EVIDENCE FOR DISASTER FAUNA, TWO PHASES OF DIVERSIFICATION OF PELAGIC FISH IN THE AFTERMATH OF THE K/PG MASS EXTINCTION

Molecular and fossil evidence suggest that a major radiation of pelagic fishes occurred around the K/Pg boundary, but the timing and dynamics of this radiation are poorly constrained. The microfossil record of fish teeth in deep-sea sediment cores preserves a high temporal resolution record of fish through time. We defined 136 unique fish tooth morphotypes present in the South Pacific Ocean (DSDP Site 596) between 73 and 42 million years ago (Ma) resolved to ~700kyr spacing, using morphological characters describing external and internal tooth morphology. Calculations of morphospace occupation and origination/extinction rates show that there is little variation in extinction rate, with only 2 of the 48 Cretaceous morphotypes making last appearances at the K/Pg. In contrast, the Paleocene is marked by two pulses of morphotype origination. The first pulse produced many new, short-lived, morphologically extreme morphotypes that represent a “disaster-fauna” restricted to the early Paleocene (66-60 Ma). The second pulse of origination occurred from 60-56 Ma and produced tooth morphotype groups which persisted through our record to at least the middle Eocene. Both pulses of origination occur prior to a significant increase in ichthyolith abundance in the early Eocene, suggesting that changes in morphotype diversity are not due to sampling intensity. Indeed, while ichthyolith abundance peaks in the Early Eocene at nearly 10x Paleocene levels, there is almost no origination during that interval.

The pulses of Paleocene innovation in fish has parallels with radiations in other fossil groups in which there is the appearance of an early group of ‘founders’ during the immediate phase of ecosystem recovery from the extinction, and a later pulse of origination as Paleogene faunas are established. The main oceanic fish radiation occurred quickly after the K/Pg, possibly due to the low levels of extinction in fishes across the K/Pg, with new morphotypes augmenting and replacing Late Cretaceous survivors over the course of the early Paleogene, changing the evolutionary trajectory of the open ocean fish community, and allowing fishes to take on more, novel roles within the open ocean ecosystem.