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 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 which suggest that the pelagic ecosystem restructuring commonly attributed to the E/O transition likely occurred much later, in the late Oligocene or Miocene.

始新世-渐新世（Eocene-Oligocene，简称E/O）界线距今约3390万年，曾被描述为地球系统的一次状态转变，其标志性特征为南极大陆永久性冰川形成，以及学界提出的海洋生产力提升。本研究利用全球7个深海沉积岩芯中的鱼类微体化石牙齿（鱼石，ichthyoliths），定量分析了鱼类生产力与生物多样性对该地质事件的响应。鱼石堆积速率（作为鱼类生物量生产力的替代指标）在E/O界线前后并未呈现同步变化趋势。南大洋与太平洋环流区站位的鱼石堆积量较赤道与大西洋站位低一个数量级，这表明无论E/O界线前后，南大洋都并非鱼类高生产力生态系统。此外，该时段内牙齿形态型多样性与组合组成始终保持稳定，表明远洋鱼类的生物多样性及其生态功能并未发生显著变化。尽管E/O界线是一次重大的全球气候变化事件，但其对远洋鱼类的影响相对微弱。本研究结果佐证了近期关于鲸类与磷虾多样性演化的研究结论，即通常被归因于E/O界线转变的远洋生态系统重构，实际上很可能发生在更晚的渐新世晚期或中新世时期。