The only known species is T. atrox, described in 2022 from fossils discovered in the Ouled Abdoun Basin, initially identified as coming from other genera such as Mosasaurus or Prognathodon.

Hypothetical Thalassotitan specimens may have been found in other corners of the world, although researchers also note the possibility that they come from distinct, related taxa.

This genus shows definitely that mosasaurs evolved to take over the apex predator niche in the oceans of the Late Cretaceous which is now filled by sharks and orcas.

The phosphate deposits of the Ouled Abdoun Basin in Morocco have been known since the beginning of the 20th century to yield various fossils of numerous aquatic vertebrates dating from the Maastrichtian stage of the Upper Cretaceous.

[1] It is on the basis of these numerous anatomical differences that Nicholas R. Longrich and his colleagues erected in 2022 a new genus and a new species of mosasaurids under the name of Thalassotitan atrox.

All formally known specimens of this taxon have been more precisely discovered at the level of Upper Couche III, dating from approximately 67 to 66 million years ago.

The genus name Thalassotitan is a portmanteau of the Ancient Greek θάλασσα (thálassa, "sea") and τιτάν (tītā́n, "giant"), referring to the mosasaur's large size.

The specific epithet atrox is a Latin word translating to "cruel" or "merciless", which references the species' trophic position as an apex predator and frequency of intraspecific bite marks on fossils.

[1] Although all fossil remains formally attributed to Thalassotitan come from the Ouled Abdoun Basin, the authors note that teeth discovered elsewhere in the world could belong to this genus, or at least to a related taxon like Prognathodon saturator.

[1] In 2024, Trevor H. Rempert and his colleagues note the fairly abundant presence of fossil teeth comparable to those of Thalassotitan having been discovered in the Pee Dee Formation in North Carolina, USA.

The body of the premaxilla contains numerous pits called neurovascular foramina, which are believed to house tactile nerves that are very sensitive to touch.

The texture of the maxilla's surface is rough, which is especially apparent in larger individual, caused by a network of veined grooves to house blood vessels.

The dorsal (back) vertebrae are slightly longer than wide with tall neural arches, rounded articular surfaces, and large rectangular transverse processes.

[1] The forelimbs formed long paddles that resembled mosasaurin mosasaurs like Mosasaurus and Plotosaurus but more primitive in possessing longer but fewer phalanges.

It is as large as the humerus and much larger than the ulna and takes on a crescent-like or subrectangular form, unlike smaller hourglass-shaped radii in typical mosasaurs.

Morphologically, it is most similar to the giant mosasaurs P. currii and P. saturator, and a phylogenetic analysis by Longrich et al. (2022) recovered Thalassotitan in a clade between the two.

[1] However, due to a high degree of convergent evolution in the relationship-determining traits among many mosasaurs (especially among prognathodontines), phylogenetic results between each study are seldom consistent, mystifying exactly which species must be revised to stabilize the Prognathodontini.

(MGGC 21876) "Magahouanga mosasaurine" Carinodens Xenodens The phosphate deposits of Morocco have revealed an extremely diverse environment of late maastrichtian age.

[8][9] It seems that Thalassotitan was an apex predator in its ecosystem, with evidence being digestive damage found on some of the fossils in the nearby vicinity including those of plesiosaurs, turtles, and large fish.

[1] In the skies flew multiple species of pterosaurs including the azhdarchid Phosphatodraco, the nyctosauromorph Alcione and Simurghia, the nyctosaurid Barbaridactylus, and the possible pteranodontid Tethydraco.