No state change in pelagic fish production and biodiversity during the Eocene–Oligocene transition

The Eocene/Oligocene (E/O) boundary (~33.9 million years ago) has been described as a state change in the Earth system marked by the permanent glaciation of Antarctica and a proposed increase in oceanic productivity. Here we quantified the response of fish production and biodiversity to this event using microfossil fish teeth (ichthyoliths) in seven deep-sea sediment cores from around the world. Ichthyolith accumulation rate (a proxy for fish biomass production) shows no synchronous trends across the E/O. Ichthyolith accumulation in the Southern Ocean and Pacific gyre sites is an order of magnitude lower than that in the equatorial and Atlantic sites, demonstrating that the Southern Ocean was not a highly productive ecosystem for fish before or after the E/O. Further, tooth morphotype diversity and assemblage composition remained stable across the interval, indicating little change in the biodiversity or ecological role of open-ocean fish. While the E/O boundary was a major global climate-change event, its impact on pelagic fish was relatively muted. Our results support recent findings of whale and krill diversification suggesting that the pelagic ecosystem restructuring commonly attributed to the E/O transition probably occurred much later, in the late Oligocene or Miocene.