Biogeography of terrestrial and freshwater vertebrates from the late Cretaceous (Campanian) Western Interior of North America

Previous biogeographic studies of late Cretaceous (late Campanian) vertebrate faunas in the Western Interior Basin (WIB) of North America have suggested the presence of faunal and floral provincialism, characterized by distinct northern and southern ‘biomes.’ However, the “provincialism hypothesis” has been questioned based largely on the contention that the investigated faunas were recovered from a series of diachronous, time-transgressive deposits, and are therefore non-correlative.Extensive work in several fossiliferous units of late Campanian age, including the Dinosaur Park, Judith River, Two Medicine, Kaiparowits, Fruitland/Kirtland, and Aguja formations, has greatly increased understanding of WIB vertebrate faunas and their chronostratigraphic relationships. Here updated and greatly expanded faunal and chronostratigraphic datasets are utilized to undertake an extensive biogeographic analysis of these six terrestrial fossiliferous formations within the WIB of North America. Quantitative biogeographic comparisons of the formations and their constituent faunas are conducted using four statistical methods: Analysis of Similarity, Q-mode cluster analysis, Parsimony Analysis of Endemicity, and Correspondence Analysis.The results of this study provide strong support for highly divergent faunas in northern and southern regions of the WIB, with a latitudinal faunal gradient as an interface. Yet the nature of the interface between these faunas remains unclear, with possibilities including: 1) two or more discrete provinces separated by a zone (or zones) of faunal mixing; and 2) a continuous latitudinal gradient or cline, with no discrete zones of endemism. Lacking evidence of any physiographic barrier to north–south dispersal, climatic variation within the WIB is regarded as the most likely explanation for the overarching biogeographic patterns observed for late Campanian vertebrate taxa.