In 1926, Charles Mortram Sternberg named "Thespesius saskatchewanensis" for NMC 8509, a skull and partial skeleton from the Wood Mountain plateau of southern Saskatchewan. He had collected this specimen in 1921, from rocks that were assigned to the Lance Formation, now the Frenchman Formation. NMC 8509 included an almost complete skull, numerous vertebrae, partial shoulder and hip girdles, and partial hind limbs, representing the first substantial dinosaur specimen recovered from Saskatchewan. Sternberg opted to assign it to "Thespesius" because that was the only hadrosaurid genus known from the Lance Formation at the time. At the time, "T. saskatchewanensis" was unusual because of its small size, estimated at in length.