IGUANODON, a large extinct herbivorous land reptile from the Wealden formation of western Europe, almost completely known by numerous skeletons from Bernissart, near Mons, Belgium. It is a typical representative of the ornithopodous (Gr. for bird-footed) Dinosauria. The head is large and laterally compressed with a blunt snout, nearly terminal nostrils and relatively small eyes. The sides of the jaws are provided with a close series of grinding teeth, which are often worn down to stumps; the front of the jaws forms a toothless beak, which would be encased originally in a horny sheath. When unworn the teeth are spatulate and crimped or serrated round the edge, closely resembling those of the existing Central American lizard, Iguana - hence the name Iguanodon (Gr. Iguana-tooth) proposed by Mantell, the discoverer of this reptile, in 1825. The bodies of the vertebrae are solid; and they are convexoconcave (i.e. opisthocoelous) in the neck and anterior part of the back, where there must have been much freedom of motion. The hindquarters are comparatively large and heavy, while the tail is long, deep and more or less laterally compressed, evidently adapted for swimming. The small and mobile forelimbs bear four complete fingers, with the thumb reduced to a bony spur. The pelvis and hind-limbs much resemble those of a running bird, such as those of an emu or the extinct moa; but the basal bones (metatarsals) of the three-toed foot remain separate throughout life, thus differing from those of the running birds, which are firmly fused together even in the young adult. No external armour has been found. The reptile doubtless frequented marshes, feeding on the succu lent vegetation, and often swimming in the water. Footprints prove that when on land it walked habitually on its hind-limbs.
The earliest remains of Iguanodon were found by Dr G. A. Mantell in the Wealden formation of Sussex, and a large part of the skeleton, lacking the head, was subsequently discovered in a block of ragstone in the Lower Greensand near i Skeleton of Iguanodon bernissartensis. (After Dollo.) Maidstone, Kent. These fossils, which are now in the British Museum, were interpreted by Dr Mantell, who made comparisons with the skeleton of Iguana, on the erroneous supposition that the resemblance in the teeth denoted some relationship to this existing lizard. Several of the bones, however, could not be understood until the much later discoveries of Mr S. H. Beckles in the Wealden cliffs near Hastings; and an accurate knowledge of the skeleton was only obtained when many complete specimens were disinterred by the Belgian government from the Wealden beds at Bernissart, near Mons, during the years 1877-1880. These skeletons, which now form the most striking feature of the Brussels Museum, evidently represent a large troop of animals which were suddenly destroyed and buried in a deep ravine or gully. The typical species, Iguanodon mantelli, measures 5 to 6 metres in length, while I. bernissartensis (see fig.) attains a length of 8 to io metres. They are found both at Bernissart and in the south of England, while other species are also known from Sussex. Nearly complete skeletons of allied reptiles have been discovered in the Jurassic and Cretaceous rocks of North America.
REFERENCES. - G. A. Mantell, Petrifactions and their Teaching (London, 1851); L. Dollo, papers in Bull. illus. Roy. d'Hist. Nat. Belg., vols. i.-iii. (1882-1884). (A. S. Wo.)
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