Iguanodon

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Iguanodon was a bulky herbivore that could shift from bipedality to quadrupedality. The best-known species, I. bernissartensis, is estimated to have weighed about 3.5 tons on average, and measured about  33 ft long as an adult, with some specimens possibly as long as 43 ft. This genus had a large, tall but narrow skull, with a toothless beak probably covered with keratin.

The arms were long (up to 75% the length of the legs in I. bernissartensis) and robust, undefined with rather inflexible hands built so that the three central fingers could bear weight. The thumbs were conical spikes that stuck out away from 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, 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 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). Overall, in body structure, it was not too dissimilar from its later relatives, the hadrosaurids.