Trachodon
| Trachodon Temporal range: Late Cretaceous, | |
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| Illustration of some of Leidy's original Trachodon teeth, including the holotype (1-6) | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ornithopoda |
| Family: | †Hadrosauridae |
| Genus: | †Trachodon Leidy, 1856 |
| Species: | †T. mirabilis |
| Binomial name | |
| †Trachodon mirabilis Leidy, 1856 | |
Trachodon (meaning "rough tooth") is a dubious genus of hadrosaurid dinosaur from the Upper Cretaceous of what is now Montana in the western United States. The only species is T. mirabilis, though many others have been assigned to the genus. T. mirabilis was described by Joseph Leidy in 1856 based on a set of teeth; however, this was later shown to be a mixture of hadrosaurid and ceratopsian teeth. These fossils were unearthed in 1856 by Ferdinand Vandeveer Hayden during an expedition to the Missouri River in Montana, USA, in rock from the Judith River Formation. This formation dates to the Campanian stage of the Late Cretaceous, which lasted from 79 to 75.3 million years ago. One of the teeth, an incomplete hadrosaurid tooth, was later chosen as the holotype.
Trachodon was the first hadrosaurid to be named. Several specimens, including complete skeletons of what is now considered Edmontosaurus annectens, were mistakenly assigned to the genus or T. mirabilis. Additionally, many species erected for Trachodon, such as T. marginatus, are now in their own genus or are nomen dubia, like T. selwyni. The taxon coexisted with other ornithischians, including hadrosaurids like Corythosaurus, ceratopsians like Lokiceratops and Medusaceratops, and pachycephalosaurids like Hanssuesia. Carnivorous theropod dinosaurs have been unearthed as well, such as the tyrannosaurid Daspletosaurus and the troodontid Troodon. Several other dubious dinosaur genera were named from the formation as well, such as Deinodon, Palaeoscincus, and possibly Troodon. During the Campanian, the Judith River Formation was a vast floodplain on along the western coast of the Western Interior Seaway and blanked by ferns, conifers, cycads, and angiosperms.