[13] 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.

[13][14] The type specimen was described by Cope in 1869 based on a fragmentary skull measuring nearly 5 feet (1.5 m) in length and thirteen vertebrae lent to him by Louis Agassiz of the Harvard Museum of Comparative Zoology.

[15] The fossil, which remains in the same museum under the catalog number MCZ 4374, was recovered from a deposit of the Niobrara Formation located in the vicinity of Monument Rocks[16] near the Union Pacific Railroad at Fort Hays, Kansas.

[13][15] The specific epithet proriger means "prow-bearing", which is in reference to the specimen's unique prow-like elongated rostrum[18][19] and is derived from the Latin word prōra (prow) and suffix -gero (I bear).

The type specimen of this species was discovered by geologist Benjamin Franklin Mudge near the Solomon River, and consists of several cranial fragments and a dorsal vertebra[27] now catalogued as AMNH 1565.

This species, whose specific epithet refers to Nepaholla, the Native American name for the Salomon River[25] is formally transferred to the genus Tylosaurus in 1894 by John Campbell Merriam.

[6] In 1885, Louis Dollo described the genus and species Hainosaurus bernardi from an almost complete but poorly preserved skeleton discovered in a phosphate quarry in the Ciply Basin in Belgium,[30] the specimen having since been catalogued as IRSNB R23.

[40] When the taxon was significantly revised in a in 2002 study, being reassigned to Tylosaurus, Lindgren and Mikael Siverson referred additional fossils to this latter that had been discovered at Ivö Klack, including cranial and vertebral remains.

[7] In 2012, Octávio Mateus and colleagues reported that an additional specimen of T. iembeensis consisting of fragmentary cranial elements was recovered during an expedition to the locality of the since-lost holotype, although it was not figured or formally described.

[45] The validity of this species was questioned as early as 2007 by Caldwell,[46] to the point that in a study published in 2016 with Jiménez-Huidobro and other authors, the latter considers it a juvenile form of T. nepaeolicus, thus making the first name a junior synonym of the second.

With visibly distinct features from other species and having been discovered approximately 55 kilometers (34 mi) northeast of Grande Prairie, Alberta, this makes the specimen the northernmost known occurrence of the genus, being then named T. 'borealis', in reference to its northernly presence.

The largest well-known specimen, a skeleton of T. proriger from the University of Kansas Natural History Museum nicknamed "Bunker" (KUVP 5033), has been estimated to measure between 12–15.8 meters (39–52 ft) long.

[25] In North America, the earliest representatives of Tylosaurus during the Turonian[70] and Coniacian (90-86 mya), which included early T. nepaeolicus and its precursors, typically measured 5–7 meters (16–23 ft) long[25] and weighed between 200–500 kilograms (440–1,100 lb).

[51] A mounted skeleton of T. pembinensis, nicknamed "Bruce," at the Canadian Fossil Discovery Centre measures at 13.05 meters (42.8 ft) long and was awarded a Guinness World Records for "Largest mosasaur on display" in 2014.

[6] The jugal forms the bottom of the orbit; in Tylosaurus, it is L-shaped and has a distinctive serif-like extension at the lower back corner of the junction between the horizontal and vertical rami (arms) called the posteroventral process.

[6] In lateral view, the quadrate resembles a hook in immature T. nepaeolicus and T. proriger individuals, but in adult forms for both species[49] and in T. bernardi,[6][35] T. pembinensis,[34] and T. saskatchweanensis it takes on a robust oval-like shape.

[53] The suprastapedial process is a hook-like extension of bone that curves posteroventrally from the apex of the shaft into an incomplete loop, and it likely served as the attachment point for the depressor mandibulae muscles that opened the lower jaw.

The North American 'proriger group' includes T. proriger and T. nepaeolicus and is characterized by teeth with smooth or faint facets, less prominent carinae, and a vein-like network of primitive striations extending to near the tip.

[99][page needed] Tylosaurus limbs are primitive relative to other mosasaurs; their stylopodia (humeri and femora) lack both the complex muscle attachment sites and extreme proximodistal shortening present in other derived taxa.

[106] Microscopic analysis of scales in a T. nepaeolicus specimen by Lindgren et al. (2014) detected high traces of the pigment eumelanin indicative of a dark coloration similar to the leatherback sea turtle in life.

[49] A slightly younger specimen is of a skull (SGM-M1) of an indeterminate Tylosaurus species similar to T. kansasensis from the Ojinaga Formation in Chihuahua, Mexico,[3] dated around ~90 million years old at earliest.

[6][54] In 2020, Madzia and Cau performed a Bayesian analysis to better understand the evolutionary influence on early mosasaurs by contemporaneous pliosaurs and polycotylids by examining the rates of evolution in mosasauroids like Tylosaurus (specifically T. proriger, T. nepaeolicus, and T. bernardi).

The analysis also generated a paraphyletic status of the genus, approximating Taniwhasaurus to have diverged from Tylosaurus around 84.65 million years ago, but this result is not consistent with previous phylogenetic analyses.

First, Jiménez-Huidobro, Simões, and Caldwell proposed in 2016 that T. proriger evolved as a paedomorph of T. nepaeolicus, in which the descendant arose as a result of morphological changes through the retention of juvenile features of the ancestor in adulthood.

[47][49] However, an ontogenetic study by Zietlow (2020) found that it was unclear whether this observation was a result of paedomorphosis, although this uncertainty may have been due that the sample size of mature T. nepaeolicus was too low to determine statistical significance.

[f] Endothermy would have provided several advantages to Tylosaurus such as increased stamina for foraging larger areas and pursuing prey, the ability to access colder waters, and better adaptation to withstand the gradual cooling of global temperatures during the Late Cretaceous.

[119] The enormous and varied appetite of Tylosaurus can be demonstrated in a 1987 find that identified fossils of a mosasaur measuring 2 meters (6.6 ft) or longer, the diving bird Hesperornis, a Bananogmius fish, and possibly a shark all within the stomach of a single T. proriger skeleton (SDSM 10439) recovered from the Pierre Shale of South Dakota.

[125] Garvey (2020) criticized the lack of conclusive evidence to support this hypothesis and ruled out T. proriger as a possible culprit, given that the species did not appear until the Santonian and is exclusive to the Western Interior Seaway.

Many of these fossils consist of healed bite marks and wounds that are concentrated around or near the head region, implying that there were the result of non-lethal interaction, but the motives of such contact remain speculative.

[127] Carlsen (2017) posited that Tylosaurus gained avascular necrosis because it lacked the necessary adaptations for deep or repetitive diving, although noted that the genus had well-developed eardrums that could protect themselves from rapid changes in pressure.