Reassessment of Xenodens calminechari with a discussion of tooth morphology in mosasaurs.

Xenodens calminechari is a mosasaurid taxon named by Longrich et al. (2021) based on the holotype MHNM.KH.331, a left maxilla with several teeth. This holotype was obtained nonscientifically (without technical supervision) from an area in Morocco that yields many manipulated or forged specimens. Examination of Longrich et al. (2021) reveals four tooth crowns occupy what appear to be two alveoli in MHNM.KH.331, and there is potential adhesive connecting the tooth crowns to the maxilla on their lateral sides. We argue that the articulated tooth crowns of this taxon were artificially placed in the maxilla, rendering at least two apomorphies of this taxon the product of forgery. Longrich et al. (2021)'s claims of fused tooth 'roots' in MNHM.KH.331 are instead calcified periodontal ligament and alveolar bone that have ankylosed, resembling the typical mosasaurid condition. Differing tooth crown morphology does not preclude the referral of the teeth of this specimen to a younger ontogenetic stage of another mosasaur (possibly Carinodens) because many extant lizard species show drastic ontogenetic changes in the dentition. We argue that Xenodens calminechari represents a nomen dubium. This specimen constitutes a confluence of two persistent problems in vertebrate paleontology: material sourced from commercial excavations that has not been adequately tested for forgery, and taxa named from tooth-based holotypes that ignore the potential for intraspecific dental variation and interspecific convergence in dental characters, as are common in squamates. We suggest that Longrich et al. CT scan MHNM.KH.331, and we supply CT examples for identifying artificially added tooth crowns to Moroccan mosasaur jaws. Finally, we provide recommendations for the designation of mosasaurid holotypes.