The research history of Tylosaurus is extensive given to its long taxonomic and cultural histories, with the earliest record of its fossils dating back to 1869, when Edward Drinker Cope described a partial specimen from Kansas, USA. Initially named as Macrosaurus proriger, Cope later moved the species to the genus Liodon. Over the following years, he described two additional species that he assigned to this genus. In 1872, Othniel Charles Marsh established the genus name Tylosaurus, literally meaning "knob lizard" in reference to the animal's rostrum, in order to include a species that he had previously described as well as two of the species described by Cope. This name began to be used the following year by Joseph Leidy, although Cope, being in rivalry with Marsh, did not adopt it. In 1894, John Campbell Merriam definitively transferred the species of Liodon described by Cope and Marsh to Tylosaurus, and one of them was placed in synonymy with the type species T. proriger by Dale A. Russell in 1967. Cope described Tylosaurus in 1870 as an imposing marine reptile sharing a morphology similar to a sea serpent and whose size would have been that of the largest cetaceans, an interpretation that was followed in various paleoarts at that time. In an 1898 revision, Samuel Wendell Williston presented a much more precise skeletal reconstruction of the animal that is still recognised as accurate for modern standards. From the 19th to the 21st century, several mosasaurids species since attributed to Tylosaurus were initially described as coming from different genera. One of these genera, Hainosaurus, is mainly recognized as a junior synonym of Tylosaurus since a 2016 study, although some authors suggested to keep it distinct and valid again. Nine species of Tylosaurus are currently recognized, namely T. proriger, T. nepaeolicus, T. bernardi, T. gaudryi, T. ivoensis, T. iembeensis, T. pembinensis, T. saskatchewanensis, and T. rex. The validity of two additional taxa remain unsettled; there is still debate whether T. kansasensis is synonymous with T. nepaeolicus, and T. "borealis" has yet to be described in a formal publication.
Initial discoveries
Possible earlier finds Several possible fossils coming from Tylosaurus were possibly discovered long before the genus was formally described and named in scientific literature, making it the first known mosasaur. The discovery of its fossils would have influenced the folk stories present among certain Native Americans peoples. The oldest possible discovery concerning it was possibly made by the North American people of the Lenapes, before the arrival of European settlers, around before the 1500s. An old legend tells about discovery made by hunters of a bone fragment coming from a monster that killed people in territories that would correspond to present-day Philadelphia, Pennsylvania or New Jersey. After the discovery, the hunters, having taken the bone to their home village, were quickly encouraged by a local wise man to find other remains of this same creature. According to the wise man, smoking the bone with tobacco would promote various positive things such as good health for children, longevity or even a successful hunt. Tylosaurus fossils have been discovered in the area where this legend may have emerged. However, remains of dinosaurs and crocodilians from the Cretaceous period are also known there, changing the origin of the story. In a slightly more recent period, around the 1600s, the Dakota and Cheyenne peoples speak of tales of an ancient era ruled by massive aquatic creatures that were in constant fight with thunderbirds. According to the tales, these aquatic monsters were petrified by lightning struck by the thunderbirds, and their remains have been found on the ground ever since. The stories provided by the Pawnee and Crow peoples depict similar aquatic creatures living in rivers. These are described as having a crocodile-like heads with serpentine bodies, and having legs or even fins. The Great Plains, where the peoples from which these legends originated, are known to provide a considerable number of fossils of large mosasaurs such as Tylosaurus and pterosaurs such as Pteranodon, which may have been the source of these legends. The first possible recorded discovery of a mosasaur in North America was of a partial skeleton described as "a fish" in 1804 by Meriwether Lewis and William Clark's Corps of Discovery during their 1804–1806 expedition across the western United States. It was found by Sergeant Patrick Gass on black sulfur bluffs near the Cedar Island alongside the Missouri River and consisted of some teeth and a disarticulated vertebral column measuring 45 feet (14 m) in length. Four members of the expedition recorded the discovery in their journals including Clark and Gass. Some parts of the fossil were collected and sent back to Washington, D.C., where it was lost before any proper documentation could be made. In 2003, Richard Ellis speculated that the remains may have belonged to Mosasaurus missouriensis. Alternatively, a 2007 study led by Robert W. Meredith and colleagues suggested that the fossils would possibly come from a tylosaurine mosasaur based on the measurements cited by Clark and Gass and the evidence of Tylosaurus fossils that have been found in the Missouri River. However, the authors also mentioned the possibility that the remains would also come from an elasmosaurid plesiosaur, which are also known from the river, although being rarer. In 1858, Ebenezer Emmons described a single tooth of a mosasaur as a new genus and species, Elliptonodon compressus. In 1967, Halsey W. Miller examined a cast of the type specimen, catalogued as USNM 7444 in the National Museum of Natural History, and tentatively referred it to Tylosaurus? sp. based on the supposed similarities between the tooth and an anterior (front) tooth from a Tylosaurus jaw, catalogued as USNM 3763 and housed in the same institution. In the same year, Dale A. Russell conversely suggested that the type and only known specimen most closely resembles the tooth of Prognathodon, but argued that it is too fragmentary with uncertain locality of origin, and concluded that E. compressus should be classified as a nomen dubium (dubious name).
First formal discoveries and naming
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