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David Norman, during his reexamination of ''Iguanodon'', was able to show that this posture was unlikely, due to the presence of a long tail stiffened with ossified tendons.<ref name=DBN80/> To get the tripodal pose, the tail would literally have to be broken.<ref name=DBN85b/> Putting the animal in a more horizontal makes many aspects of the arms and [[pectoral girdle]] more understandable. For example, the hand is relatively immobile, with the three central fingers grouped together, bearing [[hoof]]-like [[phalanx bones|phalanges]], and able to [[extension (kinesiology)|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.<ref name=DBN80/> Furthermore, it appears that ''Iguanodon'' became more quadrupedal as it got older and heavier; [[juvenile (organism)|juvenile]] ''I. bernissartensis'' have shorter arms than adults (60% of hindlimb length versus 70% for adults).<ref name=DBN04/> When walking as a quadruped, the animal's hands would have been held so that the [[palm]]s faced each other, as shown by iguanodontian trackways and the anatomy of this genus' arms and hands.<ref name=JLW96>Wright, J.L. (1996). Fossil terrestrial trackways: Preservation, taphonomy, and palaeoecological significance. Ph.D. dissertation, University of Bristol:Bristol. 300 pp.</ref><ref name=JLW99>Wright, J.L. (1999). Ichnological evidence for the use of the forelimb in iguanodontians. In: Unwin, D.W. ''Cretaceous Fossil Vertebrates.'' ''Special Papers in Palaeontology'' '''60''':209-219.</ref> The three toed [[pes (zoology)|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).<ref name=DBN04/> The maximum speed of ''Iguanodon'' has been estimated at 24 [[kilometres per hour|km/hr]] (14.9 [[miles per hour|mph]]),<ref name=WPCJ78>Coombs, W.P., Jr. (1978). Theoretical aspects of cursorial adaptations in dinosaurs. ''Quarterly Review of Biology'' '''53''':393-418.</ref> which would have been as a biped; it would not have been able to [[Horse_gait#Gallop|gallop]] as a quadruped.<ref name=DBN04/>
David Norman, during his reexamination of ''Iguanodon'', was able to show that this posture was unlikely, due to the presence of a long tail stiffened with ossified tendons.<ref name=DBN80/> To get the tripodal pose, the tail would literally have to be broken.<ref name=DBN85b/> Putting the animal in a more horizontal makes many aspects of the arms and [[pectoral girdle]] more understandable. For example, the hand is relatively immobile, with the three central fingers grouped together, bearing [[hoof]]-like [[phalanx bones|phalanges]], and able to [[extension (kinesiology)|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.<ref name=DBN80/> Furthermore, it appears that ''Iguanodon'' became more quadrupedal as it got older and heavier; [[juvenile (organism)|juvenile]] ''I. bernissartensis'' have shorter arms than adults (60% of hindlimb length versus 70% for adults).<ref name=DBN04/> When walking as a quadruped, the animal's hands would have been held so that the [[palm]]s faced each other, as shown by iguanodontian trackways and the anatomy of this genus' arms and hands.<ref name=JLW96>Wright, J.L. (1996). Fossil terrestrial trackways: Preservation, taphonomy, and palaeoecological significance. Ph.D. dissertation, University of Bristol:Bristol. 300 pp.</ref><ref name=JLW99>Wright, J.L. (1999). Ichnological evidence for the use of the forelimb in iguanodontians. In: Unwin, D.W. ''Cretaceous Fossil Vertebrates.'' ''Special Papers in Palaeontology'' '''60''':209-219.</ref> The three toed [[pes (zoology)|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).<ref name=DBN04/> The maximum speed of ''Iguanodon'' has been estimated at 24 [[kilometres per hour|km/hr]] (14.9 [[miles per hour|mph]]),<ref name=WPCJ78>Coombs, W.P., Jr. (1978). Theoretical aspects of cursorial adaptations in dinosaurs. ''Quarterly Review of Biology'' '''53''':393-418.</ref> which would have been as a biped; it would not have been able to [[Horse_gait#Gallop|gallop]] as a quadruped.<ref name=DBN04/>


