Iguanodon

At the same time, tension began to build between Mantell and Richard Owen, an ambitious scientist with much better funding and society connections in the turbulent worlds of Reform Act-era British politics and science. Owen, a firm creationist, opposed the early versions of evolutionary science ("transmutationism") then being debated and used what he would soon coin as dinosaurs as a weapon in this conflict. With the paper describing Dinosauria, he scaled down dinosaurs from lengths of over , determined that they were not simply giant lizards, and put forward that they were 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.

In 1849, a few years before his death in 1852, Mantell realised that iguanodonts were not heavy, pachyderm-like animals, as Owen was putting forward, but had slender forelimbs. However, since his passing left him unable to participate in the creation of the Crystal Palace dinosaur sculptures, Owen's vision of the dinosaurs became that seen by the public for decades.

Bernissart mine discoveries and Dollo's new reconstruction

The largest find of Iguanodon remains to that date occurred on 28 February 1878 in a coal mine at Bernissart in Belgium, at a depth of , In 1882, the holotype specimen of I. bernissartensis became one of the first ever dinosaur skeletons mounted for display. It was put together in a chapel at the Palace of Charles of Lorraine using a series of adjustable ropes attached to scaffolding so that a lifelike pose could be achieved during the mounting process. a larger and much more robust animal than the English remains had yet revealed. One specimen, IRSNB 1551, was at first referred to the nebulous, gracile I. mantelli, but is currently referred to Mantellisaurus atherfieldensis. The skeletons were some of the first complete dinosaur skeletons known. Found with the dinosaur skeletons were the remains of plants, fish, and other reptiles,

The science of conserving fossil remains was in its infancy, and new techniques had to be improvised to deal with what soon became known as "pyrite disease". Crystalline pyrite in the bones was being oxidized to iron sulphate, accompanied by an increase in volume that caused the remains to crack and crumble. When in the ground, the bones were isolated by anoxic moist clay that prevented this from happening, but when removed into the drier open air, the natural chemical conversion began to occur. To limit this effect, De Pauw immediately, in the mine-gallery, re-covered the dug-out fossils with wet clay, sealing them with paper and plaster reinforced by iron rings, forming in total about six hundred transportable blocks with a combined weight of a hundred and thirty tons. In Brussels after opening the plaster he impregnated the bones with boiling gelatine mixed with oil of cloves as a preservative. Removing most of the visible pyrite he then hardened them with hide glue, finishing with a final layer of tin foil. Damage was repaired with papier-mâché. This treatment had the unintended effect of sealing in moisture and extending the period of damage. In 1932 museum director Victor van Straelen decided that the specimens had to be completely restored again to safeguard their preservation. From December 1935 to August 1936 the staff at the museum in Brussels treated the problem with a combination of alcohol, arsenic, and 390 kilograms of shellac. This combination was intended to simultaneously penetrate the fossils (with alcohol), prevent the development of mould (with arsenic), and harden them (with shellac). The fossils entered a third round of conservation from 2003 until May 2007, when the shellac, hide glue and gelatine were removed and impregnated with polyvinyl acetate and cyanoacrylate and epoxy glues. Modern treatments of this problem typically 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 that moisture is immediately removed and pore spaces are infiltrated with polyethylene glycol to seal and strengthen the fossil. His reconstruction would prevail for a long period of time, but would later be discounted. During World War I, when the town was occupied by German forces, preparations were made to reopen the mine for palaeontology, and Otto Jaekel was sent from Berlin to supervise. Just as the first fossiliferous layer was about to be uncovered, however, the German army surrendered and had to withdraw. Further attempts to reopen the mine were hindered by financial problems and were stopped altogether in 1921 when the mine flooded.

Turn of the century and the Dinosaur Renaissance

thumb|upright|I. bernissartensis skeletal mount in modern bipedal pose, [[Übersee-Museum Bremen]]

Research on Iguanodon decreased during the early part of the 20th century as World Wars and the Great Depression enveloped Europe. A new species that would become the subject of much study and taxonomic controversy, I. atherfieldensis, was named in 1925 by R. W. Hooley, for a specimen collected at Atherfield Point on the Isle of Wight.

