CHANGING COMMUNITY DYNAMICS ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY IN SEYMOUR ISLAND, ANTARCTICA

The Cretaceous-Paleogene mass extinction (K-PG) is one of the most widely studied crises of the Phanerozoic, with most work focusing on the impact on specific taxonomic groups. A more complete understanding of the ecological implications of the K-PG and the subsequent reorganization in its aftermath requires a community-wide approach. Here, we lay the groundwork to carry out such an approach by compiling a dataset of 7 Late Cretaceous (Maastrichtian) and 3 Early Paleocene (Danian) marine paleocommunities spanning the K-PG boundary from the Lopez de Bertodano Formation of Seymour Island, Antarctica. Species richness declined from 344 to 155 species, while functional guild richness declined from 57 to 29 across the K-PG boundary. Nektonic predator guilds were heavily affected, including those containing sharks, fish, marine reptiles, and ammonites. Danian communities showed a slight rebound of species richness (210 species), but a further decrease of the number of guilds (22).

Consistent with a hypothesized environmental perturbation during the K-PG, each community was tested for its dynamic response to primary productivity collapse, which allowed us to identify a dramatic shift in the ecological dynamics of communities coincident with the K-PG. 1000 species level food webs were simulated for each community and the fraction of species that became secondarily extinct as a result of increasing levels of primary productivity collapse was estimated using the Cascading Extinction on Graphs model. Pre-extinction communities exposed to low levels of perturbation (< 70% of productivity) yielded uniformly low levels of secondary extinction (< 10%). Conversely, post-extinction communities behaved less uniformly, with greater variance of secondary extinction at all levels of perturbation. These results suggest that under normal conditions of environmental disturbance the pre-extinction communities would have been significantly less susceptible to secondary extinction than the post-extinction communities. Additionally, pre-extinction communities yielded high levels of secondary extinction relative to post-extinction communities at high levels of perturbation, suggesting that the extinction may have conditioned communities that were more capable of enduring mass extinction scale perturbations.