33-7 Fósiles Viejos y Nuevos Amigos - A Collaborative Investigation Into Fossil Invertebrates of the Rosario Formation of Baja California

Session: Borderless Geoscience: Linking Baja and Southern California

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The Rosario Formation of Southern California and Baja California represents one of the most continuous and accessible Late Cretaceous marine successions along the Pacific margin. Composed of fossiliferous shales, siltstones, and volcaniclastics deposited in a forearc setting, this study focuses on the San Antonio del Mar (SADM) locality, a well-exposed Campanian—Maastrichtian section approximately 600 m thick. More than 56 molluscan taxa have been documented, including numerous unpublished records, making SADM one of the most fossil-rich sections of the Rosario Formation.

Integrated stratigraphic work conducted over multiple field seasons, combined with historic museum collections, provides a refined framework for interpreting biodiversity and paleoenvironmental change. Cephalopods play a key bio stratigraphic role: Nostoceras hornbyense defines the Upper Campanian interval, followed by a short overlap with Baculites occidentalis, while basal Maastrichtian marine mudstones yield Pachydiscus neubergicus, the global index fossil for the stage boundary. Inoceramid bivalves (Endocostea, Platyceramus, and Mytiloides-like forms), along with diverse bivalves, gastropods, and shark teeth, further support paleoecological and paleotemperature reconstructions, aided by exceptional aragonitic preservation.

This project is the first of a series that will seek to explore sections of the Rosario Formation of Southern California and Baja California using this integrative approach, with the goal to ultimately provide a synthesis of known biodiversity from the unit. It highlights the value of binational collaboration between the Universidad Autónoma de Baja California Sur and the Natural History Museum of Los Angeles County, strengthening paleontological infrastructure, research capacity, and student training while advancing our understanding of the Late Cretaceous evolution of the Pacific margin.

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