Ichthyoliths and Pelagic Ecosystem Evolution:

How has the open ocean ecosystem changed through time?

I am broadly interested in how a functional (open ocean) marine ecosystem can exist, has existed, and has changed over geologic time, in conjunction with major changes in climate, biotic turnover, and earth system shifts. The fossil record provides a rich historical record that can be examined, in conjunction with climatic proxies and other tools (eg. phylogenies), to look at how these changes interact with evolution and ecosystem function. I am also interested in the evolution of fish, and the evolution of ecological roles played by fish and other marine megafauna within the pelagic ecosystem. My main research focus is on ichthyoliths, isolated fossil fish teeth and shark dermal scales, preserved in deep-sea sediment cores, to study how pelagic vertebrate consumers, and marine ecosystem function, has responded to these changes.

I use ichthyoliths to investigate three main aspects of pelagic ecosystem evolution throughout Earth’s history, to assess changes in fish (1) production, (2) community structure, and (3) evolutionary dynamics. Interpreted within their paleoenvironmental and paleoclimatic context, ichthyoliths can reveal patterns in the coevolution of life and climate in the ocean, during periods of global change and relative stasis alike.

Dissertation Abstract

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

Committee: Richard Norris (Chair), Lin Chao, Peter Franks, Philip Hastings, Lisa Levin

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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