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
thumb|left|[[Joseph Leidy, who named Trachodon|347x347px]]
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, infact belonged to ceratopsians.
Later in the same year, Leidy would name the genus and species Thespesius occidentalis based on hadrosaurid tail vertebrae and a 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.
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.
thumb|Restoration of "Hadrosaurus mirabilis" by [[Charles R. Knight, based on Cope's 1882 skeleton, published in an 1897 article]]
In the wake of Leidy's work, Edward Drinker Cope published much of the following work on early hadrosaurs. 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.
Wide acceptance of Trachodon
left|thumb|Skeletons displayed as Trachodon mirabilis at the [[American Museum of Natural History in 1916]]
Around the turn of the century, the nomenclature of hadrosaurs including the species mirabilis would remain inconsistent. Promotion and correspondence by the AMNH would variously use the names Hadrosaurus, Diclonius, and Claosaurus to contain the species and refer to Cope's skeleton and others being excavated in the American West. This position was shared by Oliver Perry Hay in a 1902 paper.
In 1902, John Bell Hatcher would publish a review all members of Trachodontidae (which he used over the term Hadrosauridae), concluding that of ten established genera, only Trachodon and Marsh's Claosaurus were valid. All others (Thespesius, Hadrosaurus, Ornithotarsus, Cionodon, Polyonax, Diclonius, Pteropelyx, and Claorhynchus) were considered to be synonyms of Trachodon. How many species were synonymous within the lumped Trachodon was a matter he, however, considered uncertain. Some of these, such as Claorhynchus and Polyonax, would later turn out to represent ceratopsians. Lambe would later give it the distinct genus Stephanosaurus. T. altidens, for which Lambe provisionally suggested the distinct genus Didanodon, was based on a jawbone. The two specimens were mounted in the museum's Hall of Fossil Reptiles in 1908 under the name Trachodon mirabilis.
Decline of usage and abandonment
thumb|left|The discovery of new hadrosaur genera such as [[Saurolophus caused a shift away from the usage of Trachodon]]
Brown's excavations in the American west and Alberta, Canada would continue to uncover "trachodont" (hadrosaur) skeletons, and in 1910 and 1912 respectively he established the new genera Kritosaurus and Saurolophus for two such skeletons. This represented the first major break from Hatcher's paradigm of Trachodon dominating hadrosaur taxonomy. In light of the proliferation of taxa, in 1915 Charles W. Gilmore returned to Hatcher's conclusions, regretting that Trachodon had come to be so widely used without taxonomic caution. He noted the insufficient nature of Leidy's original teeth for identification at the species level (something even Hatcher was aware of). Furthermore, he noted distinction between the number of teeth in Judith and Lancian hadrosaurs. Consequently, he considered the longstanding conclusion that Cope's specimen and others belonged to Trachodon mirabilis, and instead he placed them in a revived Thespesius occidentalis. As for Trachodon, he suggested it would be appropriate to use it as the name for one of the species from older strata (as an age difference between the Judith and Lance had now been appreciated).
thumb|The original "Trachodon" mounts still stand today, now recognized as [[Edmontosaurus annectens specimens]]
Lambe would publish a study in 1918, also cautioning against the extensive usage of Trachodon. If it was to be used, he argued it must stand based on the identification of the original single-rooted tooth. He noted that the tooth seemed distinct in anatomy from that known in any hadrosaur. Thus, he considered it possible it represented a distinct species from any other then known. However, he also noted it was possible the tooth did not represent ordinary dental anatomy, and would thus be entirely useless for taxonomic purposes, with Trachodon consequently abandoned from use. He also noted that several of the other genera and species Hatcher synonymized with Trachodon were, themselves, based on insufficient material and required extensive revision. The complete skeletons referred to T. mirabilis were suggested to be retained in the genus Diclonius, oweing to their historical association with the name. William Parks supported the disuse of Trachodon in 1920, noting that Lambe's argument against Trachodon was made with good reason.
Lambe also noted the seniority of Hadrosauridae over Trachodontidae, which had been the dominant term for hadrosaurs since Hatcher's paper. Likewise, he deprecated Trachodontinae in favor of Hadrosaurinae for the flat-crested forms.