Three-toed footprints are commonly found in [[Wealden]] beds on the Isle of Wight, which initially were difficult to interpret. [[Samuel Beckles]] in 1854 noted that they looked like bird tracks, but might have come from dinosaurs.<ref name=SB54>Beckles, S.H. (1854). On the ornithoidichnites of the Wealden. ''Quarterly Journal of the Geological Society of London'' '''10''':456-464.</ref> The identity of the trackmakers was greatly clarified upon the discovery of the hind leg of a young ''Iguanodon'', with distinctly three-toed feet, showing that such dinosaurs could have made the tracks.<ref name=RO58>Owen, R. (1858). Monograph on the Fossil Reptilia of the Wealden and Purbeck Formations. Part IV. Dinosauria (''Hylaeosaurus''). Paleontographical Society Monograph '''10''':1-26.</ref><ref name=LHL>{{cite web | author = Linda Hall Library of Science, Engineering & Technology | title = Bird-Footed Iguanodon, 1857 | url = http://www.lindahall.org/events_exhib/exhibit/exhibits/dino/owe1857.htm | accessdate = 2007-02-14}}</ref> Today, such tracks are often attributed to ''Iguanodon''.<ref name=DBN85b/> A trackway in England may show an ''Iguanodon'' moving on all fours, but the foot prints are poor, making a direct connection more difficult.<ref name=DBN80/>
Three-toed footprints are commonly found in [[Wealden]] beds on the Isle of Wight, which initially were difficult to interpret. [[Samuel Beckles]] in 1854 noted that they looked like bird tracks, but might have come from dinosaurs.<ref name=SB54>Beckles, S.H. (1854). On the ornithoidichnites of the Wealden. ''Quarterly Journal of the Geological Society of London'' '''10''':456-464.</ref> The identity of the trackmakers was greatly clarified upon the discovery of the hind leg of a young ''Iguanodon'', with distinctly three-toed feet, showing that such dinosaurs could have made the tracks.<ref name=RO58>Owen, R. (1858). Monograph on the Fossil Reptilia of the Wealden and Purbeck Formations. Part IV. Dinosauria (''Hylaeosaurus''). Paleontographical Society Monograph '''10''':1-26.</ref><ref name=LHL>{{cite web | author = Linda Hall Library of Science, Engineering & Technology | title = Bird-Footed Iguanodon, 1857 | url = http://www.lindahall.org/events_exhib/exhibit/exhibits/dino/owe1857.htm | accessdate = 2007-02-14}}</ref> Today, such tracks are often attributed to ''Iguanodon''.<ref name=DBN85b/> A trackway in England may show an ''Iguanodon'' moving on all fours, but the foot prints are poor, making a direct connection difficult.<ref name=DBN80/>


===Thumb spike===
===Thumb spike===

Revision as of 21:40, 14 February 2007

Iguanodon
Iguanodon skull from Oxford University Museum of Natural History.
Fossil
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Sauropsida
Superorder:
Dinosauria
Order:
Ornithischia
Suborder:
Cerapoda
Infraorder:
Ornithopoda
Family:
Iguanodontidae

Cope, 1869
Genus:
Iguanodon

Mantell, 1825
Species
  • I. bernissartensis (Boulenger, 1881) (neotype)
  • I. anglicus (Holl, 1829)
  • I. dawsoni (Lydekker, 1888)
  • I. fittoni (Lydekker, 1889)
  • I. lakotaensis (Weishampel & Bjork, 1989)

Iguanodon (IPA pronunciation /ɪˈgwænəˌdɒn/, meaning "Iguana tooth") is the name given to a genus of ornithopod dinosaur, which lived roughly halfway between the early hypsilophodontids and their ultimate culmination in the duck-billed dinosaurs. Most Iguanodon species lived between 140 to 120 million years ago, in the Valanginian to Barremian ages of the Early Cretaceous Period of Europe, although one dubious species is from the Late Jurassic, and possible remains are known from North America, Asia, and Africa. Iguanodon's most distinctive feature was a large 'thumb spike', possibly used for defense against predators.

