THE ROLE OF LARAMIDE UPLIFTS IN SHAPING LATE CRETACEOUS TERRESTRIAL BIODIVERSITY: A CASE STUDY IN MAMMALS FROM THE HEWETT’S FORESIGHT LOCAL FAUNA (LANCE FORMATION, BIGHORN BASIN, WYOMING)

Whereas patterns of Late Cretaceous terrestrial biodiversity and biogeography have emerged via continent-scale studies focused on latitudinal gradients, high-spatial-resolution, basin-scale studies reduce sampling bias and permit more direct tests of how environmental gradients, such as topography, influence those patterns. In modern basins, biodiversity is highest within and proximal to mountain uplifts and decreases in more distal lowlands. In the fossil record, diversity increases as tectonic activity increases, such as the Cretaceous diversification of mammals coincident with the acceleration of tectonic processes that formed the North American Cordillera. Thus, in deep time, topographic biodiversity gradients should be revealed by comparing the diversity of contemporaneous mountain-proximal vs. mountain-distal faunal assemblages. This study describes mammal teeth from the Hewett’s Foresight local fauna, a mountain-proximal site from the latest Cretaceous Lance Formation of the Bighorn Basin, northwestern Wyoming. Preliminary observations indicate that mammalian taxonomic richness at Hewett’s Foresight, particularly among multituberculates, may be higher than previously noted and may exceed that of the roughly coeval, mountain-distal Type Lance local fauna to the east. Pending quantitative analyses, these preliminary results show a striking similarity in taxonomic composition between multituberculate mammals from the Hewett’s Foresight local fauna and other coeval mountain-proximal sites, including Black Butte Station in southwestern Wyoming and Pawnee National Grassland in northeastern Colorado. We interpret the higher species richness and strong faunal similarities among mountain-proximal sites as evidence of a topographic diversity gradient in the latest Cretaceous of western North America. This topographic diversity gradient suggests tectonic processes, specifically the emergence of Laramide uplifts, were important drivers of mammalian community assembly and spatial heterogeneity in the North American Western Interior just prior to the Cretaceous-Paleogene mass extinction.