160-6 Stable Diversity amidst Population Collapse in Pelagic Fish Communities across Cretaceous Ocean Anoxic Event 2

Abstract:

Cretaceous black shales chronicle episodic ocean deoxygenation in the extreme greenhouse world of the Mesozoic. The largest of these Ocean Anoxic Events, OAE2 (Cenomanian-Turonian boundary event, ~94 Ma), dramatically reorganized marine ecosystems, driving turnover and notable extinctions in communities ranging from microscopic plankton to large predatory reptiles. Yet, the evolutionary dynamics of middle-trophic communities like fishes—which critically mediate energy transfer between such producers and predators— remain uncertain.

This enigmatic history stems largely from a sparse whole-body fossil record for fishes, despite their ecological dominance in modern open-ocean ecosystems. Recent works, however, highlights how certain disarticulated fish microfossils (“ichthyoliths”) like fish teeth offer insight into the evolutionary response of fish communities writ large to extreme marine perturbations. Because they are highly abundant and taphonomically resistant in sediments, ichthyoliths can provide a uniquely high-resolution, community-scale view of pelagic ecosystem evolution. 

Here, we present a ~3.5-million-year record of late-Cenomanian/early-Turonian ichthyoliths from the proto-North Atlantic (IODP Site U1407) to reconstruct changes in pelagic fish abundance, diversity, and turnover across OAE2. Ichthyholith accumulation rates, interpreted as a first-order proxy for fish abundance, abruptly decreased near the onset of anoxia and remained suppressed for at least ~2 million years after the recovery of oxic conditions. Nonetheless, despite this near five-fold decline in production, the morphological diversity (richness and evenness) of post-OAE2 teeth remained nearly identical to pre-OAE2 assemblages, with both intervals sharing similar “core” communities. Thus, the ichthyolith record of the proto-North Atlantic demonstrates a remarkably stable fish community in spite of its population crash in OAE2, revealing resilience in the function and architecture of pelagic ecosystems amid severe oceanographic change. Ongoing work aims to verify the spatial and temporal extent of these trends, and to constrain the cause(s) of the apparent permanency of reduced populations in spite of the biogeochemical recovery of Cretaceous oceans.

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