Temporal range: Late Cretaceous, 65.5Ma
|The syntype fossils|
|Species:||† T. occidentalis|
In 1855 geologist Ferdinand Vandiveer Hayden sent a number of fossils to paleontologist Joseph Leidy in Philadelphia. Hayden had collected them in an area within present-day South Dakota, then Nebraska Territory, near the Grand River. 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 Vandiveer 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 "deinosaurian". The specific name means "western" in Latin.
The caudal vertebrae, USNM 219 and USNM 221, and the middle toe phalanx, USNM 220, form the syntype series.
Like Trachodon, another duckbill genus named by Joseph Leidy, Thespesius is an 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, which led to a great number of existing species being assigned to Thespesius.
Two other species of duckbill started out as species of Thespesius: T. saskatchewanensis, now thought to be a species of Edmontosaurus (and included in Anatosaurus for many years); and T. edmontoni, now considered to be the same as Edmontosaurus annectens.
- Leidy, J. (1856). "Notice of extinct Vertebrata, discovered by Dr. F. V. Hayden during the expedition to the Sioux country under the command of Lieut. G.K. Warren." Proceedings of the Academy of Natural Science Philadelphia, 8(December 30): 311-312.
- Creisler, B.S. (2007). Deciphering duckbills. in: K. Carpenter (ed.), Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs. Indiana University Press: Bloomington and Indianapolis, 185-210. ISBN 0-253-34817-X
- Sternberg, C.M. (1926). A new species of Thespesius from the Lance Formation of Saskatchewan. Canada Department of Mines Geological Survey Bulletin (Geological Series) 44(46):73-84.
- Horner, J.R., Weishampel, D.B., and Forster, C.A. (2004). Hadrosauridae. In: Weishampel, D.B., Dodson, P., and Osmólska, H. (eds.). The Dinosauria (second edition). University of California Press:Berkeley, 438-463. ISBN 0-520-06727-4
- Lull, R.S., and Wright, N.E. (1942). Hadrosaurian Dinosaurs of North America. Geological Society of America Special Paper 40:1-242.
- Gilmore, C.W. (1924). A new species of hadrosaurian dinosaur from the Edmonton Formation (Cretaceous) of Alberta. Canada Department of Mines Geological Survey Bulletin (Geological Series) 38(43):13-26.
- Campione, N.E. (2009). "Cranial variation in Edmontosaurus (Hadrosauridae) from the Late Cretaceous of North America." North American Paleontological Convention (NAPC 2009): Abstracts, p. 95a.