nosed beast. However, as more bones were discovered, Mantell discovered that the forelimbs were much smaller than the hind limbs. Mantell’s rival Owen suggested all four legs were similar and pillar-like. Since Mantell’s failing health kept him from overseeing the reconstruction of the first Iguanodons, Owen’s vision formed the basis on which subsequent sculptures were modeled.  But finally, later discoveries revealed the dinosaur had a bipedal nature. However, it was inaccurately depicted with the tail dragging on the ground, giving it posture when on its hind limbs only.

This posture was deemed unlikely due to the tail stiffened with ossified tendons. To act as a tripod, the tail would literally have to be broken. Putting the animal in a horizontal posture makes many aspects of the arms and pectoral girdle more understandable. For example, the hand is relatively immobile, with the three central fingers grouped together, bearing hoof-like phalanges, and able to hyperextend. This would have allowed them to bear weight. The wrist is also relatively immobile, and the arms and shoulder bones robust. These features all suggest that the animal spent most of its time on all fours.

It appears that Iguanodon may also have become more quadrupedal as it aged and became heavier; juveniles have shorter arms than adults. When walking as a quadruped, the animal’s hands would have been held so that the palms faced each other, as shown by iguanodontian tracks and the anatomy of this genus’s arms and hands. The maximum speed of Iguanodon has been estimated at 14.9 mph, which would have been as a biped. It would not have been able to gallop as a quadruped.

It could be assumed since the Bernissart Iguanodon discoveries came with numerous skeletons in a single bed that these dinosaurs were herding animals, but this may not be the case. An argument against herding is that juvenile remains are very uncommon at the Bernissart site, unlike modern cases with herd mortality. It is more possible that these dinosaurs were the victims of flash floods whose carcasses accumulated in a lake or marshy setting. However, a group of dinosaurs found in Nehden, Germany possibly shows a sign of herding Iguanodons migrating through rivers.

Iguanodon has been popularized in culture since it was first described more than 175 years ago. Two life-size reconstructions were built in 1852 at the Crystal Palace in London, which greatly contributed to the popularity of the genus.

Several motion pictures have featured Iguanodon, including the Disney film “Dinosaur.” Iguanodon is also one of three dinosaur genera that inspired “Godzilla.” It has also made appearances in the “Land Before Time” films and the television series. Iguanodon was also featured on the BBC miniseries “Walking with Dinosaurs” and had a starring role in Sir Arthur Conan Doyle’s novel, “The Lost World.” It was also featured on the Discovery Channel series “Dinosaur Planet,” and a number of other works.

Iguanodon


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