Discovered in 1822, and described in 1825 by English geologist Gideon Mantell, Iguanodon was the first dinosaur recognized and the second dinosaur formally named. Together with Megalosaurus and Hylaeosaurus, it was one of the three originally used to define the new classification, Dinosauria. The genus continues to be analysed as new species are erected or long-standing ones are referred to other genera.

A large, bulky herbivore, Iguanodon is a member of the paraphyletic clade Iguanodontia. Scientific understanding of the dinosaur has evolved over the centuries as new information is discerned from the fossils. The numerous specimens of Iguanodon have allowed researchers to make informed hypotheses regarding many aspects of the living animal, including feeding, movement, and the question of social behavior.

Description

(for more details on the skull, hands, locomotion, and posture, see the relevant sections under Paleobiology)

The various Iguanodon species were bulky herbivores that could shift from bipedality to quadrupedality.[1] The best-known species, I. bernissartensis, is estimated to have measured about 10 metres (32.8 feet) long as an adult, with some specimens suggesting a 13 m (42.6 ft) length,[2] and massing about 3.08 tonnes (3.5 tons) on average.[3] It had a large, vertically-deep skull, with a horny toothless beak and teeth like those of an iguana. The brain was like that of the later hadrosaurids.[4][1] The arms were long (up to 75% the length of the legs) and robust,[2] with rather inflexible hands modified so that the three central digits could bear weight.[1] Iguanodon's thumb spikes were perpendicular to the three main digits. In early restorations, the spike was placed on the animal's nose. Later fossils revealed the true nature of the thumb spikes,[5] although their exact function is still debated. It could have been used for defence, or for foraging for food. The "little finger" was elongate and dexterous, and could have been used to manipulate objects. The legs were powerful, but not built for running, and there were three toes on each foot. The backbone and tail were supported and stiffened by ossified tendons.[1] Overall, in body structure, it was not too dissimilar from its later relatives, the hadrosaurids.

Classification and origins

Main article: Iguanodont
Iguanodon model Oxford University Museum of Natural History.

Iguanodon gives its name to the clade Iguanodontia, a very populous group with many species of ornithopod from the Middle Jurassic to the Late Cretaceous. Aside from Iguanodon, the best-known members of the clade include: Dryosaurus, Camptosaurus, Callovosaurus, Ouranosaurus, and the "duck-billed" hadrosaurs. In older sources, Iguanodontidae is shown as a distinct family.[6][7] Traditionally, this family included ornithopods that weren't hypsilophodonts or hadrosaurids. In practice, animals like Callovosaurus, Camptosaurus, Craspedodon, Kangnasaurus, Mochlodon, Muttaburrasaurus, Ouranosaurus, and Probactrosaurus were usually assigned to this family;[7] with the advent of cladistic analyses, Iguanodontidae was shown to be paraphyletic, and these animals are recognized to form a stepwise arrangment leading to the hadrosaurs.[4][1] Groups like Iguanodontoidea are still used as unranked clades in the scientific literature, though many traditional "iguanodontids" are now included in the superfamily Hadrosauroidea. Iguanodon ends up between Camptosaurus and Ouranosaurus, and is probably descended from a camptosaur-like animal.[1] At one point, Jack Horner suggested, based mostly on cranial characters, that hadrosaurids actually 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,[8] but this has been rejected.[4][1]

Discovery and history

Mantell and Owen: Iguanodon from giant lizard to dinosaur

Illustration of fossil Iguanodon teeth with a modern iguana jaw from Mantell's 1825 paper describing Iguanodon.

