Iguanodon is a species of ornithopod dinosaurs. It was the first dinosaur documented, the second dinosaur officially named and described, and with Megalosaurus and Hylaeosaurus, one of the three at first used to define the new classification, Dinosaurian.

Description:

Iguanodon is an ornithopod dinosaur, approximately halfway between the early hypsilophodontids and their final culmination in the duck-billed dinosaurs. They lived between 120 to 140 million years ago, in the Barremian to Valanginian ages of the early Cretaceous time, though one unknown species is from the late Jurassic. Its most characteristic feature was a large razor-sharp thumb, likely used for defense from predators.

The various Iguanodon species are large herbivores, ranging from 6 to 11 metres (20 to 36 feet) in length, and averaging about 5 tonnes (5.5 tons) in weight.

Discovery and classification:

Gideon Mantell's wife, Mary Ann, exposed the first tooth of an Iguanodon in the Tilgate Forest in England, in 1822. 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"). Based on isometric scaling, he predictable it might be up to 60 feet (18 meters) long.

A better specimen was exposed in a quarry in Maidstone, which Mantell was able to recognize as an Iguanodon based on the tooth. The Maidstone slab permitted the first skeletal reconstructions and artistic renderings of the Iguanodon. The most well-known mistake was the placement of a "horn", also exposed by Mantell's wife, on the nose of the dinosaur. The discovery of much better specimens of I. bernissartensis in 1878 revealed that the horn was in fact a modified thumb, perhaps used for defense.