Charles W. Gilmore in 1915 reassessed hadrosaurids and recommended that Thespesius be reintroduced for hadrosaurids from the Lance Formation and rock units of equivalent age, and that Trachodon, based on inadequate material, should be restricted to a hadrosaurid from the older Judith River Formation and its equivalents.
Charles W. Gilmore je 1915. godine ponovo procijenio hadrosauride i predložio da se Thespesius ponovo uvede za hadrosauride iz formacije Lance i stijena iz odgovarajućeg perioda, a da se Trachodon, zbog neadekvatnog materijala, ograniči na hadrosaurida iz starije formacije Judith River i njoj ekvivalentnih stijena.