New Information on the Morphology and Tooth Replacement of Xenodens calminechari (Squamata: Mosasauridae), a Unique Mosasaurid from the Maastrichtian Phosphates of Morocco

Xenodens calminechari is a highly derived mosasaurid from the latest Maastrichtian Phosphates of the Oulad Abdoun Basin, Morocco. Originally described based on a single maxilla, Xenodens differs from all known squamates in its closely packed, bladelike marginal teeth and modified tooth implantation and replacement. Xenodens’ relationships and anatomy remain poorly understood, and a recent study suggested that the holotype represents a composite, and furthermore that the animal might represent a juvenile of Carinodens. Evidence from a new referred specimen of Xenodens and CT scans corroborate the original description of Xenodens. Scans of the holotype and referred specimen of Xenodens reveal highly derived tooth implantation; interdental ridges are reduced in the posterior part of the jaw and teeth implant in a groove, with adjacent roots contacting and fusing. Tooth roots bear large, deep replacement pits, as is typical of derived mosasaurids, but in posterior teeth the replacement pits merge lingually to create a single large pit for two teeth. We provide an updated diagnosis of Xenodens, detailing unusual features of its tooth anatomy, implantation and replacement. Differences between Xenodens and Carinodens are numerous and no intermediate morphologies exist; furthermore, the size overlap between Carinodens and Xenodens indicates that Xenodens cannot represent a juvenile Carinodens. Xenodens highlights the remarkable diversity of mosasaurids, as well as the exceptional range of ecological niches occupied by this highly successful group of marine reptiles before their extinction.