CLIMATIC AND ENVIRONMENTAL DRIVERS OF LATE CRETACEOUS VERTEBRATE ENDEMISM

The Western Interior Basin (WIB) of western North America contains an exceptionally well-preserved sedimentary archive of climatic and biologic conditions during the Late Cretaceous hothouse (101 to 66 Ma) that provides an opportunity to study climate and biologic interactions on a continental scale. Since the 1970s, it has been suggested that Campanian and Maastrichtian non-avian dinosaur communities in the WIB were highly endemic, and could be divided into distinct northern and southern provinces or communities. Faced with a lack of physical barriers (e.g., water bodies or topographic barriers), proponents of this hypothesis have suggested that some sort of “climate barrier” was responsible for the observed provincialism; however, this hypothesis has never been formally tested. To address this long-standing question, we created a compilation of all known quantitative temperature reconstructions for the Campanian (84 to 72 Ma) and Maastrichtian (72 to 66 Ma) stages of the Late Cretaceous and created new spatially-interpolated maps of mean annual temperature across the WIB. This new dataset reveals the presence of a steep (>1°C/°latitude) temperature gradient at ~45 to 55 °N paleolatitude during the Late Cretaceous, which may have acted as a boundary between northern and southern biomes. We present additional similarity analyses of fossil pollen, fossil leaves, and dinosaur assemblages found north and south of this steep temperature gradient, and show that the similarity values for the dinosaur localities follow trends in pollen similarity values. Based on our findings, we suggest that the presence of a steep temperature gradient in the central WIB led to partitioning of Late Cretaceous floral communities, which in turn led to the partitioning of dinosaur communities into distinct northern and southern provinces. These findings have implications for understanding climate controls on Late Cretaceous paleobiogeography and provide an important new dataset for climate model validation and paleoenvironmental studies in the WIB.