DECLINE AND FALL? EXAMINING LATE CRETACEOUS AMMONOID DIVERSITY PATTERNS AND THEIR DRIVERS USING A BAYESIAN FRAMEWORK

Ammonoid cephalopod molluscs were integral components of marine ecosystems for over 300 million years. Their evolutionary success is often considered to have waned through the Late Cretaceous prior to their final extinction at the end of the Mesozoic. The pattern of this hypothesised decline and its drivers (abiotic or biotic) are unresolved, and require disentanglement from the pervasive spatial and geographical sampling biases that affect the fossil record. We compiled data from the published literature, new and unpublished museum collections, and the Paleobiology Database (PBDB), to generate a large database (>20,000 occurrences) of Late Cretaceous (Cenomanian – Turonian) ammonoids, employing statistical methods to detect and resolve stratigraphically and taxonomically spurious occurrences. We divide our database into spatially-standardised subsamples, eliminating geographic sampling bias and providing a regionalised perspective on the apparent global decline of ammonoids. We then infer regionalised diversification dynamics in a Bayesian framework that corrects for uneven sampling through time (PyRate). We uncover a mosaic of ammonoid diversity patterns, demonstrating that a global trend of ammonoid decline is not regionally pervasive. Preliminary analyses suggest that spatially varied palaeogeographic and environmental drivers are responsible for this mosaic, highlighting strong biogeographic nuances in the history of this iconic and well-sampled fossil group prior to the end-Cretaceous (K-Pg) mass extinction.