Early Campanian mosasaurs (Reptilia; Mosasauridae) from the Kristianstad Basin, southern Sweden

Abstract: Marine strata of latest early Campanian age (sensu germanico) in the Kristianstad Basin, Skåne (Scania), southern Sweden, have yielded a diverse mosasaur fauna comprising six to eight taxa, including Tylosaurus ivoensis, Clidastes propython, Hainosaurus minor n. sp., Halisaurus sternbergii, Platecarpus cf. somenensis, Platecarpus? sp., Dollosaurus sp., and possibly yet another mosasaurine or halisaurine. The material consists mainly of isolated shed tooth-crowns, although a number of vertebrae and other, fragmentary bones have been recorded as well. The assemblage is similar in composition to approximately coeval mosasaur faunas from the Western Interior and the Gulf Coast of North America. The giant T. ivoensis was formerly believed to be a species of Mosasaurus (i.e. M. hoffmanni ivoensis and M. ivoensis of earlier authors). Nevertheless, its dental and vertebral morphology correspond perfectly to those of tylosaurine mosasaurs. Tylosaurus pembinensis (a nominal species originally assigned to Hainosaurus) from the late early Campanian of southern Manitoba, Canada, may be a junior synonym of T. ivoensis, as its teeth (both marginal and pterygoid) and vertebrae are very similar to those of the latter taxon. The occurrence of C. propython in the early Campanian of southern Sweden represents the first record of both the genus and species outside of North America. In the Kristianstad Basin area remains of Clidastes are found in deposits representing a shallow water archipelago environment. This is in stark contrast to the deeper water, outer shelf environment advocated by one author as the preferred habitat for this species. Another intercontinental mosasaur; H. sternbergii, was originally described from an unspecified horizon in the late Coniacian-earliest Campanian Smoky Hill Chalk in western Kansas, USA. The Kristianstad Basin population of H. sternbergii was probably derived from individuals that migrated from the Mississippi Embayment sometime during the early Campanian. A new species of Hainosaurus; H. minor n. sp., has been identified from very well preserved marginal tooth-crowns. The nominal species can be distinguished from the early Maastrichtian H. bernardi (the only other valid nominal species of Hainosaurus) by its relatively small-sized and heavily faceted teeth. At, or near, the early/late Campanian boundary, the mosasaur faunas in North America and northern Europe were severely decimated. New low-diversity faunas with Mosasaurus, Plioplatecarpus and Prognathodon replaced species-rich assemblages dominated by Clidastes, Platecarpus and Tylosaurus. The great distance and markedly different palaeoenvironments between different areas affected by the reorganization (e.g. sub-tropical outer shelf in Alabama vs. warm temperate shallow water archipelago in the Kristianstad Basin area) indicate that the faunal turnover was caused by a (undetermined) global catastrophic event, or a series of events, rather than by local, independent deteriorations of the environment.