Halisaurine mosasaurs are poorly known, represented by a small number of specimens from the Santonian-Maastrichtian (~86 Ma – ~66 Ma), but enjoyed broad palaeobiogeographic distribution during that time. They are important for understanding mosasaur evolution because certain aspects of their morphology retain the relatively plesiomorphic or minimally modified squamate conditions; however, existing material is limited and certain anatomical details are lacking. We report here two new specimens of Halisaurus arambourgi including a well-preserved, nearly complete skull and postcranial skeleton, and a partial skull that preserves details of the braincase and quadrate. We focus our description on morphology that augments the original description of this species and provides comparisons with other halisaurines. Braincase and temporal arcade characters confirm the plesiomorphic nature of Halisaurus, supporting a relatively basal position of Halisaurinae within Mosasauridae. Comparisons of cranial morphology support reconstruction of relationships within Halisaurinae, indicating that H. arambourgi is most closely related to H. platyspondylus, Phosphorosaurus (= H. ortliebi) is the sister taxon to those taxa, and Eonatator is the most basal described halisaurine. The proportions of the epipodials and the caudal vertebral centrum morphometrics indicate H. arambourgi is more derived than the Santonian to early Campanian Eonatator sternbergii but less derived than a Halisaurus sp. specimen from the mid-Maastrichtian of the Moreno Formation of California, USA. Moreover, vertebral morphometrics reveals that H. arambourgi possessed a downturned tail that likely supported a crescent-like fluke.

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