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.
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The ichthyolith accumulation data generated in this study, along with all age model and accumulation rate calculations, are available in Supplementary Tables 1–20, and on the Pangaea Database at https://doi.org/10.1594/PANGAEA.910379. We have also included an appendix with photographs of all tooth morphotypes identified in this study, and high-resolution digital images of each microfossil are available via Dryad at https://doi.org/10.5061/dryad.nk98sf7q5.
All code used in this study is available at https://github.com/esibert/EO_Fish/.
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The authors thank the International Ocean Discovery Program (IODP) for providing samples. This work was supported by the Scripps Institution of Oceanography Graduate Division, the Harvard Society of Fellows, the Digital Imaging Facility at the Museum of Comparative Zoology at Harvard University and a William F. Milton Grant to E. Sibert.
The authors declare no competing interests.
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Extended Data Fig. 1 A compilation of palaeoproductivity proxies (barium in brown and silica in blue-green), compared to the ichthyolith accumulation rates produced in this study.
All data were re-converted from the reported raw concentration values of the original studies to mass accumulation rates, based on GTS2012 age models50 and shipboard-reported dry bulk density. Barium and silica data from ODP Site 1217 was reported by the shipboard scientific party and downloaded from the JANUS Database18. Barium and silica data for DSDP Site 596 was reported in Zhou and Kyte (1992)16. Barium and silica Data for ODP Site 689 was reported in Faul and Delaney (2010)26. ODP Site 748 has no reported silica or barium data. Nearby ODP Site 744 silica data was reported by Salamy and Zachos (1999)21. Please see the supplemental data tables for additional details on the reported productivity proxy values.
Extended Data Fig. 2 A comparison of sediment mass accumulation rate (gray) with the raw concentration values for ichthyoliths per gram (black), barium concentration (parts per million) and opal abundance (% silica) for ODP Site 1217.
The Yellow box highlights a two-million year period of elevated sediment MAR that is associated with an increase in the concentration of ichthyoliths and barium but not silica.
Both sites are underestimating total morphotype diversity, as range-through diversity is not accounted for in this analysis, though it is clear that the two oldest samples (>40 Ma) in ODP Site 689 have lower rarefied diversity than the younger samples, despite having much higher total numbers of ichthyoliths. Further, DSDP Site 596 has overall higher rarefied diversity estimated at each sample size, showing that the South Pacific Gyre had a higher species richness overall than the Antarctic during this study interval. There is no systematic shift in total richness across the E/O at either site.
Extended Data Fig. 4 Capture-mark-recapture analysis output showing constant rates of origination (blue) and extinction (red) throughout the study interval for DSDP Site 596 and ODP Site 689.
Note that at both locations, origination slightly outpaces extinction, revealing an overall trend towards increasing species richness during the Late Eocene and Early Oligocene.
Extended Data Fig. 5 Various diversity metrics from DSDP Site 596 and ODP Site 689 showing little change in sampled tooth morphotype diversity across the E/O.
Note that calculation of rarefied richness allows for a maximum resample size of the smallest sample in the dataset, so the richness estimates for ODP Site 689 are lower than they may otherwise be due to the small sample size. Rarefaction curves for both sites are seen in Extended Data Fig. 3.
Type catalogue for all tooth morphotypes considered in this study.
All raw data used to construct the figures in the manuscript, including ichthyolith metrics generated in this study, as well as age models updated to GTS 2012, barium accumulation and silica accumulation. Each tab includes one supplementary table.
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Sibert, E.C., Zill, M.E., Frigyik, E.T. et al. No state change in pelagic fish production and biodiversity during the Eocene–Oligocene transition. Nat. Geosci. 13, 238–242 (2020). https://doi.org/10.1038/s41561-020-0540-2