Mosasaur remains from the Maastrichtian of Denmark: implications for the diversity, ecology, and demise of a clade (Squamata: Mosasauridae)

Abstract Mosasaurs are a group of aquatic squamates that dominated marine environments during the Late Cretaceous (∼100–66 Mya) and radiated dramatically during the latest Cretaceous (Campanian–Maastrichtian, 83.6–66 Mya). Mosasaur fossils from the Maastrichtian of Denmark are scarce and have been studied only sporadically, despite substantial interest in vertebrate biodiversity prior to the Cretaceous–Palaeogene mass extinction. A comprehensive taxonomic re-examination of the Danish material reveals the coexistence of four mosasaurines (Mosasaurus hoffmanni, Mosasaurus lemonnieri?, Prognathodon sp., and Carinodens minalmamar) and one plioplatecarpine (Plioplatecarpus sp.), with M. lemonnieri? previously unknown from the assemblage. The relatively high taxonomic richness, in addition to new evidence for neonatal mosasaurs, indicates that a diverse mosasaur fauna inhabited the Boreal ‘Chalk Sea’ throughout the Maastrichtian. Tooth shape variation hints at the occupation of several ecological guilds. Spectrometry-based analyses of enamel samples, examining strontium and barium concentrations relative to calcium, provide further evidence for partitioning between presumed macrophagous apex predators and taxa occupying lower trophic levels. By integrating geochemical and morphological data, our findings point to the persistence of high ecological disparity in Boreal Realm mosasaurs through to the final 50 000 years of the Cretaceous, abruptly terminating at the Cretaceous–Palaeogene boundary.