Iguanodon

Iguanodon (its name means 'iguana-tooth') was named in 1825. It is a genus of iguanodontian dinosaur. Many species found around the world have been placed in this genus. They lived from the Late Jurassic to the Early Cretaceous. In the early 21st century, scientists defined Iguanodon around at least one well-known species, Iguanodon bernissartensis. This species lived during the Barremian to early Aptian ages of the Early Cretaceous in Belgium, Germany, England, and Spain, about 126 to 122 million years ago.

Iguanodon was a large plant-eater, or herbivore. It could grow up to 9–11 metres (30–36 ft) long and weigh 4.5 metric tons (5.0 short tons). It had large thumb spikes, which may have been used to defend against predators. It also had long fifth fingers that could grip, which helped it gather food.

English geologist Gideon Mantell named the genus in 1825, based on fossils found in England. He gave it the species name I. anglicus. Iguanodon was the second kind of dinosaur formally named from fossils, after Megalosaurus. Together with Megalosaurus and Hylaeosaurus, it was one of the three genera first used to define Dinosauria. It belongs to the larger group Iguanodontia, which also includes the duck-billed hadrosaurs. Scientists are still studying how this genus is classified, as new species are named and older ones are moved to other genera.

In 1878, much more complete remains of Iguanodon were found in Belgium. Louis Dollo studied them and named the new species I. bernissartensis. In the early 21st century, scientists found that the remains called Iguanodon in England actually belonged to four different species that were not closely related. These were moved into Mantellisaurus, Barilium, and Hypselospinus. The original type species, I. anglicus, was found to be a nomen dubium, meaning its name is not valid. So the name "Iguanodon" was kept for the well-known species based mainly on the Belgian specimens. In 2015, another species, Iguanodon galvensis, was named based on fossils found in the Iberian Peninsula.

Scientists have learned more about Iguanodon over time as new fossils were found. Many specimens have been discovered, including nearly complete skeletons from two well-known bone beds. These finds have helped researchers form hypotheses about many parts of the living animal, such as how it fed, moved, and lived with others of its kind. As one of the first dinosaurs to be scientifically well known, Iguanodon has held a small but important place in people's picture of dinosaurs. How artists draw it has changed a lot as scientists learned new things from its remains.

Discovery and history

Iguanodon was first recognised from teeth found in England in the 1820s. A popular legend says that the wife of Gideon Mantell, Mary Ann, found the first teeth. There is no evidence for this, and Mantell himself later said that he had found them. Mantell first showed the teeth to the Royal Society of London, but its members thought they belonged to fish or to a rhinoceros. The French naturalist Georges Cuvier later admitted his mistake. Mantell named the animal Iguanodon, which means 'iguana-tooth', because its teeth looked like those of an iguana. Over the following decades, scientists changed their ideas about Iguanodon many times. One famous mistake was about a "horn" on its nose. This was later found to be a thumb.

The largest find of Iguanodon remains came in 1878. Two mineworkers in Belgium found a skeleton in a coal mine. Scientists later uncovered many more individuals, and Louis Dollo rebuilt the skeletons in 1882. Research continued into the 20th and 21st centuries. Scientists learned more about how the animal walked and what it ate. Over time, some species once placed in Iguanodon were moved into other groups. Today, Iguanodon is understood to be based on one well-studied species.

Description

Iguanodon were large plant-eaters. They could walk on two legs or on all four. The best-known species, I. bernissartensis, is estimated to have been about 9 metres (30 feet) long as an adult. Some large individuals may have reached 11 metres (36 feet). Estimates of their weight vary. One method based on allometry suggests that a smaller individual of about 8 metres (26 feet) weighed about 3.8 metric tons (4.2 short tons).

Their arms were long and strong, but the hands were not very flexible. The three middle fingers could bear weight. The thumbs were pointed spikes. Scientists still debate what they were used for, and they may have been used for defense or for finding food. The little finger was long and skilful, and it could have been used to handle objects. The legs were powerful but not built for running. Each foot had three toes. The backbone and tail were stiffened by ossified tendons, which turned to bone while the animal was alive.

These animals had large, tall, narrow skulls with toothless beaks. The beaks were probably covered with keratin. Their teeth looked like those of iguanas, but they were much larger and packed more closely together. Unlike hadrosaurids, Iguanodon had only one replacement tooth at a time for each position. The upper jaw held up to 29 teeth on each side, but none were at the front of the jaw. The lower jaw had 25 teeth. Scientists believe Iguanodon had a cheek-like structure, cheek, that helped keep food in the mouth.

