291-3 BIOGEOGRAPHICAL RESPONSE OF LATE CRETACEOUS AMMONITES DURING OCEAN ANOXIC EVENT 3 SUBEVENTS

Session: Cephalopods Through Time: Insights into Evolution, Ecology, and Environmental Reconstruction

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Ammonites were globally expansive, are easily identifiable, and had volatile turnover rates, which make them excellent biostratigraphic markers. This study focuses on the paleobiogeographic response of three Late Cretaceous ammonite superfamilies during Ocean Anoxic Event 3 (OAE3). We grouped fossil occurrences into nine different regions to study changes in biodiversity, turnover rates, and geospatial distributions through three subevents that comprise OAE3. These subevents are the Coniacian Kingsdown event (OAE3a), the Santonian Horseshoe Bay event (OAE3b), and the Santonian/Campanian Boundary Event (SCBE; OAE3c). Fossil occurrences of three ammonoid superfamilies (Acanthoceratoidea, Hoplitoidea, and Scaphitoidea) were compiled from published data sets and the Paleobiology Database. Occurrences from the upper Turonian through lower Campanian were extracted. Ammonite occurrences were then manually screened to pinpoint their stratigraphic position as precisely as possible, creating a dataset with temporal resolution sufficient to track diversity through the three OAE3 subevents. After spatial standardization, diversity dynamics and spatial distributions were analyzed at both genus and species levels. Geographic ranges were quantified using minimum spanning tree lengths, and longitude and latitude ranges. Global genus diversity shows peaks during OAE3a and OAE3c with a significant drop during OAE3b. As predicted, genus level extinction rates increased during all three anoxic intervals for Acanthoceratoids and Scaphitoids, and during OAE3c for Hoplitoids. Origination rates are more variable across all three events in these three clades. Both genus- and species-level analyses show a similar decrease in geographic ranges after most OAE3 subevents. In Europe, latitudinal and longitudinal ranges shifted in concert, while in the Western Interior Seaway, there is a strong inverse relationship between latitudinal and longitudinal range shifts except prior to OAE3a and during OAE3b. Interestingly, we show a relatively increased sampled-in-bin richness in the upper Coniacian coinciding with a decrease in geospatial ranges in the different regions with sufficient data. This high-resolution dataset reveals diversity and geospatial patterns that normally would not be observable from biodiversity studies using coarser stratigraphic intervals. It also highlights the need for more sampling in the middle and lower upper Santonian.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10879

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