Richness Trends of Mosasaurs (Diapsida, Squamata) During the Late Cretaceous

Marcus R. Ross, Liberty University

Document Type Dissertation


Late Cretaceous marine sedimentary deposits from North America, The Netherlands, and Belgium are correlated on the basis of ammonites, calcareous nannofossils, sequence stratigraphy, and absolute age methods. These correlations provide the rationale for the construction of fifteen stratigraphically correlated assemblages (SCAs), informal sub-stage level subdivisions designed to provide a broad framework for compiling and organizing marine vertebrate fossils from far-ranging localities with high levels of biostratigraphic resolution.

A database of 1,805 globally distributed mosasaur specimens, each assigned to one of the fifteen SCAs and consisting of a minimum number of 354 individuals, is quantitatively analyzed to illuminate trends within the group. Four trends are recognized. First, mosasaur generic richness cannot be shown to change during the Maastrichtian stage, at the close of which all mosasaurs are extirpated. This trend is incompatible with long-term extinction scenarios, but is compatible with geologically instantaneous extinction hypotheses, such as a bolide impact at the end of the Cretaceous. The extinction of mosasaurs may be explained by a collapse of the primary production-dependant marine food web, in which mosasaurs were apical predators. Second, mosasaur richness decreases along poleward-trending latitudinal gradients, confirming earlier hypotheses. Third, mosasaurs experienced a significant expansion during the Coniacian through Santonian, followed by stable high richness levels from the early Campanian through latest Maastrichtian. This indicates that mosasaurs did not experience long-term richness declines in the latest Cretaceous. Fourth, despite stable generic richness levels, mosasaurs continually exploited new predatory modes recognized via accumulation of novel tooth forms. At the time of their extinction at the end of the Cretaceous, mosasaur were at the zenith of their morphological and ecological diversity, the extent of which was unparalleled by any other group of marine reptiles.