Tylosaurus was a species of large Mosasaur inhabiting the late cretaceous sea, it was one the largest known predators at around 12-15m with larger, common species like T. Proriger & T. Pembinensis from The Western Interior Seaway of Kansas & Canada, appearing around the same time as Mosasaurus Hoffmanni from the maastrichtian age with lengths up to 15-18m. T. Bernardi, formerly known as Hainosaurus, a 12m subspecies from Europe, likely belong to the same genus. Tylosaurus belonged to the subgroup of mosasaurs called Tylosaurinae which included its cousins: Pannoniasaurus(6m), Taniwhasaurus(7m, though it has 3 specimens: T. Oweni, T. Antarcticus & T. Mikasaensis), Kaikaifilu(10m) and even Tylosaurus itself.
Tylosaurus remains are largely from North America where it inhabited the Western Interior Seaway. Tylosaurus stomach remains have been found with fish and other smaller marine reptiles, indicating they were likely Apex predators of their areas feeding on a large array of prey. A number of predators shared the Cretaceous oceans with Tylosaurus such as Cretoxyrhina, an 8-meter shark known formerly as the Ginsu Shark, Cretodus Crassidens, a species of 11.28 meter-long Mackerel Shark that lived during the Cenomanian-Coniacian Stages of the Late Cretaceous(Most likely it was found in the Western Interior Seaway, Europe, Africa and even Asia), Megacephalosaurus, a species of 9-meter Pliosaur that lived in the Western Interior Seaway during the Turonian Stage(It was also one of the last of the Pliosaurs), Ptychodus, a species of 10-meter shark that fed on turtles and ammonites, Xiphactinus, a large predatory fish, the giant ammonite Parapuzosia Seppenradensis and even Deinosuchus, a species of 12-meter Alligatoroid Crocodilian from the Campanian Stage of Texas, Montana, the Eastern Coast of North America and also even the Western Interior Seaway. These predators may have competed with Tylosaurus while the large plesiosaur Elasmosaurus may have taken smaller prey to its body size. the adult Tylosaurus were likely the apex predators of their ecosystems. Hainosaurus Bernardi was the smaller, lesser, poorly known, European subspecies of Tylosaurus. Reaching a size of 12 meters, it was named for the River Haine(A river on the Netherlands-Belgium border line), where it was first discovered, along with more remains being found in Belgium & the Netherlands. This all proves that Tylosaurus was a very common Mosasaur species.