Anatomy and variation in the paraphyletic Mosasaurus lemonnieri assemblage from the Craie phosphatée de ciply, Belgium.

Nineteenth century excavations of the Ciply-Malogne Phosphatic Chalk Formation in Hainaut, Belgium, yielded over 50 partial mosasaur skeletons, the majority of which were referred to the species Mosasaurus lemonnieri Dollo, 1889. In the years since, this taxon has experienced a complicated taxonomic history, and its exceptional hypodigm has gone understudied. Here, the osteology and variation of these specimens are described and figured in detail, with the goal of elucidating the taxonomy of these mosasaurs and improving anatomical understandings of mosasaurs in general. These specimens mostly show a unique and stable group of apomorphies, supporting their position as a distinct taxon in the genus Mosasaurus. Variation abounds, particularly in entheseal development, allometric robusticity, and cranial ossification patterns. Most of these specimens are juveniles or subadults, which has contributed to problematic past diagnoses for this species. We conclude that M. lemonnieri is a valid taxon, but that one specimen belongs to a distinct taxon of mosasaurine, here named Thoraxion tahlequahae gen. et sp. nov. The high degree of intraspecific variation in M. lemonnieri argues against the usage of several past taxonomic and phylogenetic characters. Several well-preserved specimens allow for 3D reconstructions of the palate, rostral cranial cartilages, and ribcages of derived mosasaurs, and new putative pythonomorph synapomorphies of the cranial soft tissues.