Nineteen million years ago, sharks nearly disappeared from Earth’s oceans, according to a new study, which provides evidence of a previously unknown mass ocean extinction event. Sharks as a species never recovered, say the study’s authors; their diversity today is only a fraction of what it once was, the data suggests.

Much of what is known about ancient ocean ecosystems comes from rock and fossil records, which are usually limited to shallow water deposits and give only a small insight into the history of marine species at the ocean scale. Here, using a different dataset – small fossils in global cores of deep-water sediment – Elizabeth Sibert and Leah Rubin offer a new insight into changes in the abundance and diversity of one of the largest predators of the ocean.

Using microfossils in sediment cores called ichthyolites – scales and teeth lost by sharks and other bony fish that naturally accumulate on the seabed – Seibert and Rubin built a record of diversity and abundance sharks spanning nearly 40 million years.

According to the results, sharks virtually disappeared from records in the early Miocene around 19 million years ago, declining in abundance by over 90% and morphological diversity by over 70%. This puzzling extinction event appears to have occurred independently of any known global climate event or land mass extinction.

Although the drivers remain unknown, the authors suggest that this event fundamentally altered the ecology of pelagic predators and subsequently set the stage for the large lineages of migrating sharks that now dominate Earth’s oceans.

“Despite recent improvements in conservation actions, few countries impose restrictions that target oceanic sharks,” write Catalina Pimiento and Nicholas Pyenson from a related perspective.

According to Pimiento and Pyenson, the parallels between the extinction event of the early Miocene and the declines induced by human pressures today show a striking similarity. “Pelagic shark communities have 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.

The references:

“An early Miocene extinction in pelagic sharks” by Elizabeth C. Sibert and Leah D. Rubin, June 3, 2021, Science.
DOI: 10.1126 / science.aaz3549

“When Sharks Almost Extinct: A previously unidentified extinction event in the open ocean wiped out pelagic sharks” by Catalina Pimiento and Nicholas D. Pyenson, June 3, 2021, Science.
DOI: 10.1126 / science.abj2088