Abstract The Cretaceous–Paleogene (K–Pg) transition saw mass extinctions in terrestrial
and marine ecosystems. Terrestrial vertebrate diversity patterns across the K–Pg boundary
have seen extensive study, but less is known about marine vertebrates. We describe a new
mosasaurid from the latest Maastrichtian phosphatic beds of Morocco, showing how
mosasaurids evolved to become apex predators in the latest Cretaceous. Thalassotitan
atrox gen. et sp. nov., from the Oulad Abdoun Basin of Khouribga Province, Morocco is …