Iguanodon was not part of the initial work of the dinosaur renaissance that began with the description of Deinonychus in 1969, but it was not neglected for long. David B. Weishampel's work on ornithopod feeding mechanisms provided a better understanding of how it fed, and David B. Norman's work on numerous aspects of the genus has made it one of the best-known dinosaurs. In addition, a further find of numerous disarticulated Iguanodon bones in Nehden, Nordrhein-Westphalen, Germany, has provided evidence for gregariousness<!--not quite the same thing, but it'll have to do--> in this genus, as the animals in this areally restricted find appear to have been killed by flash floods. At least 15 individuals, from long, have been found here, most of the individuals belong to the related Mantellisaurus (described as I. atherfieldensis, at that time believed to be another species of Iguanodon). but some are of I. bernissartensis.

One major revision to Iguanodon brought by the Renaissance would be another re-thinking of how to reconstruct the animal. A major flaw with Dollo's reconstruction was the bend he introduced into the tail. This organ was more or less straight, as shown by the skeletons he was excavating, and the presence of ossified tendons. In fact, to get the bend in the tail for a more wallaby or kangaroo-like posture, the tail would have had to be broken. With its correct, straight tail and back, the animal would have walked with its body held horizontal to the ground, arms in place to support the body if needed.

21st century research and the splitting of the genus

thumb|right|I. bernissartensis from the Isle of Wight, [[Dinosaur Isle Museum]]

In the 21st century, Iguanodon material has been used in the search for dinosaur biomolecules. In research by Graham Embery et al. (2003), Iguanodon bones were processed to look for remnant proteins. In this research, identifiable remains of typical bone proteins, such as phosphoproteins and proteoglycans, were found in a rib. In 2007, Gregory S. Paul split I. atherfieldensis into a new, separate genus, Mantellisaurus which has been generally accepted. In 2009 fragmentary iguanodontid material was described from upper Barremian Paris Basin deposits in Auxerre, Burgundy. While not definitively diagnosable to the genus/species level, the specimen shares "obvious morphological and dimensional affinities" with I. bernissartensis.

In 2010, David Norman split the Valanginian species I. dawsoni and I. fittoni into Barilium and Hypselospinus respectively.

In 2011, a new genus Delapparentia was named for a specimen in Spain that was originally thought to belong to I. bernissartensis. In 2025, Franco Sancarlo, Davide Mandorlo and Tracy L. Ford assigned I. galvensis to a new genus, Paulodon, based on putative differences between it and I. bernissartensis; although paleontologists have informally disagreed with the methodology behind the paper. In 2026, Rotatori and colleagues suggested that Delapperantia is a distinct taxon more closely related to Magnamanus and "I." galvensis, the latter of which they suggested that a new genus name would be required.

Description

thumb|Size of Iguanodon bernissartensis compared to a human

Iguanodon were bulky herbivores that could shift from bipedality to quadrupedality. The only well-supported species, I. bernissartensis, is estimated to have measured about long as an adult, with some specimens possibly as long as , although this is likely an overestimate, given that the maximum body length of I. bernissartensis is reported to be . constructing a 3D mathematical model and employing allometry-based estimate suggests an I. bernissartensis close to long (smaller than average) weighs close to in body mass. Specimens of relatively large individuals have been reported in the 2020s: a specimen referred to as I. cf. galvensis was measured up to in length, while a new specimen of I. bernissartensis from the upper Barremian of the Iberian Peninsula was measured up to in length. Such large individuals would have weighed approximately .

thumb|left|Life restoration of Iguanodon bernissartensis

The arms of I. bernissartensis were long (up to 75% the length of the legs) and robust, although their exact function is still debated. 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 phalangeal formula is 2-3-3-2-4, meaning that the innermost finger (phalange) has two bones, the next has three, etc. 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).

