Nineteen million years ago, sharks nearly disappeared from Earth’s oceans, according to a new study, which provides evidence for a previously unknown mass ocean extinction event. Sharks as a species never recovered from this, the study’s authors say; their diversity today represents only a fraction of what it once was, the data suggest.
Much of what is known about ancient ocean ecosystems is derived from rock and fossil records, which are generally limited to shallow-water deposits and provide only a small glimpse into the ocean-wide history of marine species. Here, using a different dataset — small fossils in global deep-sea sediment cores — Elizabeth Sibert and Leah Rubin provide a new view into changes in the abundance and diversity of one of the ocean’s greatest predators.
Using microfossils in the sediment cores called ichthyoliths — scales and teeth shed from sharks and other bony fishes that naturally accumulate on the seafloor — Seibert and Rubin constructed a record of shark diversity and abundance spanning nearly the last 40 million years.
According to the findings, sharks all but vanished from the record during the early Miocene roughly 19 million years ago, declining in abundance by more than 90% and in morphological diversity by more than 70%. This puzzling extinction event appears to have occurred independently of any known global climate event or terrestrial mass extinction.
While the drivers remain unknown, the authors suggest that this event fundamentally altered pelagic predator ecology and subsequently set the stage for the large, migratory shark lineages that now dominate Earth’s oceans.
“Despite recent improvements in conservation actions, few countries impose restrictions that target oceanic sharks,” writes Catalina Pimiento and Nicholas Pyenson in a related Perspective.
According to Pimiento and Pyenson, the parallels between the early Miocene extinction event and the declines driven by human pressures today bear a striking similarity. “Pelagic shark communities never recovered from a mysterious extinction event 19 million years ago; the ecological fate of what remains is now in our hands,” they write.
For more on this research, read Massive Die-Off Discovered by Accident: A Shark Mystery Millions of Years in the Making.
“An early Miocene extinction in pelagic sharks” by Elizabeth C. Sibert and Leah D. Rubin, 3 June 2021, Science.
“When sharks nearly disappeared: A previously unidentified extinction event in the open ocean decimated pelagic sharks” by Catalina Pimiento and Nicholas D. Pyenson, 3 June 2021, Science.