Sharks swimming
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Sharks have survived in Earth's seas since before the age of dinosaurs, but this ancient and diverse lineage has suffered serious blows over the past 20 million years, a new study has found. In fact, humans are right now coexisting with only a fraction of the sharks that used to exist on the planet.

Not only are many sharks at risk of extinction due to the activities of a far younger species — our own — they also experienced a devastating evolutionary bottleneck some 19 million years ago that reduced their numbers by a staggering 90 percent. This "previously unknown major extinction event in sharks" occurred in the early Miocene, a murky period in geological history, and its root causes remain unexplained, according to a paper published on Thursday in Science.

"Sharks have been around for 400 million years of Earth's history," said lead author Elizabeth Sibert, a Hutchinson Postdoctoral Fellow at the Yale Institute for Biospheric Sciences at Yale University, in a call. "They've weathered almost all of the big mass extinctions. They made it through the end-Permian that wiped out 95 percent of all species, they made it through the asteroid impacts, they've made it through global warming, global cooling, and all sorts of things."

"And yet here, this event that we didn't know about wiped out 90 percent of them," she added.

Sibert first discovered evidence of this cataclysmic die-off while studying ichthyoliths, tiny fossils of fish scales and teeth, as a Junior Fellow in the Harvard Society of Fellows at Harvard University. Using trace fossils extracted from a deep-sea sediment core from the South Pacific Ocean, she was able to reconstruct a record of fish and shark abundances that covered the past 80 million years.

The sediments contained a roughly equal number of scales from both fish and sharks until an asteroid hit Earth 66 million years ago, killing off the dinosaurs as well as many sharks. This extinction event reduced the fish-shark ratio to 5:1, a figure that remained relatively stable until, suddenly, in the early Miocene, traces of sharks suddenly decreased tenfold.

Sibert zeroed in on this bizarre vanishing of sharks with the help of Leah Rubin, who is an incoming doctoral student at the State University of New York College of Environmental Science and Forestry, and was an undergraduate student at the College of the Atlantic when the new paper was written.

The pair combed through 798 shark scales (or "denticles") from the South Pacific site, along with 465 denticles from a location in the North Pacific, and meticulously categorized them into 88 related morphological groups. While all 88 groups were present before the Miocene extinction event, only eight of them were detected in sediments on the other side of it. This finding revealed that the sharks were not only decimated in sheer numbers, but that they also experienced a 70 percent drop in the biodiversity of shark species.

"This disrupted 45 million years of stability, and it happened in the blink of a geologic eye," Sibert said. "We don't know quite how fast it happened. It happened faster than about 100,000 years, but other than that, we can't say. It could have been a day, it could have been 1,000 years, or it could have been 100,000 years."

The mystery of the sudden shark decline is only deepened by the lack of obvious explanations for its potential cause. No major climatic changes are known from this period and there's an apparent gap in the sedimentary record that obscures other leads about environmental disruptions that might have decimated sharks. As a result, the new study has shone a spotlight on the early Miocene that should spark interdisciplinary research into the relatively under-studied era.

"Like most research endeavors, this first paper offers more questions than it can answer and we plan on investigating the breadth of data denticles offer through a varied set of lenses, from hydrodynamics to ecology," Rubin said in an emailed statement.

While the drivers of this past extinction event remain uncertain, humans are very clearly the cause of the modern die-off of sharks. Tens of millions of sharks are killed every year as a result of our activity, a trend that is even more worrying in light of the fact that this iconic and important family has not recovered since its rapid and mysterious Miocene decline.

It's crucial to understand what happened to sharks 19 million years ago to help inform their conservation today: both periods underscore that even animals with ancient evolutionary origins and a reputation for predatory prowess can be edged out of existence forever.

"I'm really hopeful that this paper will inspire some other folks to be excited about this time period, because I think the sharks are trying to tell us that something happened here," Sibert concluded. "And I think we've got to listen to them."