The original reconstruction of Hadrosaurus foulkii featured a creature in a kangaroo stance -- an animal that used its tail as a third leg. At the same time, while the excavated fossil was nearly complete, it lacked a skull. Because parts of its skeleton resembled those of an iguana, the skull of a modern iguana was used as a model for the skull created for the original display in 1868. That sculpted skull (above, right) is part of the collection of the Academy of Natural Sciences in Philadelphia.
It was later believed that Hadrosaurus foulkii did not use its tail as a third leg but rather as something of a low-slung counterweight that allowed it to walk upright on two legs. Above is the full-size Hadrosaurus mount currently on display at the New Jersey State Museum in Trenton. The posture is now recognized as incorrect. At the same time the skeleton is fitted with the wrong skull of another type of duck-bill dinosaur. Signs at the exhibit acknowledge that both the mounted skeleton as well as nearby illustrated depictions of what the living animal looked like are both wrong. Both are slated for correction at some unspecified future date.
The latest evidence indicates that the huge creature held its entire rear body aloft as a counterweight while projecting its upper body forward to move in a manner amazingly similar to that of a modern bird. The front limbs were used for foraging. This Hadrosaurus foulkii posture is detailed in one of the Academy of Natural Sciences' recent displays. The skull shape is based on the latest information from studies of Hadrosaurs.