The discovery of Iguanodon has long been accompanied by an enduring legend. The story goes that Gideon Mantell's wife, Mary Ann, discovered the first teeth of an Iguanodon in the strata of Tilgate Forest in Cuckfield, Sussex, England, in 1822 while Mantell was visiting a patient. However, this anecdote was probably just a story Mantell told; there is no evidence he took his wife with while visiting patients, and admitted in 1851 that he himself had found the teeth.[9] Not everyone agrees that the story is false, though.[10] Regardless of the exact circumstances, he combed the area for more fossils, and consulted the fossil experts of the time as to what the bones might belong. Most of the scientists, such as William Buckland and Georges Cuvier, thought that the teeth were from fish or mammals, but Samuel Stutchbury, a naturalist from the Royal College of Surgeons, recognized that they resembled those of an iguana.[3] Mantell waited to describe it until 1825, when he presented a paper to the Royal Society of London.[11][9]

Fossil remains found in Maidstone in 1840.
File:Crystal palace iguanodon.gif
The famous (crowded) banquet in Waterhouse Hawkins's mold for a Crystal Palace Iguanodon.

The tooth resembled that of an iguana, but was twenty times larger, so he named it Iguanodon or "iguana-toothed", from iguana and the Greek word odontos ("tooth").[2] Based on isometric scaling, he estimated that the creature might have been up to 40 feet (12 metres) long.[11] His initial idea for a name was Iguanasaurus, however his friend William Daniel Conybeare suggested the name was applicable to the iguana lizard itself, so a better name would be Iguanoides ("Iguana-like") or Iguanodon.[12] He neglected to add a species name to form a proper binomial, so one was supplied in 1829: I. anglicum, which was later amended to I. anglicus.[13]

A better specimen was discovered in a quarry in Maidstone, Kent, England, which Mantell was able to identify, after acquiring it in 1834, as an Iguanodon, based on the tooth. The Maidstone slab allowed the first skeletal reconstructions and artistic renderings of the Iguanodon. The most famous mistake was the placement of a "horn", which he put on the nose.[14] The discovery of much better specimens of I. bernissartensis in 1878 in Belgium revealed that the horn was actually a modified thumb, perhaps used for defense. Still encased in rock, the Maidstone slab is currently displayed at the Natural History Museum, in London. The borough of Maidstone commemorated this find by adding an Iguanodon to their coat of arms, in 1949.[15] This specimen has become associated with I. mantelli, a species named in 1832 by Christian Erich Hermann von Meyer in place of I anglicus, but actually comes from younger strata.[12]

File:Arms-maidstone.jpg
Borough of Maidstone coat of arms (note the Iguanodon on the left).

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 floated, and used what he would soon coin as dinosaurs as a weapon in this struggle. With the paper describing Dinosauria, he scaled down dinosaurs from lengths of over 61 meters (200 feet), determined that they were not simply giant lizards, and put forward that they were advanced and mammal-like, characteristics given to them by God, as, according to the understanding of the time, they could not have been "transmuted" from reptiles to mammal-like creatures.[16][17] Shortly before his death in 1852, Mantell realized that Iguanodon was not a heavy, pachyderm-like animal,[18] as Owen was putting forward, but his passing left him unable to participate in the creation of the Crystal Palace dinosaur sculptures, and so Owen's vision of the dinosaurs became that seen by the public for decades.[16] With Waterhouse Hawkins, he set up nearly two dozen sculptures, including an Iguanodon; before the mold for the "lifesize" sculpture was completed, he held a banquet in it for twenty.[19][20]

Bernissart

The largest find of Iguanodon remains to date occurred in 1878 in a coal mine at Bernissart in Belgium, at a depth of 322 m (1056 ft), along with plant remains, fish, and other reptiles,[5] including the crocodilian Bernissartia. With the encouragement of Alphonse Briart, supervisor of mines at nearby Morlanwelz, Louis Dollo, with Louis de Pauw, oversaw excavation and reconstructed the skeletons. At least 38 Iguanodon individuals were uncovered,[1] most of which were adults.[21] Many of them went on public display beginning in 1882; 11 are displayed as standing mounts, and 20 as they were (approximately) found.[5] The completed restoration makes an impressive display in the Royal Belgian Institute of Natural Sciences, in Brussels. A replica of one of these is on display at the Oxford University Museum of Natural History.

