EVIDENCE FOR THE PRE-K/T BOUNDARY EXTINCTION OF CRETACEOUS MARINE REPTILES AND DINOSAURS, MORENO FORMATION, CALIFORNIA

Detailed stratigraphic examination of the upper Maastrichtian Moreno Formation of central California indicates that marine reptiles and dinosaurs, including mosasaurs, pleisiosaurs, and hadrosaurs, were dying out before the Cretaceous/Tertiary terminal event. The Moreno Formation represents continuous deposition across the Cretaceous/Tertiary boundary, well documented by several microfauna studies (e.g. McGuire, 1988). Based on these studies, 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. In the study area it is not evidenced by a visible layer, thus the boundary is estimated based on previous microfaunal studies.

Utilizing the stratigraphic locations of 36 marine reptile and dinosaur specimens, each locality was measured stratigraphically with respect to the underlying or overlying member. These data were then compiled in a stratigraphic column. The highest fossil locality found in the Moreno formation is that of a hadrosaur. The site is located in the Marca member, 45 meters above the Tierra Loma/Marca contact, 104 meters below the Marca/Dos Palos contact. The data indicate that these organisms were dying out locally before the Cretaceous-Tertiary terminal event.