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.
History and classification
Discovery and early nomenclature
In 1856, Joseph Leidy received fragmentary remains from the Judith River Formation, collected by Ferdinand Vandeveer Hayden. From these bones, he provided the first names for North American dinosaurs: Deinodon, Palaeoscincus, Trachodon, and Troodon (then spelled Troödon). The type species of Trachodon is T. mirabilis. The generic name is derived from Greek τραχυς, trakhys, "rough", and όδον, odon, "tooth", referring to the granulate inner surface of one of the teeth. The specific name means "marvelous" in Latin. Trachodon was based on several unassociated teeth, of which the tooth ANSP 9260 has been taken as the holotype. Leidy believed the teeth to belong to a relative of Iguanodon, and Trachodon would come to be recognized as the first known hadrosaur. However, only two of the teeth (including ANSP 9260), possessing a single root, are truly hadrosaurian. Others, with double roots, in fact belonged to ceratopsians. Later in the same year, Leidy would name the genus and species Thespesius occidentalis based on hadrosaurid tail vertebrae and a phalanx from the Lance Formation of South Dakota, and in 1859 he named Hadrosaurus foulkii based on a partial skeleton from the Woodbury Formation of New Jersey. By 1860, however, he began to question the relationship between these remains. He published on these concerns in 1865, speculating that Trachodon and Thespesius may represent the same animal, with the distinct tail vertebrae of Hadrosaurus maintaining its distinctiveness. Leidy's further statements, which were often contradictory, would go on to confuse later palaeontologists. By a short 1868 publication he had come to regret the name Trachodon, feeling that Hadrosaurus (meaning "bulky lizard") was a more appropriate name. Furthermore, he had by now become aware the single and double rooted teeth of Trachodon belonged to different animals, suggesting that Trachodon could come to refer to the double rooted (ceratopsian) teeth, with Hadrosaurus referring to the single rooted (hadrosaur) ones and inheritingt he anem Hadrosaurus mirabilis. Whether he intended this to be a formal taxonomic action was, however, unclear. In 1870 he made another short publication, noting tooth characteristics that may indicate Hadrosaurus mirabilis (the single rooted "Trachodon" teeth) was a distinct genus from Hadrosaurus foulkii; seemingly, he did not believe Trachodon an appropriate name for the former. As in 1860, he wondered if Thespesius represented the same animal as the original Trachodon teeth. However, he noted that differing vertebral characteristics demonstrated Thespesius was distinct from both Hadrosaurus and Trachodon, in spite of the latter not being associated with any vertebrae.
In the wake of Leidy's work, Edward Drinker Cope published much of the following work on early hadrosaurs. In an 1869 paper he assigned all of Leidy's species to the genus Hadrosaurus, including H. mirabilis, and named the family Hadrosauridae to include Hadrosaurus, its later synonym Ornithotarsus, and Hypsibema. In 1876 he named the genus Diclonius, with three species, each based also on teeth. The following year, he split Leidy's original sample of Trachodon teeth between the species Trachodon mirabilis, Diclonius perangulatus, and Dysganus haydenianus. In 1882 fossil collectors R. S. Hill and J. L. Wortman, working for Cope, collected the first complete skeleton of a hadrosaur. Cope would produce a preliminary publication on the specimen in 1883, and assigned it to the species mirabilis. However, viewing Leidy as having abandoned the genus Trachodon, Cope used his own genus name Diclonius, producing the binomial Diclonius mirabilis for the species. Why Cope believed the skeleton belonged to the same species as Leidy's original hadrosaur teeth, and why he allied it to Diclonius instead of creating a new genus, was not entirely clear and subject to speculations of later authors. In 1888, Richard Lydekker named the species Trachodon cantabridgiensis based on a tooth from the Cambridge Greensand of England, recognizing it alongside T. mirabilis in a distinct Trachodon of which Hadrosaurus was a synonym. Consequently, he established the name Trachodontidae as a replacement for Hadrosauridae.
Wide acceptance of Trachodon
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