The upper arm (humerus) is massive, rather pillar-like, out door animatronic dinosaur and shows evidence that it anchored huge arm and shoulder muscles. This is also unusually long, over three-quarters the length of the hind limb. The true size of the arms is somewhat masked in the original skeletal reconstructions, because they were folded against the chest and always seemed shorter than they really are.
Finally, the shoulder bones are large and powerfully built, which makes perfect sense if the arms are functioning as legs. But the shoulders show another unexpected feature. In the centre of the chest of the larger skeletons at Bernissart there is an irregular bone that grew in the soft tissues across the centre of the chest between the shoulder joints. This bone is pathological in origin – it formed as a response to strain within the chest created while the animal was walking on all fours (and is called an intersternal ossification).
Reassessing the posture of Iguanodon in the light of these observations, walking dinosaur costume for adult it seems clear that a more natural pose of the backbone was horizontal, with the body weight distributed along the backbone and largely balanced at the hips and supported by the massive and strong hind legs. The ossified tendons distributed along the spine, above the chest, hip, and tail, clearly acted as tensioners to distribute body weight along the backbone. This pose allowed the front limbs to reach the ground, and these were used for weight support while these animals were stationary. Iguanodon probably moved slowly on all fours at least part of the time.