Nineteen million years ago, 90% of sharks mysteriously became extinct and never regained their numbers

Sharks have been living in the Earth’s seas since before the dinosaur era, but over the past 20 million years, this ancient and diverse species has been dealt a serious blow, according to a new study. In fact, humans now coexist with only a fraction of the sharks that once existed on the planet.

Not only are many sharks threatened with extinction by the activities of a much younger species – our own – they also experienced a devastating event about 19 million years ago that reduced their numbers by a staggering 90 percent. This “previously unknown major shark extinction event” occurred in the early Miocene, a complex period of geologic history, and its underlying causes remain unexplained, according to a paper published Thursday in the journal Science.

“Sharks have been around for 400 million years of Earth history,” said lead author Elizabeth Siebert, a Hutchinson postdoctoral fellow at the Yale Institute for Biospheric Sciences at Yale University. “They survived almost all of the major mass extinctions. They survived the end of the Permian, which wiped out 95 percent of all species, they survived asteroid collisions, they survived global warming, global cooling and all sorts of other phenomena.”

“And yet this event that we didn’t know about wiped out 90 percent of them,” she added.

Siebert first discovered evidence of this cataclysmic extinction while studying ichthyolites – tiny fossils of fish scales and teeth – as a Harvard Fellowship Junior Fellow at Harvard University. Using trace fossils recovered from a core of deep-sea sediments in the South Pacific, she was able to reconstruct the population history of fish and sharks over the past 80 million years.

The sediments contained roughly equal amounts of both fish and shark scales until an asteroid hit Earth 66 million years ago, wiping out the dinosaurs as well as many sharks. As a result of this extinction, the ratio of fish to sharks dropped to 5:1, a figure that remained relatively stable until, at the beginning of the Miocene, shark tracks suddenly decreased tenfold.

Sibert discovered this strange disappearance of sharks with the help of Leah Rubin, who is a future doctoral student in the College of Environmental Science and Forestry at the State University of New York and was a student at the College of the Atlantic at the time of writing the new paper.

The pair examined 798 shark scales (or “denticles”) from the South Pacific as well as 465 denticles from the North Pacific and carefully divided them into 88 related morphological groups. Although all 88 groups existed before the Miocene extinction event, only eight of them were found in sediments on the other side of it. This finding showed that not only had the sharks declined in numbers, but the biodiversity of shark species had declined by 70 percent.

“It disrupted 45 million years of stability, and it happened in the blink of an eye,” Siebert said. “We don’t know how fast it happened. It happened faster than about 100,000 years, but we can’t say any more than that. It could have been a day, it could have been 1,000 years or 100,000 years.”

The mystery of the sudden decline in shark numbers is only compounded by the lack of obvious explanations for its possible cause. No major climatic changes are known during this period, and there is a clear gap in the sediments that obscures other theories about environmental disturbances that may have led to the shark population decline. As a result, the new study sheds light on the early Miocene, which should spur interdisciplinary research into a relatively understudied era.

“Like most studies, this first paper offers more questions than it can answer,” Rubin said.

While the causes of past extinctions remain unclear, humans are clearly the cause of modern shark extinctions. Our activities are killing tens of millions of sharks every year, a trend that is even more alarming in light of the fact that this iconic and important family of sharks to nature has never recovered from the rapid and mysterious decline in numbers in the Miocene.

It is crucial to understand what happened to sharks 19 million years ago to help with their conservation today: these events underscore that even animals with ancient evolutionary origins and reputations as predators can be permanently wiped out.

“I really hope this article inspires others to study this time period, because I think sharks are trying to tell us that something happened here,” Siebert concluded. “And I think we should listen to them.”

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