A High-Resolution Timeline for the Late Cretaceous Evolution of Dinosaur Ecosystems Based on CA-ID-TIMS U-Pb Geochronology of the North American Western Interior Basin

The Western Interior Basin (WIB) of North America preserves a remarkable record of continental fauna, flora and associated paleoenvironments during the Late Cretaceous and across the Cretaceous-Paleogene transition. Century-long (and ongoing) discoveries of vertebrate fossil assemblages out of the extensive WIB foreland successions have provided a unique opportunity to investigate the diversification and provinciality of dinosaur fauna during the zenith of their diversity in the Campanian and Maastrichtian. Of particular scientific interest are possible relationships of dinosaur paleogeography and evolution to the global climate change and the tectonic evolution of the North American Cordillera (e.g., uplift and volcanism). Tectonic convergence associated with the mountain building stage of the Laramide Orogeny resulted in basement thrusting, uplift and foreland basin transformation, evidently starting in the mid-Campanian. These roughly coincided with elevated volcanic activity due to a magmatic arc ‘flare-up’, as indicated by an increase in the abundance of bentonites and volcanogenic sediments throughout the WIB at ~76 Ma. Concomitantly, a global cooling trend following the Turonian peak greenhouse climate resulted in ocean water temperatures up to 7°C cooler during the Campanian, presumably driven by declining atmospheric CO2 and reconfiguration of ocean gateways and in the absence of polar ice. Exploring ecosystem changes controlled by climate, tectonism (landscape evolution) and volcanism, as well as their possible linkages to dinosaur evolution are complicated by laterally discontinuous continental stratigraphy, its inherent depositional gaps and time-transgressive lithostratigraphic units. These stratigraphic complexities, along with the absence of diagnostic marine fossils, hamper any reliable correlation of distant fossil beds necessary for accurately deciphering evolutionary trends. A recent systematic geochronologic study of the Campanian fossil-rich formations across the WIB based on U-Pb CA-ID-TIMS dating of bentonites succeeded in constructing a high-resolution chronostratigraphic framework for these units over a 1600 km latitudinal distance and spanning ca. 83 – 73 Ma. This framework allows key dinosaur assemblages throughout the Kaiparowits Formation (southern Utah), Judith River Formation (central Montana), Two Medicine Formation (western Montana), Dinosaur Park Formation (southern Alberta) and their age-equivalents to be correlated in absolute time within a ±30 k.y. uncertainty. The results point to a close temporal relationship between dinosaur abundance/diversity, increased volcanism and changes in the overall basin architecture in the Campanian. Although the upper Maastrichtian of the WIB has been investigated extensively in connection to the end-Cretaceous mass extinction, the lower Maastrichtian remains poorly studied in comparison, despite its invaluable fossil and paleoenvironmental records. Our integrated stratigraphic and U-Pb geochronologic work in progress is aimed at filling the knowledge gap in the mode and tempo of dinosaur evolution during this crucial period in comparison to that in the Campanian. A long-standing question we aim to address is whether or not there was an appreciable decline in dinosaur biodiversity in the run up to the end-Cretaceous mass extinction.