SPECIES DISTRIBUTION MODELING OF WESTERN INTERIOR AMMONOIDS DURING OCEAN ANOXIC EVENT 2

The Cretaceous Western Interior Seaway (WIS) of North America provides one of the best-documented records of Ocean Anoxic Event 2 (OAE2), a critical interval in Earth history. Late Cenomanian perturbations to the carbon cycle led to warming, sea level rise, and the spread of anoxic conditions into shallow marine basins. These changes led to widespread extinction of marine organisms, including ammonoid cephalopods. While ammonoids were common within the WIS, the habitat requirements of the various species are poorly known, limiting our ability to interpret extinction drivers during OAE2. Species distribution modeling (SDM) provides a powerful quantitative tool for relating the geographic distribution of species to environmental parameters. SDM was used here to explore environmental controls on ammonoid species distributions within the WIS and how suitable habitat changed through OAE2 in the Late Cenomanian. Occurrences within the WIS for 13 common ammonoid species, including nine acanthoceratids and four heteromorphs, were included in the dataset. Spatial models for five environmental parameters, including percents of clay, silt, and carbonates, benthic oxygenation, and depositional environment (inner shelf to basin), were compiled for three ammonoid biozones representing the time intervals just before OAE2, onset of OAE2, and peak OAE2. The maximum entropy algorithm Maxent was used to create species distribution models for each ammonoid species. Generally, different environmental parameters were important for different ammonoid species, including congeners, suggesting that WIS ammonoids varied in their ecologies. During the initial onset of OAE2, oxygenation was the most important environmental parameter for all ammonoid species, regardless of morphology or inferred ecology. At peak OAE2, shallow areas with high silt and clay content showed the highest suitability for ammonoid species. Habitat suitability for the southwestern margin of the WIS remained high for most ammonoid species during all three time intervals studied, supporting previous work identifying this region as a possible refugium during OAE2.