Photograph of a Bernissart Iguanodon skeleton being mounted.

Most of the remains were referred to the new species I. bernissartensis, a larger and much more robust animal than the English remains had yet revealed, but one specimen was referred to the nebulous, gracile I. mantelli. The skeletons became among the first complete dinosaur skeletons known. Unfortunately, the science of conserving fossil remains was in its infancy, and was ill-equipped to deal with what soon became known as "pyrite disease", a "condition" where pyrite, already in the bones, is altered to iron sulfate, and the bones are damaged. When in the ground, the bones were exposed to moisture that prevented this from happening, but when removed into the drier open air, the natural chemical conversion began to occur. To stop this, the Museum in Brussels attempted to "treat" the "disease" with a combination of alcohol, arsenic, and shellac, to simultaneously penetrate, kill, and harden. This had the effect of sealing in moisture and extending the period of damage. Modern treatments instead 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 moisture is immediately removed and pore space is infiltrated with polyethelene glycol to seal and strengthen the fossil.[5]

File:Iguanodonskel.jpg
Cast of a Bernissart Iguanodon mounted in the Reptile Gallery of the British Museum in 1895.

Dollo's specimens allowed him to show that Owen's prehistoric pachyderms were not correct for Iguanodon. He instead modeled the skeletal mounts after the emu and wallaby, and put the spike that had been on the nose firmly on the thumb.[22][23] He was not completely correct, but he also had the disadvantage of being faced with some of the first complete dinosaur remains. A major problem was with 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, he had to break it. 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.[5]

Excavations at the quarry were stopped in 1881, although it was not exhausted of fossils, as recent drilling operations show.[24] During World War I, when the town was occupied by German forces, preparations were made to reopen the mine for paleontology, and Otto Jaekel was sent from Berlin to supervise. The Allies recaptured Bernissart just as the first fossiliferous layer was about to be uncovered. Further attempts to reopen the mine were hindered by financial problems and were stopped altogether in 1921 when the mine flooded.[5]

To the present: Iguanodon worldwide

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 controversey, I. atherfieldensis, was named in 1925 by R. W. Hooley, for a specimen collected at Atherfield Point on the Isle of Wight.[25] However, what had been a European genus was now going worldwide, with material in Africa (teeth from Tunisia[26] and elsewhere in the Sahara Desert),[27] Mongolia (I. orientalis),[28] and North America (I. ottingeri from Utah[29] and I. lakotaensis from South Dakota).[30]

Iguanodon was not part of the initial work of the "Dinosaur Renaissance" that began after 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 ate,[31] and David Norman's work on numerous aspects of the genus has made it one of the best-known dinosaurs.[32][5][33][1] In addition, a further find of numerous Iguanodon skeletons, in Nehden, Nordrhein-Westphalen, Germany, has provided evidence for gregariousness in this genus, as the animals in this areally-restricted find appear to have been killed by flash floods. At least 15 individuals, from 2 to 8 m (6.6 to 26.2 ft) long, have been found here, although at least some of them belong to the closely related Mantellisaurus atherfieldensis (at that time considered to be another species of Iguanodon).[21] Iguanodon bones have also been 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.[34]

Species of Iguanodon

As one of the first named dinosaurs, numerous species have been assigned to Iguanodon. While never becoming the wastebasket taxon several other early dinosaurs became (such as Megalosaurus and Pelorosaurus), this genus has had a complicated history, and continues to undergo revision.[35][36][37] Remains of the best-known Iguanodon species have been found predominantly in Belgium, England, Germany, Spain and France. Remains of similar animals have been found in Tunisia and Mongolia, and distinct species have been found in Utah and South Dakota.

