SEQUENTIAL SHIFTS IN FISH COMMUNITY STRUCTURE AND ECOLOGICAL ROLE FOLLOWING THE K/PG MASS EXTINCTION

The Cretaceous-Paleogene (K/Pg) mass extinction profoundly disrupted marine ecosystems. However, marine fishes thrived in the aftermath of the extinction event, rapidly diversifying in the early Paleogene open ocean. Here we use ichthyoliths – microfossil fish teeth – preserved in a deep-sea sediment core from the South Pacific Ocean to investigate the structure of open ocean fish communities from the Cretaceous to the middle Eocene (72-43 Ma), across this rapid diversification in the clade. Prior work using ichthyoliths has shown that there were two pulses of elevated origination rate of fish tooth morphotypes during the Paleocene, and each had different effects on tooth morphospace occupation. However, morphospace occupation does not consider the relative abundance or importance of a particular group within the community as a whole. Using statistical methods traditionally employed in ecological studies, we calculated fish community dissimilarity at 1 million year intervals, and found that the Cretaceous, and Eocene had distinct but stable fish community structures, which were linked by a v-shaped trajectory through NMDS-space during the Paleocene. Furthermore, the inflection point of this trajectory, occurred between the two pulses of elevated origination rate during the Paleocene. We then used both k-means and hierarchical clustering to group tooth morphotypes with similar shapes, creating clusters which were treated as equivalent through time, and repeated the dissimilarity analyses. The overall pattern of changing community structure remains consistent across a wide range of cluster numbers and across different clustering algorithms, suggesting that there were fundamental shifts in the composition of fish communities across the K/Pg, and during the Paleocene, and that different fish tooth morphotypes thrived during each interval. Together, these results shed light on the interaction between origination rate and community structure of fish following the K/Pg mass extinction, and suggest that there were several distinct shifts in the role that fish played within the Paleocene marine ecosystem, both individually, and as an ecological group into the Cenozoic.