| Thespesius Temporal range: Late Cretaceous,
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| The syntype fossils | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Neornithischia |
| Clade: | †Ornithopoda |
| Family: | †Hadrosauridae |
| Subfamily: | †Saurolophinae |
| Genus: | †Thespesius Leidy, 1856 |
| Species: | †T. occidentalis
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| Binomial name | |
| †Thespesius occidentalis Leidy, 1856
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| Synonyms | |
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Thespesius (meaning "wondrous one") is a dubious genus of hadrosaurid dinosaur from the late Maastrichtian-age Upper Cretaceous Lance Formation of South Dakota.