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 of 2000, the International Commission on Zoological Nomenclature changed the type species to I. bernissartensis, which is known from dozens of skeletons found in Benissart (see below), some of which are essentially complete. The original Iguanodon tooth is held at Te Papa Tongarewa, the Museum of New Zealand in Wellington, although it is not on display.

Species currently accepted as valid

Only a few of the many species assigned to Iguanodon are still considered to be valid and to fall within the genus Iguanodon.[1][37]

Sculpture outside the museum of the Royal Belgian Institute of Natural Sciences, Brussels.

Iguanodon bernissartensis, described by George Albert Boulenger in 1881, is the neotype for the genus. This species is best known for the many skeletons discovered in Bernissart, but is also known from remains across Europe.

I. dawsoni, described by Richard Lydekker in 1889,[38] is known from two partial skeletons from the Valanginian-Barremian-age Lower Cretaceous Hastings Beds of East Sussex, England.[1]

I. fittoni was also described by Lydekker, in 1888.[39] Like I dawsoni, this species is known from the Hastings Beds of East Sussex. It may also have been found in Spain. Three partial skeletons are known.[1]

I. lakotaensis was described by David B. Weishampel and Philip R. Bjork in 1989.[30] The only accepted North American species of Iguanodon, I. lakotaensis was described based on a partial skull from the Barremian-age Lower Cretaceous Lakota Formation of South Dakota. Some researchers suggest that it was more basal, and related to Theiophytalia.[40]

Reassigned species

Iguanodon atherfieldensis, described by R.W. Hooley in 1925,[25] was smaller and less robust than Iguanodon bernissartensis. It was renamed Mantellisaurus atherfieldensis in 2006,[37] but it remains to be seen to what degree this will be accepted by paleontologists. I. exogyrarum (also spelled Iguanodon exogirarum or Iguanodon exogirarus) was described by Fritsch in 1878. It is a nomen dubium and has been reassigned (by George Olshevsky) to Ponerosteus. I. foxii (also spelled I. foxi), was described by Thomas Huxley in 1869. It has been assigned to Hypsilophodon. I. hoggi (also spelled Iguanodon boggii or hoggii), described by Owen in 1874, has been referred to Camptosaurus.[36] I. hollingtoniensis (also spelled as Iguanodon hollingtonensis), described by Lydekker in 1889, is I. fittoni. I. major, a vertebra from the Isle of Wight described by Owen in 1842 as a species of Streptospondylus, is a nomen dubium which has been referred to I. anglicus.[1]

I. mantelli (also spelled as Iguanodon manteli or Iguanodon mantellii), described by von Meyer in 1832, is actually based on the same material as I. anglicus;[12] however, several skeletons, including the Maidstone specimen, and one of the Bernissart skeletons, have been assigned here over the years, and their disposal is not complete. The gracile Bernissart skeleton, for example, has been reassigned to Mantellisaurus atherfieldensis.[33] I. orientalis, described by Rozhdestvensky in 1952,[28] was based on poor material, but a skull with a distinctive arched snout that had been referred to it was renamed Altirhinus kurzanovi in 1998. In the same paper, I. orientalis was considered to be indistinguishable from I. bernissartensis.[35] I. phillipsi (also spelled Iguanodon phillipsii), described by Harry Seeley in 1869,[41] has been reassigned to Priodontognathus.[42]

I. prestwichii (misspelled as Iguanodon prestwichi or Iguanodon prestwichii), described by Hulke in 1880, has been reassigned to Camptosaurus prestwichii. I. seelyi (also spelled Iguanodon seeleyi), described by Hulke two years after I. prestwichii, was moved to I. bernissartensis. I. suessii, described by Bunzel in 1871, has been reassigned to Mochlodon suessi.

