Iguanodon

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

Iguanodon was a large, bulky herbivore, a plant-eater. 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 were possibly used to defend against predators. It also had long fifth fingers that could grip, which helped it gather food.

The genus was named in 1825 by English geologist Gideon Mantell, 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 originally used to define Dinosauria. The genus belongs to the larger group Iguanodontia, which also includes the duck-billed hadrosaurs. The classification of this genus is still a topic of study, 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 and studied by Louis Dollo. These were given the new species name I. bernissartensis. In the early 21st century, scientists realised that the remains called Iguanodon in England actually belonged to four different species, which were not closely related. Those species 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" became fixed around the well-known species based mainly on the Belgian specimens. Another species, Iguanodon galvensis, was named in 2015 based on fossils found in the Iberian Peninsula.

Scientific understanding of Iguanodon has grown over time as new information came from fossils. Many specimens of this genus have been found, including nearly complete skeletons from two well-known bone beds. These have helped researchers form hypotheses about many parts of the living animal, such as how it fed, moved, and behaved 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 the public's picture of dinosaurs. Its artistic likeness has changed a great deal as scientists have 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, but 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 famous French naturalist Georges Cuvier later admitted his mistake, and Mantell named the animal Iguanodon, or 'iguana-tooth', because its teeth looked like those of an iguana. Over the following decades, scientists made many changes to how Iguanodon was understood, including a famous mistake about a "horn" on its nose, which was later found to be a thumb.

The largest find of Iguanodon remains came in 1878, when two mineworkers in Belgium found a skeleton in a coal mine. Scientists later uncovered at least 38 individuals, and Louis Dollo reconstructed the skeletons in 1882. Research continued into the 20th and 21st centuries, with scientists learning more about how the animal walked and fed. Over time, some of the species once placed in Iguanodon were moved into new groups, so today Iguanodon is understood to be based on one well-studied species.

Description

Iguanodon were bulky plant-eaters that could shift from bipedality to quadrupedality. The best-known species, I. bernissartensis, is estimated to have been about 9 metres (30 feet) long as an adult, and some large individuals may have reached 11 metres (36 feet). Estimates of their weight vary. One method based on allometry suggests a smaller individual of about 8 metres (26 feet) weighed about 3.8 metric tons (4.2 short tons).

The arms were long and sturdy, and the hands were not very flexible. The three central fingers could bear weight. The thumbs were conical spikes, and scientists still debate their exact use, which may have been defense or finding food. The little finger was long and skilful, and could have been used to handle objects. The legs were powerful but not built for running, and each foot had three toes. The backbone and tail were stiffened by ossified tendons, which turned to bone during life.

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

Classification and evolution

Iguanodon gives 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, or hadrosaurs. In older books, Iguanodontidae was shown as a separate family. This family was often a bit of 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 using family tree analysis, they found that the old idea of Iguanodontidae was not a true single group, as described by the term 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 descended 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 been rejected. The cladogram below follows an analysis by Andrew McDonald, 2012.

Species

Because Iguanodon was one of the first dinosaur genera to be named, many species have been placed in it over time. Its history has been complicated, and scientists still revise 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 are also known from across Europe. The original type species, I. anglicus, was based on a single tooth, so in 2000 the International Commission on Zoological Nomenclature changed the type species to I. bernissartensis. The original tooth is held at Te Papa Tongarewa, the national museum of New Zealand in Wellington, though 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, and these are listed here.

Palaeobiology

Early scientists noticed 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, which Mantell compared to the two-toed sloth and the extinct ground sloth Mylodon. Mantell also suggested that it had a prehensile tongue like a giraffe, but more complete remains showed this was probably an error. The skull allowed the upper and lower teeth to grind food as the jaws closed, similar to chewing in mammals. Because the teeth were replaced throughout life, Iguanodon could eat tough plant material, including twigs and shoots cut off by a beak covered in a tough material. Its size would have let it reach food from ground level up to tree foliage about 4–5 metres (13–16 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, and the Bernissart skeletons revealed 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, and that the animal probably spent much of its time on all fours. Young juvenile animals had shorter arms than adults, which 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, but the Bernissart skeletons 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 record several deaths over time rather than one single disaster, and some evidence suggests that these animals may have sometimes lived in groups, though this is not certain. Some bones also show injuries and disease, including a fractured hip bone and signs of osteoarthritis in two other individuals.

In popular culture

Since it was described in 1825, Iguanodon has been a feature of popular culture around the world. Two life-size models of Mantellodon, which was considered Iguanodon at the time, were built at the Crystal Palace in London in 1852. They helped make the genus popular. Their thumb spikes were mistaken for horns, and they were shown as elephant-like animals walking on four legs. In 1910, Heinrich Harder portrayed a group of Iguanodon in Tiere der Urwelt, a classic 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 dinosaur was featured in the BBC documentary miniseries Walking with Dinosaurs (1999) and played a starring role in Sir Arthur Conan Doyle's book The Lost World. Because it is one of the first dinosaurs described and one of the best-known, Iguanodon shows how public and scientific ideas about dinosaurs have changed. Its reconstructions moved from an elephant-like, horn-snouted animal, to a two-legged reptile that used its tail for support, to its current, more agile form that can move on two legs or four.