"Tylosaurus" was among the earliest known derived mosasaurs. The oldest fossil attributed to the genus is of a skull (SGM-M1) recovered from Upper Turonian deposits of Ojinaga Formation in Chihuahua, Mexico, which may date around 90 million years old at earliest based on correlations with index fossils. This skull has been identified as an indeterminate species with significant affinities with "T. kansasensis", which was previously considered the most primitive species. A tooth from a Late Maastrichtian deposit in Nasiłów, Poland dating close to the Cretaceous–Paleogene boundary has been attributed to "Hainosaurus sp." With the incorporation of "Hainosaurus" as a synonym of "Tylosaurus", this also makes the genus one of the last mosasaurs. Currently, eight species of "Tylosaurus" are recognized by scientists as taxonomically valid. They are as follow: "T. proriger" and "T. nepaeolicus" from North America, "T. bernardi" from Europe, "T. gaudryi" from France, "T. ivoensis" from Sweden, "T. iembeensis" from Angola, and "T. pembinensis" and "T. saskatchewanensis" from Canada. 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.