Evidence for pre-Cretaceous/Tertiary extirpation of marine reptiles and dinosaurs of the Moreno Formation, Fresno County, California

Detailed stratigraphic examination of the Maastrichtian-Danian aged Moreno Formation of central California indicates that marine reptiles and dinosaurs, including mosasaurs, plesiosaurs, and hadrosaurs, were locally extirpated before the Cretaceous/Tertiary (K/T) boundary event. The Moreno Formation represents continuous deposition across the Cretaceous/Tertiary boundary. The boundary is placed within a 2.4-meter thick interval approximately 6.7 meters above the contact between the Marca Shale Member and the overlying Dos Palos Member of the Moreno Formation.; Forty marine reptile and dinosaur localities were measured to determine stratigraphic positions relative to the Cretaceous/Tertiary boundary. The highest fossil locality is that of a mosasaur. The site is located in the Marca Member, 7.5 meters below the Marca/Dos Palos contact. Based on an average sedimentation rate of 89.5m/my, these organisms were dying out approximately 218,000 years before the K/T boundary.