Thespesius (meaning "wondrous one") is a dubious genus of hadrosaurid dinosaur from the late Maastrichtian-age Upper Cretaceous Lance Formation of South Dakota. History In 1855 geologist Ferdinand Vandeveer Hayden sent a number of fossils to paleontologist Joseph Leidy in Philadelphia. Hayden had collected them from the surface of a rock formation then known as the Great Lignite Formation (now recognized as part of the Lance Formation) in the Nebraska Territory, near the Grand River (present-day South Dakota). Among them were two caudal vertebrae and a phalanx. In 1856 Leidy named the type species Thespesius occidentalis for these three bones. The generic name is derived from Greek θεσπεσιος, thespesios, "wondrous", because of the colossal size of the remains. Leidy avoided using the suffix "saurus" in the genus name because Vandeveer Hayden had claimed the bones came from a layer from the Miocene so there was a chance that the animal would turn out to be a mammal, though Leidy himself was convinced it was a dinosaurian. The specific name means "western" in Latin. The caudal vertebrae, USNM 219 and USNM 221, and the middle toe phalanx, USNM 220, form the original syntype series. Classification Like Trachodon, another duckbill genus named by Joseph Leidy, Thespesius is a historically-important genus with a convoluted taxonomy that has been all but abandoned by modern dinosaur paleontologists. Around 1900 the name was used by some authors to indicate all late Maastrichtian hadrosaurids in North America. In 1875, E.D. Cope stated that he considered Agathaumas milo, known from partial limb bones and some vertebrae, to be a synonym of T. occidentalis (which he considered a species of Hadrosaurus at the time). In 1900, a short piece published in Science by F.A. Lucas noted that Leidy's original Thespesius occidentalis fossils were indistinguishable from more complete specimens which had been referred in the late 1800s to the species Claosaurus annectens. Therefore, Lucas argued, the name T. occidentalis should be used for this animal. Lucas' opinion was supported by Charles W. Gilmore in a 1915 paper for Science re-evaluating the use of the genus Trachodon. A wide variety of hadrosaurid species had been classified as Trachodon or "trachodonts", most notably the large "duck-billed" specimens collected by E.D. Cope and mounted in the American Museum of Natural History. Gilmore noted that the holotype fossils of T. occidentalis were "inadequate", but that geologic work showed that they undoubtedly came from the same