Halisaurus arambourgi including a well-preserved, nearly complete skull and with lower jaws skeleton, and a partial skull that preserves details of the braincase and quadrate. We focus our description on the morphology of this spectacular Halisaurus discovery from Ouled Bouali, Khourigba, Morocco, North Africa.
Halisaurus appeared relatively early in the evolutionary history of the mosasaurs, during the Santonian. As such, the genus retains many primitive characteristics, as does the Halisaurinae at large. Both H. platyspondylus and H. arambourgi reached lengths of between 3 and 4 metres (9.8 - 13.1 feet). The length of the dubious H. onchognathus is difficult to tell due to the lack of remains but was likely similar.
As in other halisaurines, the flippers of Halisaurus are poorly differentiated which means that the genus lacked the hydrophalangy of more advanced mosasaurs. That Halisaurus was a relatively poor swimmer is relatively surprising considering its small size, since other small and medium-sized mosasaurs were mostly adept swimmers. The description of Phosphorosaurus ponpetelegans revealed that Phosphorosaurus, another member of the Halisaurinae, was highly specialized to compensate for its lack of hydrophalangy.
Classification and Species:
The exact position of Halisaurus within the larger mosasaur family tree has long been considered controversial. The type specimen of H. platyspondylus is not comprehensive and only preserves an angular and a basicranium fragment. Though the specimen does not reveal much about the mosasaur, it was noted by D.A. Russell in his Systematics and Morphology of American Mosasaurs (1967) that the angular resembles that of Platecarpus but is more symmetrically heartshaped in anterior aspect and appears slightly inflated in later profile with a convex anteroventral outline that is continuous with the anteroexternal margin of the articular surface, as in the genus Clidastes. The similarities with both Platecarpus and Clidastes were problematic, as said genera have always been classified in separate subfamilies, the Plioplatecarpinae and Mosasaurinae respectively. Russell referred the genus to the Plioplatecarpinae on the basis of a Platecarpus-like suprastapedial process in specimens referred to H. onchognathus.
Several discoveries throughout the 1980s and 1990s helped shed light on Halisaurus, with more complete specimens of the type species H. platyspondylus being discovered and Phosphorosaurus ortliebi being momentarily reassigned to the genus by Lingham-Soliar (1996). In 2005, the species Halisaurus sternbergii was reassigned to its own genus, Eonatator, along with the description of the new species Halisaurus arambourgi by Nathalie Bardet and colleagues. With the description of Eonatator as a closely related genus to Halisaurus, the two genera were grouped into the new subfamily Halisaurinae, which was then believed to be a sister-group to more advanced mosasaurs.
The recent most major phylogenetic analysis of mosasaurs, conducted by Tiago R. Simões and colleagues in May 2017, recovered Halisaurus and the rest of the Halisaurinae as a sister group to the Mosasaurinae. This would mean that the halisaurines are more closely related to the mosasaurines than the russellosaurines (genera such as Tylosaurus and Plesiopatecarpus) are.