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Iguanodon bernissartensis, tooth in matrix

Iguanodon was named in 1825 by English geologist Gideon Mantell. Iguanodon was the second type of dinosaur formally named based on fossil specimens, after Megalosaurus. Together with Megalosaurus and Hylaeosaurus, it was one of the three genera originally used to define Dinosauria.

Notice the serrations on this tooth.

Iguanodon were bulky herbivores that could shift from bipedalism to quadrupedal. The only well-supported species, Iguanodon. bernissartensis, is estimated to have weighed about 3.4 tons on average, and measured about 10 meters (33 feet) long as an adult, with some specimens possibly as long as 13 meters (43 feet). These animals had large, tall but narrow skulls, with toothless beaks probably covered with keratin, and teeth like those of iguanas, but much larger and more closely packed.

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; 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 most famous feature of Iguanodon is the large thumb spike that was probably used for defense against predators.