Classification and evolution

thumb|left|Restoration of I. bernissartensis (second from left) among other [[ornithopods]]

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. In older sources, Iguanodontidae was shown as a distinct family. This family traditionally has been something of a wastebasket taxon, including ornithopods that were neither hypsilophodontids or hadrosaurids. In practice, animals like Callovosaurus, Camptosaurus, Craspedodon, Kangnasaurus, Mochlodon, Muttaburrasaurus, Ouranosaurus, and Probactrosaurus were usually assigned to this family. but his proposal has been rejected.

The cladogram below follows an analysis by Andrew McDonald, 2012.

Species

thumb|Skeletal restoration of I. bernissartensis by [[Othniel Charles Marsh|O. C. Marsh, 1896]]

Because Iguanodon is one of the first dinosaur genera to have been named, numerous species have been assigned to it. While never becoming the wastebasket taxon several other early genera of dinosaurs (such as Megalosaurus) became, Iguanodon has had a complicated history, and its taxonomy continues to undergo revisions. Although Gregory S. Paul recommended restricting I. bernissartensis to the famous sample from Bernissart, ornithopod workers like Norman and McDonald have disagreed with Paul's recommendations, except exercising caution when accepting records of Iguanodon from France and Spain as valid.

I. anglicus was the original type species, but the lectotype 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, with the new holotype being IRSNB 1534. 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.

Species currently accepted as valid

Only one species assigned to Iguanodon is still considered to be valid. based on a specimen previously assigned to Iguanodon bernissartensis, was argued to be distinct from the latter based on the relative height of its neural spines. However, a 2017 study noted that this is easily within the range of individual variation, and that the difference may also arise from D. turolensis being an adult older than other specimens of I. bernissartensis.

• I. seelyi (also incorrectly spelled I. seeleyi), described by John Hulke in 1882, has also been synonymised with Iguanodon bernissartensis, though this is not universally accepted. It was discovered in Brook, on the Isle of Wight, and named after Charles Seely MP, Liberal politician and philanthropist, on whose estate it was found.
• David Norman has suggested that I. bernissartensis includes the dubious Mongolian I. orientalis (see also below), but this has not been followed by other researchers.
• I. atherfieldensis, described by R.W. Hooley in 1925,
• I. dawsoni, described by Lydekker in 1888, is known from two partial skeletons found in East Sussex, England,
• I. exogyrarum was described by Fritsch in 1878. It is a nomen dubium based on very poor material and was renamed Ponerosteus in 2000.
• I. fittoni was described by Lydekker in 1889. Like I. dawsoni, this species was described from the Wadhurst Clay However, recent work places it as indeterminate beyond Hadrosauroidea outside Hadrosauridae.
• I. hoggi (also spelled I. boggii or hoggii), named by Owen for a lower jaw from the Tithonian–Berriasian-age Upper Jurassic–Lower Cretaceous Purbeck Beds of Dorset in 1874, has been reassigned to its own genus, Owenodon.
• I. hollingtoniensis (also spelled I. hollingtonensis), described by Lydekker in 1889, has variously been considered a synonym of Hypselospinus fittoni A specimen from the Valanginian Wadhurst Clay Formation, The only well-accepted North American species of Iguanodon, I. lakotaensis was described from a partial skull from the Barremian-age Lower Cretaceous Lakota Formation of South Dakota. Its assignment has been controversial. Some researchers suggest that it was more basal than I. bernissartensis, and related to Theiophytalia, but David Norman has suggested that it was a synonym of I. bernissartensis. Several taxa, including the holotype of Mantellisaurus and Mantellodon, but also the dubious hadrosauroid Trachodon cantabrigiensis the hypsilophodont Hypsilophodon, and Valdosaurus, were previously mis-assigned to I. mantelli.
• "I. mongolensis" is a nomen nudum from a photo caption in a book by Whitfield in 1992 of remains that would later be named Altirhinus.
• I. orientalis, described by A. K. Rozhdestvensky in 1952, was based on poor material, but a skull with a distinctive arched snout that had been assigned to it was renamed Altirhinus kurzanovi in 1998. but he later reassigned it to Priodontognathus.
• I. praecursor (also spelled I. precursor), described by E. Sauvage in 1876 from teeth from an unnamed Kimmeridgian (Late Jurassic) formation in Pas-de-Calais, France, is actually a sauropod, sometimes assigned to Neosodon, although the two come from different formations.
• I. prestwichii (also spelled I. prestwichi), described by John Hulke in 1880, has been reassigned to Camptosaurus prestwichii or to its own genus Cumnoria.
• I. suessii, described by Emanuel Bunzel in 1871, has been reassigned to Mochlodon suessi.
• I. gracilis, named by Lydekker in 1888 as the type species of Sphenospondylus and assigned to Iguanodon in 1969 by Rodney Steel, has been suggested to be a synonym of Mantellisaurus atherfieldensis, based on vertebrae from the Isle of Wight and Sussex originally described by Owen in 1842 as a species of Streptospondylus, S. major, is a nomen dubium. Originally named by Hulke as a distinct genus in 1879 based on vertebral and pelvic remains, it was from the Barremian stage of the Isle of Wight. It was considered a juvenile specimen of Mantellisaurus atherfieldensis, or an undetermined species of Mantellisaurus,

