The hadrosaurids, or "duck-billed" dinosaurs, were large, bipedal dinosaurs, with body lengths up to fifteen meters, that lived during the Upper Cretaceous across North and Central America, South America, Europe, and Asia. It is thought that they originated in Asia, but once they arose, they spread and diversified worldwide, quickly becoming the primary constituent of herbivorous dinosaur faunas. The first hadrosaur remains to be found were represented by a few fragments from Montana and South Dakota, described by Joseph Leidy in 1856. Shortly afterward, a partial skeleton was found in New Jersey, the first nearly complete dinosaur found anywhere, and described by Leidy in 1858 as Hadrosaurus foulkii. Hadrosaurs are ornithopod (bird-footed) dinosaurs, a term that refers to their three-toed feet. They are thus part of the Ornithischia, one of the two major dinosaur subdivisions, and characterized by a pubis (one of the three bones of the pelvis) that is inclined backward. In cladistic terms, they are considered to be a monophyletic group; that is, they are all derived from a common ancestor. They are particularly characterized by a toothless front to the mouth, which is flared outward to form a broad, flat "bill", prompting early researchers to dub them the duck-billed dinosaurs. This bill was covered by a thick, horny sheath, and the rest of the jaws bore closely packed batteries of grinding teeth, up to four hundred on each side of each jaw. The postcranial anatomy of hadrosaurs is generally very similar and the really obvious differences lie in the crests that many species bear on the top of their heads. These are formed of outgrowths of the nasal bones, are frequently hollow, and are found particularly in the hadrosaurs known as lambeosaurines.
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