A European mosasaurine (Squamata: Mosasauridae) with affinities to a clade from new zealand

The Royal Belgian Institute of Natural Sciences houses an extensive collection of Maastrichtian-aged mosasaur fossils. Among the numerous specimens of these marine reptiles is a skull and associated vertebral column, IRSNB 3211. This specimen was originally assigned to Plioplatecarpus houzeaui, but more recent studies have suggested that it represents an immature individual of Mosasaurus lemonnieri. New three-dimensional surface scans of IRSNB 3211 contribute to a detailed description of the specimen. Several characters strongly disagree with the diagnosis for Plioplatecarpus houzeaui, including the height of the coronoid process and the presence of zygosphenes and zygrantra. The proportions of the quadrate, the complex frontal/parietal suture, and the morphology of the posterior mandibular elements identify this specimen as belonging to Mosasaurinae. However, the morphology of the skull and vertebrae differs from that of Mosasaurus lemonnieri. Skeletal immaturity could account for some of these differences, such as the relatively longer rostrum or shorter quadrate of IRSNB 3211. However, ontogenetic change does not explain the differences in the morphology of the maxilla or braincase elements.
Phylogenetic analyses do not find a sister-group relationship between Mosasaurus lemonnieri and IRSNB 3211. The results of these analyses instead indicate a relationship between this specimen and Moanasaurus mangahouangae within a clade of mosasaurines from the Pacific Ocean. Characters that unite IRSNB 3211 with Moanasaurus mangahouangae include dorsal excavation of the maxilla for the external naris and cervical vertebrae with laterally extended postzygapophyses. However, skull proportions and differences in dental morphology do not support assigning this specimen to Moanasaurus mangahouangae. The qualitative results of morphological comparisons and the quantitative results of phylogenetic analysis support assigning IRSNB 3211 to a new taxon. Moanasaurus mangahouangae and its closest relatives have previously been considered to have been endemic to New Zealand. The New Zealand fossils are from Campanian strata, indicating the clade originated in the Southern Pacific and migrated to the Tethys during the Maastrichtian. Other fossils in European collections are likely representatives of this new taxon, but general similarities between this moanasaur-like taxon and Mosasaurus lemonnieri masked the diversity of latest Cretaceous mosasaurines from Europe.