Dubious species

thumb|Original I. anglicus teeth and thumb spike described by Mantell

• I. anglicus, described by Friedrich Holl in 1829, is the original type species of Iguanodon, but, as discussed above, was replaced by I. bernissartensis. In the past, it has been spelled as I. angelicus (Lessem and Glut, 1993) and I. anglicum (Holl, 1829 emend. Bronn, 1850). It is possible teeth ascribed to this species belong to the genus now called Barilium. The name Therosaurus (Fitzinger, 1840), is a junior objective synonym, a later name for the material of I. anglicus.
• I. ottingeri, described by Peter Galton and James A. Jensen in 1979, is a nomen dubium based on teeth from the possibly Aptian-age lower Cedar Mountain Formation of Utah.

Palaeobiology

Feeding

thumb|left|I. bernissartensis skull and neck

One of the first details noted about Iguanodon was that it had the teeth of a herbivorous reptile, like a giraffe. More complete remains have shown this to be an error; for example, the hyoid bones that supported the tongue are heavily built, implying a muscular, non-prehensile tongue used for moving food around in the mouth. The giraffe-tongue idea has also been incorrectly attributed to Dollo via a broken lower jaw.

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. Additionally, the front ends of the animal's jaws were toothless and tipped with bony nodes, both upper and lower, The evidence is not conclusive, though. Whatever its exact diet, due to its size and abundance, Iguanodon is regarded as a dominant medium to large herbivore for its ecological communities.

Posture and movement

thumb|upright|left|Nineteenth-century painting showing I. bernissartensis in outdated tripod pose

Early fossil remains were fragmentary, which led to much speculation on the posture and nature of Iguanodon. Iguanodon was initially portrayed as a quadrupedal horn-nosed beast. However, as more bones were discovered, Mantell observed that the forelimbs were much smaller than the hindlimbs. His rival Owen was of the opinion 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 bipedal nature was revealed with the discovery of the Bernissart skeletons. However, it was depicted in an upright posture, with the tail dragging along the ground, acting as the third leg of a tripod. 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 (walking on the fingers and toes). which would have been as a biped; it would not have been able to gallop as a quadruped. and Samuel Beckles noted in 1854 that they looked like bird tracks, but might have come from dinosaurs. The identity of the trackmakers was greatly clarified upon the discovery in 1857 of the hind leg of a young Iguanodon, with distinctly three-toed feet, showing that such dinosaurs could have made the tracks. Despite the lack of direct evidence, these tracks are often attributed to Iguanodon.

Thumb spike

thumb|I. bernissartensis hand with spike

The thumb spike is one of the best-known features of Iguanodon. Although it was originally placed on the animal's nose by Mantell, the complete Bernissart specimens allowed Dollo to place it correctly 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, but this has not been accepted, as the spike was not hollow,