Nearly all sharks disappeared in a 100,000-year period about 19 million years ago, research shows.
The total number of sharks declined by 90%, and the number of shark species decreased by 70%.
Scientists don't know why the mass extinction happened - it didn't coincide with changes in climate.
Sharks are the consummate survivors. They've been around for more than 400 million years, surviving all five of the major mass extinctions in Earth's history.
But the animals almost disappeared completely 19 million years ago, according to a study published this week in the journal Science. During a span of less than 100,000 years, the total number of sharks in the world's oceans decreased by 90%. The number of shark species, meanwhile, dropped by at least 70%.
The finding comes from an examination of shark fossils from two sites deep in the Pacific Ocean - researchers analyzed changes in the variety and number of shark fossils there over time. They concluded that this previously unknown die-off hit shark species harder than the mass extinction that wiped out the dinosaurs and 75% of all life on Earth 66 million years ago.
"This event, for sharks, was a much bigger deal than the end-Cretaceous event that killed the dinosaurs," Elizabeth Sibert, an ecologist at Yale University who co-authored the new study, told Insider. "It was twice as extreme."
According to the study, sharks never recovered.
"Sharks don't come back and magically re-diversify," Sibert said.
Had this extinction not occurred, she added, "we'd see a whole lot more different sharks and relatives of sharks in the open ocean today."
Scientists used scales the size of a human hair to reconstruct the past
Marine fossils from the early Miocene - the geologic epoch that occurred between 23 million and 16 million years ago - are rare. That could explain why scientists didn't know about this extinction before.
Sibert's research involved examining microscopic fish teeth and shark scales (called denticles) preserved in sediment cores collected in the Pacific Ocean. Denticles, which are no wider than a human hair, ensconce sharks in an armor-like coating of scales that slough off regularly.
Deep sea drillers collected the cores at two sites - one in the north Pacific, the other in the south Pacific - up to 3 miles under the waves. The cores, cylinders of earth and sand, act like a geologic hour glass: Fossils found in deeper layers are older.
The layers revealed a striking tale. Up until 19 million years ago, there was about one shark fossil for every five fish fossils preserved, suggesting sharks were about 20% as plentiful as fish. But after that, the ratio changed dramatically. Sibert could only find one shark denticle for every 100 fish teeth.
Given that the cores came from sites 2,000 miles apart yet told the same story, the devastation likely hit sharks everywhere, according to a letter accompanying the study. The shark extinction was a "global event," it says.
Sibert said she thinks the extinction event "could've taken one day, 100 years, or 1,000 years, but probably less than 100,000 years."
Some shark species got hit harder
In order to determine which sharks, and how many species, had disappeared, Sibert enlisted the help of Leah Rubin, an undergraduate at the College of the Atlantic in Maine at the time.
"My focus was really on teasing out the 'who' of this decline," Rubin, who co-authored the paper, told Insider.
Sibert and Rubin categorized more than 1,200 denticles into 88 groups of species - the more groups, the greater the total number of shark species. But after that mysterious transition point 19 million years ago, less than 10 of those groups remained in the fossil record. That suggests shark diversity plummeted by between 70% and 90%.
Further analysis of the preserved denticles offered insight into which types of shark species survived while others perished.
Denticles are not all the same. Sharks that migrate long distances along coastlines, like great whites, have denticles with parallel ridges that help reduce drag as the animals glide through water. Sharks like cookie cutters - which stick to the same patches of open ocean, preferring to ambush their prey - have geometric denticles with ridges that resemble a Jackson Pollock painting.
Sibert and Rubin found that fewer species with these geometric denticles made it through. That could explain why only 53 open-ocean shark species are alive today, compared to hundreds of coastal shark species.
This mass extinction likely affected more than just sharks
Rubin said "the million-dollar question" left unanswered in this research is why sharks nearly disappeared.
Most mass extinctions are caused by climate change, asteroid impacts, volcanic eruptions, or a combination thereof. But none of those events happened in the early Miocene, and there wasn't a corresponding extinction on land during that time.
Sometimes, species can also vanish if another predator that relies on the same prey arrives on scene. But according to Sibert, migratory whales and tuna - which would have competed with sharks for the same resources - didn't show up until at least 2 million years after the big die-off.
"How is it that sharks that ruled the oceans for many millions of year could go extinct so thoroughly?" Sibert said.
She's working on follow-up studies that aim to identify other marine species hit by this same mass extinction.
"The more we look, the more we find it wasn't isolated to sharks and it was not isolated to open oceans," Sibert said.
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