Classification and evolution

Iguanodon gave its name to Iguanodontia, a very large group of plant-eating dinosaurs called ornithopods. Its members lived from the Middle Jurassic to the Late Cretaceous. Other well-known members include Dryosaurus, Camptosaurus, Ouranosaurus, and the duck-billed dinosaurs, called hadrosaurs. In older books, Iguanodontidae was shown as a separate family. This family was often a catch-all group for dinosaurs that did not fit anywhere else. Animals such as Callovosaurus, Camptosaurus, Craspedodon, Kangnasaurus, Mochlodon, Muttaburrasaurus, Ouranosaurus, and Probactrosaurus were usually placed in it.

When scientists began studying family trees more closely, they found that the old idea of Iguanodontidae was not a true single group. This kind of group is called paraphyletic. These animals sit at different points compared with hadrosaurs on a family tree diagram, called a cladogram. Today, Iguanodontidae includes only Iguanodon. Some groups, such as Iguanodontoidea, are still used by scientists, though many older iguanodontids are now placed in the superfamily Hadrosauroidea. Iguanodon sits between Camptosaurus and Ouranosaurus on these diagrams and probably came from a camptosaur-like animal. Jack Horner once suggested, mostly based on skull features, that hadrosaurs formed two separate groups: one leading to the flat-headed hadrosaurines with Iguanodon, and one leading to the crested lambeosaurines with Ouranosaurus. This idea has since been rejected. The cladogram below follows an analysis by Andrew McDonald, 2012.

Species

Iguanodon was one of the first dinosaur genera to be named, so many species have been placed in it over time. Its history is complicated, and scientists still change how it is classified. Today, only one species, I. bernissartensis, is still considered valid. It is best known from the many skeletons found at Bernissart, and its remains have also been found across Europe. The first type species, I. anglicus, was based on a single tooth. In 2000, the International Commission on Zoological Nomenclature changed the type species to I. bernissartensis. The original tooth is kept at Te Papa Tongarewa, the national museum of New Zealand in Wellington, but it is not on display.

Many other species once placed in Iguanodon have since been moved to different genera or are now considered doubtful. Some of these are listed below.

Some species that were once thought to be Iguanodon are now grouped as doubtful. They are listed here.

Palaeobiology

Early scientists saw that Iguanodon had the teeth of a herbivorous reptile, but they were not always sure how it ate. Its lower jaw had a toothless, scoop-shaped front. Mantell compared this to the two-toed sloth and the extinct ground sloth Mylodon. Mantell also thought it may have had a prehensile tongue, like a giraffe. Later, more complete remains showed this was probably a mistake. The skull let the upper and lower teeth grind food as the jaws closed, a bit like chewing in mammals. The teeth were replaced throughout life, so Iguanodon could eat tough plant material, including twigs and shoots. A beak covered in a tough material cut them off. Its size would have let it reach food from the ground up to tree foliage about 4 to 5 metres (13 to 16 ft) high.

Because early fossils were incomplete, scientists argued for a long time about how Iguanodon stood. Later finds showed that its front legs were much smaller than its back legs. The Bernissart skeletons showed that it could walk on two legs. David Norman later showed that its long tail was too stiff to drag on the ground as a third leg. He thought the animal probably spent much of its time on all fours. Young juvenile animals had shorter arms than adults. This suggests that Iguanodon became more four-legged as it grew heavier. Its top speed has been estimated at 24 km/h (15 mph) while walking on two legs.

Its famous thumb spike was first placed on the nose by Mantell. The Bernissart skeletons later showed that it was a modified thumb. It was most likely a close-range stiletto-like weapon against predators.

Fossil remains from Bernissart seem to show several deaths over time, rather than one single event. Some evidence suggests that these animals may sometimes have lived in groups, though this is not certain.

Some bones also show injuries and illness. These include a fractured hip bone and signs of osteoarthritis in two other individuals.

In popular culture

Since it was first described in 1825, Iguanodon has been part of popular culture around the world. In 1852, two life-size models of Mantellodon, which was thought to be Iguanodon at the time, were built at the Crystal Palace in London. They helped make the dinosaur well known. People thought its thumb spikes were horns, so the models looked like big elephant-like animals walking on four legs. In 1910, Heinrich Harder drew a group of Iguanodon for Tiere der Urwelt, a German collecting card game about extinct and prehistoric animals.

Iguanodon has appeared in several motion pictures. In the 2000 Disney animated film Dinosaur, an Iguanodon named Aladar is the main character. A loosely related ride of the same name at Disney's Animal Kingdom is based on bringing an Iguanodon back to the present. Iguanodon is one of three dinosaurs that inspired Godzilla, along with Tyrannosaurus rex and Stegosaurus. It has also appeared in some of the The Land Before Time films and television series. The BBC documentary miniseries Walking with Dinosaurs (1999) featured the dinosaur, and it had a starring role in Sir Arthur Conan Doyle's book The Lost World. Because Iguanodon was one of the first dinosaurs to be described and is one of the best known, it shows how ideas about dinosaurs have changed over time. Its pictures changed from an elephant-like animal with a horn on its nose, to a two-legged reptile that used its tail for support, to its current form, which can move on two legs or four.