A Giant Halisaurine from the Late Maastrichtian of Morocco

The Late Cretaceous deposits of Morocco have yielded one of the richest and most diverse assemblages of marine reptiles in the world, with the mosasaurids representing the dominant group. Among the most common mosasaurs are members of the subfamily Halisaurinae. Halisaurines ranged in size from the relatively small Halisaurus, which reached 4–5 m in length, to the larger Pluridens serpentis, which may have reached 7.5 m in length. Here we report a new, giant species of Pluridens, Pluridens imelaki. The new Pluridens is characterized by a slender, rectangular snout, a T-shaped premaxilla–maxilla junction, interlocking premaxilla–maxilla joint, a prominent dorsal ridge on the premaxilla, an exceptionally long and slender mandible, a tooth count of ~25 dentary teeth, straight, triangular tooth crowns that are strongly bent back just above the tooth–root junction, a low coronoid process and a tall and slender retroarticular process. The skull is 1.25 m long, suggesting a body length of ~9 m or more, comparable in size to large predators such as Thalassotitan. Differences between P. imaleki and P. serpentis in the jaw and tooth structure, eye size and innervation of the rostrum, as well as overall size, suggest they had different foraging strategies and occupied distinct ecological niches. Pluridens imelaki reveals that Halisaurinae were not only more species-rich than previously recognized, but also exhibited greater diversity in tooth morphology, jaw shape, and body size than previously thought. Rather than simply being outcompeted by Mosasaurinae, the Halisaurinae staged a minor adaptive radiation in the Late Cretaceous and were important members of the ecosystem in low latitudes. Pluridens imelaki appears to have been exceptionally rare in the phosphates, being documented by only a single specimen among the many hundreds of mosasaur remains recovered over many years. This underscores how the species richness of the phosphates and other diverse assemblages is driven by rare taxa that are only revealed through extensive sampling.