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Ichthyoliths as a paleoceanographic and paleoecological proxy and the response of open-ocean fish to Cretaceous and Cenozoic global change


Ichthyoliths, isolated fossil fish teeth and shark dermal scales preserved in deep-sea sediment cores, can reveal how marine vertebrate consumers (sharks and fish) have responded to major global change events in Earth’s history. In this dissertation, I first develop methods for the isolation and curation of ichthyoliths from a variety of marine sediment types. I then use ichthyoliths to assess how (1) total fish production, (2) pelagic fish community structure, and (3) fish evolution have responded to select global change events in Earth’s history.

The Cretaceous/Paleogene (K/Pg) Mass Extinction 66 million years ago (Ma) catalyzed the diversification of fish in the open ocean. Cretaceous oceans (>66 Ma) were relatively devoid of fish teeth, and at the K/Pg, fish abundance declined only in the Atlantic Ocean, while in the Pacific, fish abundance stayed constant or increased immediately following the extinction. Yet the event caused a global shift in the marine vertebrate community, with the relative abundance of teeth increasing compared to that of denticles in marine sediments. Further, the size structure of the fish tooth assemblages shifted towards larger, rather than smaller individuals, suggesting that the group was resilient to the extinction event. Bony fishes rose to ecological dominance in the open ocean following the K/Pg extinction, rapidly radiating in morphological diversity after the extinction, while other open ocean groups lagged behind. Extreme global warmth in the Early Eocene (~52-48 Ma) is associated with an increase in fish and shark abundance, but not diversity. Fish abundance broadly follows global temperature gradients in the Paleogene (66-20 Ma), with the highest abundance of fish in the warmest part of the Cenozoic. The most recent 20 million years is characterized by highly variable ichthyolith production and low abundances of sharks and other elasmobranchs in the gyres. This shift is temporally correlated with the diversification of open-ocean whales and seabirds, groups which may have out-competed sharks for fish prey in the modern open ocean. Together, these results show that that fishes were consistently able to adapt to Cenozoic global change, both ecologically and evolutionarily, allowing the Cenozoic to truly become an “Age of Fishes”.

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