Old Earth Ministries Online Dinosaur Curriculum

Free online curriculum for homeschools and private schools

From Old Earth Ministries (We Believe in an Old Earth...and God!)

NOTE:  If you found this page through a search engine, please visit the intro page first. 


Lesson 55 - Iguanodon

Iguanodon (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 21st century suggests that there is only one well-substantiated species: I. bernissartensis, that lived from the late Barremian to the earliest Aptian ages (Early Cretaceous) in Europe, between about 126 and 125 million years ago. Iguanodon's most distinctive features were its large thumb spikes, which were possibly used for defence against predators, combined with long prehensile fifth fingers able to forage for food.


Quick Facts


Length:  33 feet

Weight:   7,000 lbs

Date Range:   161 - 100 Ma



Cast of an I. bernissartensis skeleton mounted in a bipedal, horizontal posture (Picture Source)

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.


Iguanodon was a bulky herbivore that could shift from bipedality to quadrupedality. The
Iguanodon scale
(Picture Source)
 best-known species, I. bernissartensis, is estimated to have weighed about 3 tonnes (3.5 tons) on average, and measured about 10 metres long (33 ft) as an adult, with some specimens possibly as long as 13 metres (43 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.

The arms were long (up to 75% the length of the legs in I. bernissartensis) and robust, with rather inflexible hands built so that the three central fingers could bear weight. The thumbs were conical spikes that stuck out away from the three main digits. They could have been used for defense, or for foraging for food. The little finger was elongated and dextrous, and could have been used to manipulate objects. The legs were powerful, but not built for running, and each foot had three toes. 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). Overall, in body structure, it was not too dissimilar from its later relatives, the hadrosaurids.

Classification and origins

Iguanodon gives its name to the unranked clade Iguanodontia, a very populous group of ornithopods with many species known from the Middle Jurassic to the Late Cretaceous. Aside from Iguanodon, the best-known members of the clade include Dryosaurus, Camptosaurus, Ouranosaurus, and the duck-bills, or hadrosaurs.


Because Iguanodon is one of the first dinosaur genera to have been named, numerous
Modern sculpture in a German park  (Picture Source)
 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. Remains of the best-known species have come from Belgium, England, Germany, Spain, and France. Remains of similar animals possibly belonging to this genus have been found in Tunisia and Mongolia, and a distinct species is present in Utah, USA. Gregory Paul has recommended limiting use of I. bernissartensis to the Bernissart finds, and using I. sp. (meaning undetermined species) for robust iguanodontian remains from Barremian-age rocks of Europe. Thus, after thorough restudy, what had been seen as a quintessentially British dinosaur may in fact be poorly known from England.

I. anglicus was the original type species, but the holotype was based on a single tooth and only partial remains of the species have been recovered since. In March 2000, the International Commission on Zoological Nomenclature changed the type species to the much better known I. bernissartensis. The original Iguanodon tooth is held at Te Papa Tongarewa, the national museum of New Zealand in Wellington, although it is not on display. The fossil arrived in New Zealand following the move of Gideon Mantell's son Walter there; after the elder Mantell's death, his fossils went to Walter.


Feeding and diet

One of the first details noted about Iguanodon was that it had the teeth of a herbivorous reptile, although there has not always been consensus on how it ate. As Mantell noted, the remains he was working with were unlike any modern reptile, especially in the toothless, scoop-shaped form of the lower jaw symphysis, which he found best compared to that of the two-toed sloth and the extinct ground sloth Mylodon.

Iguanodon teeth are, as the name suggests, like those of an iguana, but larger. Unlike hadrosaurids, which had columns of replacement teeth, Iguanodon only had one
Iguanodon skull
I. bernissartensis skull, Muséum national d'Histoire naturelle, Paris  (Picture Source
 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, and the lower jaw 25; the numbers differ because teeth in the lower jaw are broader than those in the upper. Because the tooth rows are deeply inset from the outside of the jaws, and because of other anatomical details, it is believed that, as with most other ornithischians, Iguanodon had some sort of cheek-like structure, muscular or non-muscular, to retain food in the mouth.

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. Because the teeth were always replaced, the animal could have used this mechanism throughout its life, and could eat tough plant material. 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. Its food gathering would have been aided by its flexible little finger, which could have been used to manipulate objects, unlike the other fingers.

Exactly what Iguanodon ate with its well-developed jaws is not known. The size of the larger species, such as I. bernissartensis, would have allowed them access to food from ground level to tree foliage at 4–5 metres high (13–16.5 ft).  Whatever its exact diet, due to its size and abundance, Iguanodon is regarded as a dominant medium to large herbivore for its ecological communities. In England, this included the small predator Aristosuchus, larger predators Eotyrannus, Baryonyx, and Neovenator, low-feeding herbivores Hypsilophodon and Valdosaurus, fellow "iguanodontid" Mantellisaurus, the armoured herbivore Polacanthus, and sauropods like Pelorosaurus.

Posture and movement

Iguanodon was initially portrayed as a quadrupedal horn-nosed beast. However, 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
Iguanodon hand 
Hand of Iguanodon shown in the Natural History Museum  (Picture Source
 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 time on all fours.

Furthermore, it appears that Iguanodon became more quadrupedal as it got older and heavier; juvenile I. bernissartensis have shorter arms than adults (60% of hindlimb length versus 70% for 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 trackways and the anatomy of this genus's arms and hands. 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). The maximum speed of Iguanodon has been estimated at 24 km/h (14.9 mph), which would have been as a biped; it would not have been able to gallop as a quadruped.

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-quarter stiletto-like weapon against predators, although it could also have been used to break into seeds and fruits, or against other Iguanodon. One author has suggested that the spike was attached to a venom gland, but this has not been accepted, as the spike was not hollow, nor were there any grooves on the spike for conducting venom.

In popular culture

Since its description in 1825, Iguanodon has been a feature of worldwide popular culture. Two lifesize reconstructions of Iguanodon built at the Crystal Palace in London in 1852 greatly contributed to the popularity of the genus. 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 as well as featuring in an episode of the Discovery Channel documentary, Dinosaur Planet. It also was present in Bob Bakker's Raptor Red (1995), as a Utahraptor prey item. A main belt asteroid, 1989 CB3, 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 early 20th century, but was slowly overturned during the 1960s.

Return to the Old Earth Ministries Online Dinosaur Curriculum homepage.

horizontal rule


Bay State Replicas - Thumb spike, tooth

Black Hills Institute - none