Iguanodon

***Iguanodon*** is a genus of the iguanodont family, which was named after it. It lived in the early Cretaceous in Europe; specimens from Asia and North America have been reassigned to *Altirhinus* and *Dakotadon*, respectively. It lived 125/126 million years ago. Its most famous feature was the spikes on each opposable thumb. It could switch from being bipedal to quadrupedal.

*Iguanodon* was discovered in 1822 and described three years later by English doctor and fossil-collector Gideon Mantell. It's name means "iguana tooth".

A large, bulky herbivore, *Iguanodon* is thought by some to be in the same family as the duck-billed hadrosaurs. The taxonomy of the genus continues to be a topic of study as new species are named or long-standing ones reassigned to other genera.

## Description

*Iguanodon* was a large herbivore that could shift from two legs to four legs. The best-known one, *I. bernissartensis*, weighed about 3.5 tons, and was about 33 ft long as an adult; some may have been as long as 43 ft. It had a large, tall but slim skull, with a toothless beak probably covered with keratin, and teeth like those of an iguana, but much larger and more closely packed.

The arms were long (up to 75% the length of the legs in *I. bernissartensis*) and stout, with hands that were hard to bend and built so that the three central fingers could bear weight. The thumbs were cone-shaped spikes that stuck out from the three main digits. In early restorations, the spike was placed on its nose. Later fossils revealed the true place of the thumb spikes, but their true role is still debated. They could have been used for defense, or to get food. The little finger was slim and dextrous, and could have been used to operate objects. The legs were strong, but not built to run, and each foot had three toes. The spine and tail were supported and stiffened by ossified tendons, which were tendons that turned to bone during life (these rod-like bones are left out from most skeletal mounts and drawings). As a whole, its form was a lot like that of its later relatives, the hadrosaurids.

## Classification

*Iguanodon* gives its name to Iguanodontia, a large group of ornithopods from the Middle Jurassic to the Late Cretaceous. Aside from *Iguanodon*, some of the best-known of the clade are *Dryosaurus*, *Camptosaurus*, *Ouranosaurus*, and the duck-bills, or hadrosaurs. In old sources, Iguanodontidae was shown as a distinct family. Iguanodontidae now just has *Iguanodon*. *Iguanodon* lies between *Camptosaurus* and *Ouranosaurus* in cladograms, and is likely descended from a camptosaur-like animal. At one point, Jack Horner thought, based on skull traits, that hadrosaurids formed two more distantly related groups, with *Iguanodon* on the line to the flat-headed hadrosaurines, and *Ouranosaurus* on the line to the crested lambeosaurines, but this has been rejected.

## History

The first trace of Iguanodontia was a fossilized tooth found by Gideon Mantell's wife, Mary Ann. Mantell named the creature *Iguanodon* as the teeth found were similar to those of an iguana.

Years later he found a full specimen of Iguanodontia in a quarry in Maidstone. The Maidstone slab allowed the first skeletal rebuilds and artistic renderings of the Iguanodontia. The most famous mistake was putting a horn, also discovered by Mantell's wife, on the nose of the dinosaur. The finding of a lot of top specimens of I. bernissartensis in 1878 showed that the horn was in fact a modified thumb, that may have been used for defense.

## Paleobiology

### Posture and movement

The first fossil remains were incomplete, which led to much speculation on the gait and life of *Iguanodon*. *Iguanodon* was first shown as a four-legged horn-nosed beast, but as more bones were found, Mantell saw that the front legs weren't as long as the back legs. His rival Owen thought 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 two-legged nature was revealed with the discovery of the Bernissart skeletons. But, it was depicted in an upright posture, with the tail dragging on the ground, acting as the third leg of a tripod.

When David Norman looked at *Iguanodon*, he showed that this posture was unlikely, as its long tail was stiff from ossified tendons. If Iguanodon tried to lean on its tail, it would break. If you put it in a horizontal stance, it explains many traits of the arms and pectoral girdle. For example, the hand is relatively stiff, with the three central fingers fused in a hoof-like structure. This would have let them bear weight. The wrist is quite still, and the arms and shoulder bones strong. All these traits show that they spent time on all fours.

It also seems that *Iguanodon* spent more time on all fours as it got old and heavier; young *I. bernissartensis* have shorter arms (60% of hind limb length) than adults (70%). When they walked on all fours, their hands would have been held so that the palms faced each other, as shown by tracks and the form of their arms and hands. The three toed foot of *Iguanodon* was long, and when they walked, both the hand and the foot would have been used on the fingers and toes. The top speed of *Iguanodon* may have been 14.9 mph, which would have been on two legs; it could not have galloped on four legs.

Large three-toed footprints are known in Early Cretaceous rocks of England, like Wealden beds on the Isle of Wight. Some authors linked them with dinosaurs early on. Samuel Beckles said in 1854 that they looked like bird tracks, but might have come from dinosaurs. In 1857, the hind leg of a young *Iguanodon*, with three-toed feet, was found and these tracks may be from the *Iguanodon*. While there's no firm proof, these tracks are often said to be *Iguanodon*. Footprints in England show what may be an *Iguanodon* on all fours, but the foot prints are poor, which makes a direct connection hard.

## External links

* The Bernissart Iguanodons (*Iguanodon* herd found in Belgium).
* *Iguanodon* DinoData.
* Mantell's *Iguanodon* tooth in the collection of the Museum of New Zealand Te Papa Tongarewa

1. ↑ Norman, David B. 2004. "Basal Iguanodontia". In Weishampel D.B., Dodson P., and Osmólska H. (eds) The Dinosauria. 2nd ed, Berkeley: University of California Press. pp413–437 ISBN 0-520-24209-2
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7. ↑ Norman, David B. "Iguanodontidae". *The Illustrated Encyclopedia of Dinosaurs*, 110–115.
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10. ↑ 10.0 10.1 10.2 Norman, David B. (2004). "Basal Iguanodontia". In Weishampel, D.B., Dodson, P., and Osmólska, H. (eds.). *The Dinosauria* (2nd ed.). Berkeley: University of California Press. pp. 413–437. ISBN 0-520-24209-2.
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