Dubious species

Iguanodon albinus, described by Fritsch in 1893, is a nomen dubium. It is nondinosaurian; it was the original name of Albisaurus albinus. I. anglicus Holl, 1829 is the type species of Iguanodon, but, as discussed above, was replaced by the much better known I. bernissartensis. It has been spelled as I. angelicus (Lessem and Glut, 1993) and Iguanodon anglicum (Holl, 1829; there some debate over when it was amended) in the past.

I. hillii, described by Newton in 1892, is an early hadrosaurid of some sort. I. gracilis, described by Lydekker in 1888, Steel, 1969 (originally Sphenospondylus) may belong to Mantellisaurus atherfieldensis. "I. mongolensis" Whitfield, 1992 is a nomen nudum which has never been fully described.

I. ottingeri Galton & Jensen, 1979,[29] is a nomen dubium. I. praecursor (misspelled as Iguanodon precursor), described by Sauvage in 1876, is actually a possible sauropod, sometimes assigned to Neosodon,[43] although the two come from different formations. I. valdensis, described by Lydekker in 1889, van den Broeck, 1900 is a nomen dubium which was originally Vectisaurus; it may be another specimen of Mantellisaurus atherfieldensis.

Paleobiology

Feeding and diet

Nineteenth century painting showing Iguanodon feeding on tree fern leaves.

One of the first details noted about Iguanodon was that it had the teeth of a herbivorous reptile,[11] although there has not always been consensus on how it ate. At one point, Mantell thought that it had a prehensile tongue which issued through a spout-like hole at the end of the lower jaw, allowing it to gather food like a giraffe,[44] but this was later shown to be in error; there was no such opening, and the hyoid bones that supported the tongue are heavily built, implying a muscular, non-prehensile tongue used for moving food around in the mouth.[32] This "giraffe tongue" idea has also been incorrectly attributed to Dollo.[45]

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 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 (teeth in the lower jaw are broader than those in the upper).[4] 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.[46][47] 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 inside surfaces of the upper jaw teeth to rub against the outside surface of the lower jaw's teeth, grinding anything caught in between and providing an action that is the rough equivalent of mammalian chewing.[31] Because the teeth were always replaced, the animal could have used this mechanism throughout its entire life, and could eat tough plant material.[48] Additionally, the front ends of the animal's jaws were toothless and tipped with bony nodes, both upper and lower,[1] providing a rough margin that was likely covered and lengthened by a horny material to form a cropping beak for biting off twigs and shoots.[5] Its food gathering would have been aided by its very flexible little finger, which could have been used to manipulate objects, unlike the other fingers.[1]

Exactly what it was eating 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 at ground level to tree foliage at 4-5 meters (13-16.5 feet) high.[2] Horsetails, cycads, and conifers is the diet suggested by David Norman,[5] although iguanodonts in general have been tied to the advance of angiosperm plants in the Cretaceous due to the dinosaurs' inferred low browsing habits. Angiosperm growth, according to this hypothesis, would be encouraged by iguanodont feeding because gymnosperms would be removed, allowing more space for the weed-like early angiosperms to grow.[49] The evidence is not conclusive, though.[50][1] Whatever its exact diet, due to its size and abundance, Iguanodon is regarded as a dominant medium to large herbivore for its ecological communities.[1] In England, this included the small predator Aristosuchus, larger predators Eotyrannus, Baryonyx, and Neovenator, low-feeding herbivores Hypsilophodon and Valdosaurus, fellow "iguanodontid" Mantellisaurus, the armored herbivore Polacanthus, and sauropods like Pelorosaurus.[51]

Posture and movement

Iguanodon hind feet, Oxford University Museum of Natural History.
Mantell's Iguanodon restoration based on the Maidstone remains.

Early fossil remains were fragmentary, which led to much speculation on the posture and nature of Iguanodon. As discussed, initially Iguanodon was 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 life size reconstruction of dinosaurs was initially offered to Mantell, who declined due to ill-health, and Owen's vision subsequently formed the basis on which the sculptures took shape. Then, with the discovery of the Bernissart skeletons, its bipedal nature was revealed. However, it was depicted in an upright posture, with the tail dragging along the ground, acting as the third leg of a tripod.

