Applications of High-Precision Geochronology in Evolution and Mass Extinctions.

High-precision geochronology provides important age constraints for the timing and tempo of important evolutionary events in the history of Earth's biota, including mass extinctions. Herein, I use 40Ar/ 39Ar and U/Pb radioisotopic dating and paleomagnetism to address temporal questions related to the origin of angiosperms, feathered dinosaurs, primitive birds, early mammals and the end Permian mass extinction. In the first chapter, I present the most accurate 40Ar/39Ar age calibration of the Jehol Biota within the Yixian Formation and the overlying Jiufotang Formation of northeastern China. The new age results provide the best age constraint on the major radiation of the Jehol Biota and indicate that the Yixian Formation was deposited during the Early Cretaceous, over a time span of less than 7 Ma. In the second chapter, I present high-precision 40Ar/39Ar data for two tuffs from the lowest part of the Lanqi Formation near the main outcrop of floral fossils in Beipiao City, Liaoning, China. The robust age results indicate the whole Lanqi Formation was deposited in the Late Jurassic; consequently, the underlying Haifanggou Formation and Schmeissneria sinensis are at least Middle Jurassic in age, suggesting that the oldest angiosperm is at least 40 Ma older than previously thought. The age results for the basal Lanqi Formation indicate that the paleoenvironment in the northern margin of the North China Craton was dry and hot in the Late Jurassic. In the third and the fourth chapters, 40Ar/39Ar and U/Pb ages of the tuffs from the lower Dewey Lake Formation from western Texas provide the first robust evidence to place the Permo-Triassic boundary on land assuming that the boundary was globally synchronous. Furthermore, magnetostragraphy of samples from the lower Dewey Lake Formation in two sections from Caprock Canyons and Palo Duro Canyon State Park combined with previous magnetostratigraphic data indicate that magnetic polarity reversals were relatively frequent in the latest Permian. My new data combined with previous results from the Dewey Lake Formation provide the most accurate Permo-Triassic paleomagnetic pole yet determined for the North American continent, thereby enabling enhanced detection of discrepancies such as those arising from the translation or rotation of exotic tectonostratigraphic terranes.