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 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
Tylosaurus was the third new genus of mosasaur to be described from North America behind Clidastes and Platecarpus and the first in Kansas. The early history of the genus as a taxon was subject to complications spurred by the infamous rivalry between American paleontologists Edward Drinker Cope and Othniel Charles Marsh during the Bone Wars. The type specimen consisted of a partial skull around in length associated with thirteen vertebrae. The history of its discovery remains partly uncertain due to conflicting accounts and differing information regarding its type locality. The earliest contemporary mentions date to late 1868 in two regional Kansas newspapers, where the discovery was attributed to the fossil collector William E. Webb, who was then the first documented owner of the specimen. All sources agree that the specimen most likely originated from the Smoky Hill Chalk of the Niobrara Formation. Museum records indicate that it was discovered near Monument Rocks, in western Gove County, Kansas. However, in 1875, Cope stated that the fossils had actually been discovered by the U.S. Army colonel John Butler Conyngham and a certain Mr. Minor, near the Monument station along the Kansas Pacific Railroad. This latter locality is generally regarded as the most probable, although further studies are still required to confirm it. Only after this discovery was the specimen acquired by Webb in 1868, most likely through Conyngham, although the exact circumstances surrounding this transfer remain uncertain.
In August of the same year, the Swiss naturalist Louis Agassiz and several other prominent figures passed through the town of Monument. It was probably on this occasion that Agassiz met Webb, and perhaps also Conyngham, subsequently acquiring the specimen and depositing it in the Harvard's Museum of Comparative Zoology under the catalog number MCZ VPRA 4374, where it is still housed today. The receipt documenting Agassiz's payment of $500 to Webb, dated April 6, 1869, is also preserved with the specimen. In addition, the sixth tooth of the right maxilla was mistakenly catalogued under the number MCZ 1596, whereas the correct catalog number appears on the remaining fossil elements. At the December 21, 1868 meeting of the Academy of Natural Sciences of Philadelphia, the American paleontologist Joseph Leidy presented photographs of the specimen that had been sent to him by Webb, provisionally identifying it as belonging to the genus Mosasaurus.. It was most likely during this same meeting that Cope first saw these photographs, after which he borrowed the specimen from Agassiz and presented its first scientific description at another meeting of the Academy in June of the following year. Cope's first publication of the fossil was very brief and was named Macrosaurus proriger, the genus being a preexisting European mosasaur taxon. The specific epithet proriger means "prow-bearing", which is in reference to the specimen's unique prow-like elongated rostrum and is derived from the Latin word prōra (prow) and suffix -gero (I bear). In 1870, Cope published a more thorough description of MCZ 4374. Without explanation, he moved the species into another European genus Liodon and declared his original Macrosaurus proriger a synonym. In his 1870 and 1875 descriptions, Cope described and illustrated several additional vertebrae that he referred to the holotype specimen. However, as these elements are not associated with the specimen as it is currently preserved, it remains uncertain whether they were subsequently lost or were, in fact, never part of the holotype.
In 1871, Cope identified a second species of North American Liodon based on several vertebrae and limb fragments, which he named Liodon dyspelor. The fossils came to Cope under the label as having been collected by an army doctor named William B. Lyon at Fort McRae, New Mexico. However, a 1997 study led by David C. Parris of the New Jersey State Museum found that the chemical composition of the fossil did not match any known geological deposit in New Mexico and is instead identical to that of the Niobrara Formation; the study speculated from examinations of multiple letters between Lyon and the Smithsonian Institution that the Liodon dyspelor fossils were actually recovered from somewhere in Western Kansas and were mislabeled as being collected by Lyon in New Mexico (who was shipping a different fossil from Fort McRae around the same time) during transit. The type specimen of the taxon is split between two museums; some of the vertebrae went to Cope's American Museum of Natural History as AMNH 1580 while the rest were sent to the Smithsonian as USNM 41.