Reconstruction by Samuel Griswold Goodrich from Illustrated Natural History of the Animal Kingdom.[52]

David Norman, during his reexamination of Iguanodon, was able to show that this posture was unlikely, due to the presence of a long tail stiffened with ossified tendons.[32] To get the tripodal pose, the tail would literally have to be broken.[5] Putting the animal in a more horizontal makes many aspects of the arms and pectoral girdle more understandable. For example, the hand is relatively immobile, with the three central fingers 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.[32] 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).[1] 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' arms and hands.[53][54] 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).[1] The maximum speed of Iguanodon has been estimated at 24 km/hr (14.9 mph),[55] which would have been as a biped; it would not have been able to gallop as a quadruped.[1]

Three-toed footprints are commonly found in Wealden beds on the Isle of Wight, which initially were difficult to interpret. Samuel Beckles in 1854 noted that they looked like bird tracks, but might have come from dinosaurs.[56] The identity of the trackmakers was greatly clarified upon the discovery of the hind leg of a young Iguanodon, with distinctly three-toed feet, showing that such dinosaurs could have made the tracks.[57][58] Today, such tracks are often attributed to Iguanodon.[5] A trackway in England may show an Iguanodon moving on all fours, but the foot prints are poor, making a direct connection difficult.[32]

Thumb spike

The original thumb spike found in Maidstone in 1840.

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, Dollo's complete Bernissart specimens allowed him 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.) The typical interpretation of this thumb is as a close-quarters stiletto-like weapon against predators,[5][1] although it could also have been used to break into seeds and fruits.[1] One author has suggested that the spike was attached to a venom gland,[59] but this has not been accepted, as the spike was not hollow, nor were there any grooves on the spike for conducting venom.[60]

Social behaviour?

Although sometimes interpreted as the result of a single catastrophe, the Bernissart finds instead record multiple episodes; at least three occasions of mortality are recorded, and though numerous individuals would have died in a geologically short time span (?10-100 years),[21] this does not necessarily mean these Iguanodon were herding.[1] For one thing, juvenile remains are very uncommon at this site, unlike modern cases with herd mortality. They more likely were the periodic victims of flash floods.[21] The Nehden find, however, with its greater span of individual ages, more even mix of Mantellisaurus to Iguanodon bernissartensis, and confined geographic nature, may record mortality of herding animals migrating through rivers.[21]

Iguanodon does not appear at this point to have been sexually dimorphic, with one gender appreciably different from the other. At one point, it was suggested that the Bernissart I. "mantelli", or I. atherfieldensis (both now Mantellisaurus) represented a gender (female?) of the larger and more robust I. bernissartensis (male?),[61] but this is not supported today.[32][5][37]

In popular culture

File:Iguanadon wwd.jpg
BBC's Walking with Dinosaurs reconstruction of Iguanodon.

Since its description in 1825, Iguanodon has been a feature of worldwide popular culture. Two life-sized 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 quadrupedal, yet this was the first time an attempt was made at constructing full-size dinosaur models.

Several motion picture series have featured Iguanodon. In the Disney film Dinosaur, an Iguanodon named Aladar served as the protagonist with three other iguanodonts as other main characters. Iguanodon is one of the three dinosaur genera which inspired Godzilla; the other two were Tyrannosaurus and Stegosaurus. Iguanodon also made appearances in the sequels of The Land Before Time.

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. An asteroid, 1989 CB3, has been named Iguanodon in honor of the genus.[62]

Changing images of Iguanodon

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 which is usually represented as standing in one place, doing nothing in particular; and finally, its current, more agile and dynamic representation, able to shift from two legs to all fours. The second representation dominated the twentieth century, and was only overturned beginning in the 1960s.[63]


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External links

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