In 1942, Richard Swann Lull and Nelda Wright published a monograph titled Hadrosaurian Dinosaurs of North America, based on an unrealized monograph on Ornithopoda planned by Cope. As with prior authors, they disassociated the original Trachodon teeth from later skeletons. Like prior authors, they noticed the distinct rough texture of the tooth. However, they made the novel observation of similar but much fainter rough texturing on teeth belonging to Hadrosaurus. In the wake of this observation, it became clear that the tooth of Trachodon was a freshly deposited tooth, and its rough texture was not a trait identifying it taxonomically but instead due to the tooth not having been worn down by use in chewing. Without this distinguishing trait, it was considered impossible to determine which of several hadrosaurs from the same time period Trachodon belonged to. In the realm of pop culture, however, the name would persist in contexts like books well into future decades.
Species
thumb|right|T. selwyni teethNumerous species have been referred to this genus.
Type species:
T. mirabilis Leidy, 1856 However, recent studies consider it either a nomen dubium and indeterminate hadrosaurid or as a synonym of Lambeosaurus lambei.
- T. amurense was described by Russian paleontologist Anatoly Riabinin in 1925, from a partial skeleton that was unearthed in Upper Cretaceous rocks of the Amur River banks of Heilongjiang in Manchuria Province, northern China. It was later amended to T. amurensis and now is the type species of Mandschurosaurus. However, some authors state it is a nomen dubium.
- T. cantabrigiensis was named by British paleontologist Richard Lydekker in 1888 based on a dentary tooth that was collected from the strata of the Cambridge Greensand, which dates to the Lower Cretaceous, in Cambridgeshire, England. It is now believed to be a synonym of Edmontosaurus annectens. It has since been classified as its own genus, Stephanosaurus, or an indeterminate hadrosaurid.
- T. imperfectus, <small>(Kuhn, 1964) Young 1944</small> = Sanpasaurus imperfectus <small>Young 1944</small>
Description
Trachodon mirabilis is known definitively from a single, isolated tooth (ANSP 9260), likely from the mandible (lower jaw). sound amplification, and/or species recognition. Skin impressions are known from several lambeosaurines, including Parasaurolophus, Lambeosaurus, and Corythosaurus, which indicate that they were covered in small scales with an arrangement of feature scales (larger scales within a matrix of smaller scales) as well. Most depictions of Trachodon are based on specimens now assigned to Edmontosaurus, a genus of saurolophine rather than lambeosaurine. Edmontosaurus skull is long and low, lacks a bony crest, and bears a soft-tissue crest, unlike those of lambeosaurines. Similar conditions exist today in East Africa. Volcanic eruptions from the west periodically blanketed the region with ash, also resulting in large-scale mortality, while simultaneously enriching the soil for future plant growth. It is these ash beds that allow precise radiometric dating as well. Fluctuating sea levels also resulted in a variety of other environments at different times and places within the Judith River Group, including offshore and nearshore marine habitats, coastal wetlands, deltas, and lagoons, in addition to the inland floodplains.
The excellent vertebrate fossil record of Judith River rocks resulted from a combination of abundant animal life, periodic natural disasters, and the deposition of large amounts of sediment. Many types of freshwater and estuarine fish are represented, including sharks, rays, sturgeons, gars, and others. The Judith River Formation preserves the remains of many aquatic amphibians and reptiles, including frogs, salamanders, turtles, Champsosaurus and crocodilians. Terrestrial lizards, including whiptails, parasaniwids, and knob-scaled lizards have also been discovered.
As for dinosaurs, a menagerie are known. Theropods are represented by tyrannosaurids like Daspletosaurus and Gorgosaurus, dromaeosaurs like Saurornitholestes and Dromaeosaurus, the possibly valid troodontid Troodon,, an indeterminate, Deinocheirus-like ornithomimosaur, the bird Hesperornis, and the enigmatic Richardoestesia. Many other ornithischians have been unearthed from Judith River, such as the ankylosaurid Zuul, the hadrosaurids Brachylophosaurus, Corythosaurus, and Probrachylophosaurus, the ceratopsians Judiceratops, Lokiceratops, Medusaceratops, and Spiclypeus, and the pachycephalosaurids Colepiocephale and Hanssuesia. Multiple other dubious dinosaurs that were described from Judith River by Leidy or during the Bone Wars, a scientific feud between paleontologists Edward Drinker Cope and Othniel Charles Marsh, include Aublysodon, Ceratops, Deinodon, and Pteropelyx.