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. 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 fossil beds as Claosaurus annectens, and that therefore the older name should be used for the Lance-aged "trachodonts." Many later researchers, including L.S. Russell and Charles M. Sternberg, continued to use the names Thespesius occidentalis or Thespesius annectens for the Lance hadrosaurids through the 1920s and 1930s. However, as early as 1913, paleontologist Lawrence Lambe regarded the type fossils of Thespesius occidentalis as inadequate and that any inferences based on them were too conjectural, as was the case for Trachodon. In an influential 1942 paper on hadrosaurids by Richard S. Lull and Nelda E. Wright, the authors classified most specimens of Thespesius annectens in the new genus Anatosaurus, and referred Cope's giant "duck-billed" specimens to Anatosaurus copei. Though they noted that T. occidentalis could possibly be distinguished from Anatosaurus based on its shorter tail vertebrae, they ultimately agreed with Lambe that, despite its historical importance, Thespesius occidentalis was too incomplete for good comparison. It has been generally ignored as a nomen dubium ever since. A referred species of Thespesius, T. saskatchewanensis, was named by Sternberg in 1926, but Nicolás Campione and David Evans found that it was a synonym of Edmontosaurus annectens in a 2011 study of edmontosaur diversity. Campione and Evans also found Thespesius edmontoni, named by Gilmore in 1924, to be a synonym of Edmontosaurus regalis.
