Though many species of Tylosaurus have been named over the years, only a few are now recognized by scientists as taxonomically valid.
They are as follows: Tylosaurus proriger (Cope, 1869), from the Santonian and lower to middle Campanian of North America (Kansas, Alabama, Nebraska, etc.)
and Tylosaurus nepaeolicus (Cope, 1874), from the Santonian of North America (Kansas).
Tylosaurus kansasensis, named by Everhart in 2005 from the late Coniacian of Kansas, has been shown to be based on juvenile specimens of T. nepaeolicus.
It is likely that T. proriger evolved as a paedomorphic variety of T. nepaeolicus, retaining juvenile features into adulthood and attaining much larger adult size.

A closely related genus, Hainosaurus, is known from the Cretaceous of North America and Europe.
Both Tylosaurus and Hainosaurus are members of the group Tylosaurinae and are referred to informally as "tylosaurines" or "tylosaurs."
Research published in 2016, however, indicates that Hainosaurus is likely congeneric with Tylosaurus.
Bell placed the tylosaurines together with the plioplatecarpine mosasaurs (Platecarpus, Plioplatecarpus, etc.)
in an informal monophyletic grouping which he called the "Russellosaurinae."

The cladogram is modified from a phylogenetic analysis by Jiménez-Huidobro & Caldwell (2019) using Tylosaurus species with sufficiently known material to model accurate relationships.
Though many species of Tylosaurus have been named over the years, only a few are now recognized by scientists as taxonomically valid. They are as follows: Tylosaurus proriger (Cope, 1869), from the Santonian and lower to middle Campanian of North America (Kansas, Alabama, Nebraska, etc.) and Tylosaurus nepaeolicus (Cope, 1874), from the Santonian of North America (Kansas). Tylosaurus kansasensis, named by Everhart in 2005 from the late Coniacian of Kansas, has been shown to be based on juvenile specimens of T. nepaeolicus. It is likely that T. proriger evolved as a paedomorphic variety of T. nepaeolicus, retaining juvenile features into adulthood and attaining much larger adult size.
A closely related genus, Hainosaurus, is known from the Cretaceous of North America and Europe. Both Tylosaurus and Hainosaurus are members of the group Tylosaurinae and are referred to informally as "tylosaurines" or "tylosaurs." Research published in 2016, however, indicates that Hainosaurus is likely congeneric with Tylosaurus. Bell placed the tylosaurines together with the plioplatecarpine mosasaurs (Platecarpus, Plioplatecarpus, etc.) in an informal monophyletic grouping which he called the "Russellosaurinae."
The cladogram is modified from a phylogenetic analysis by Jiménez-Huidobro & Caldwell (2019) using Tylosaurus species with sufficiently known material to model accurate relationships.
Tylosaurus