Trachodon is a dubious

Nomen dubium

In zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...

 genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of hadrosaurid

Hadrosaurid

Hadrosaurids or duck-billed dinosaurs are members of the family Hadrosauridae, and include ornithopods such as Edmontosaurus and Parasaurolophus. They were common herbivores in the Upper Cretaceous Period of what are now Asia, Europe and North America. They are descendants of the Upper...

 dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 based on teeth from the Campanian

Campanian

The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...

-age Upper Cretaceous Judith River Formation

Judith River Formation

The Judith River Formation is a fossil-bearing geologic formation in Montana, and is part of the Judith River Group. It dates to the upper Cretaceous, between 80 and 75 million years ago, corresponding to the "Judithian" land vertebrate age...

 of Montana

Montana

Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

, U.S.A. It is a historically important genus with a convoluted taxonomy

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 that has been all but abandoned by modern dinosaur paleontologists.

Despite being used for decades as the iconic duckbill

Hadrosaurid

Hadrosaurids or duck-billed dinosaurs are members of the family Hadrosauridae, and include ornithopods such as Edmontosaurus and Parasaurolophus. They were common herbivores in the Upper Cretaceous Period of what are now Asia, Europe and North America. They are descendants of the Upper...

 dinosaur per antonomasia

Antonomasia

In rhetoric, antonomasia is a substitution of any epithet or phrase for a proper name, such as "the little corporal" for Napoleon I. The reverse process is also sometimes called antonomasia. The word derives from the Greek verb , meaning "to name differently"...

 the material it is based on is composed of teeth from both duckbills and ceratopsids (their teeth have a distinctive double root), and its describer, Joseph Leidy

Joseph Leidy

Joseph Leidy was an American paleontologist.Leidy was professor of anatomy at the University of Pennsylvania, and later was a professor of natural history at Swarthmore College. His book Extinct Fauna of Dakota and Nebraska contained many species not previously described and many previously...

, came to recognize the difference and suggested limiting the genus to what would now be seen as ceratopsid teeth.