THE CRETACEOUS-PALEOGENE BOUNDARY: BEGINNING THE AGE OF BONY FISHES

While fishes have a long evolutionary history, the Actinopterygians (ray-finned bony fish) that dominate the modern ocean were a relatively small, insignificant group until the Late Cretaceous and early Paleogene. However, the exact timing and mechanism that drove their diversification and subsequent dominance in the marine realm is poorly constrained. Here we use a relatively untapped fossil resource, microfossil teeth and shark dermal scales (ichthyoliths) preserved in deep sea sediments, to study the changes in fish production and community structure continuously across the Cretaceous-Paleogene (K/Pg) mass extinction. We find that while other groups were decimated by the K/Pg extinction, fishes – as represented by accumulation rate of ichthyoliths – thrived. The K/Pg event marks a turning point in the ichthyolith community: where the Cretaceous open ocean is shark-dominated, the Paleogene pelagic ecosystem is ruled by bony fishes. While shark abundance does not change significantly across the boundary, the accumulation of bony fishes increased in the Pacific Ocean, in some cases to 2-5 times Cretaceous levels. Additionally, while Cretaceous samples have a small range of bony fish tooth sizes, with the majority <1 mm in length, Paleocene assemblages have a much wider range of sizes, and a sustained high relative abundance of large teeth (>1 mm) for 8 million years post-extinction. We interpret this as a shift to a more complex bony fish community with increased numbers of large predatory fishes in the Paleocene. The increase in Paleocene tooth size range and both relative and absolute abundance suggests that bony fish were able to take advantage of newly opened ecological niche space vacated by victims of the mass extinction. We conclude that the K/Pg extinction played an important role in restructuring pelagic marine ecosystems to favor bony fish evolution and ecological expansion, and indeed may have been the harbinger of the modern “Age of Bony Fishes”.