Fin-credible

Scientists discover biggest mass extinction since the dinosaurs

“They never recovered.”

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Thanks to the movie Jaws, we’re well aware of sharks every time we splash and surf in the shallows.

But despite any fear of attack, you’re far more likely to die from an errant firework or a lightning strike than a shark bite. The likelihood of encountering a shark is low — and their numbers are dwindling.

In ancient times, however, you’d be smart to avoid diving in. According to new research, the ocean was home to massive, thriving shark populations approximately 19 million years ago.

That was until they were decimated by a previously unknown extinction event.

“There were somewhere between 10 and 50 times more sharks present in the open ocean before this extinction, and they were much more diverse — there were many many more species,” Elizabeth Sibert tells Inverse.

Sibert is a Hutchinson Postdoctoral Fellow at the Yale Institute for Biospheric Sciences at Yale University and one of the co-authors of the new study. These findings were published Thursday in the journal Science.

What’s new — Sibert and co-author Leah Rubin — an incoming doctoral student at the State University of New York College of Environmental Science and Forestry — found one of the greatest known extinctions on Earth since dinosaurs disappeared during the Cretaceous period, about 65 million years ago.

“It is fair to say that, from a shark’s perspective anyway, that this extinction was bigger than the Cretaceous extinction event,” Sibert says.

The newly uncovered extinction event occurred roughly 19 million years ago during the early Miocene period, leading to the decimation of 90 percent of the ocean’s overall shark population. However, what caused the sharks to disappear isn’t exactly known.

“There were somewhere between 10 and 50 times more sharks ...”

The event was brief — in the grand scheme of Earth — lasting fewer than 100,000 years and leading to the extinction of 70 percent of shark species. The researchers write these sharks were “never recovered.”

“We don’t know exactly what all these sharks were doing,” Sibert says.

“They’re extinct now, and we are just learning about their existence, but it’s a safe guess to say there were many times more species present, and that means there was more ecological diversity as well.”

A figure from the study compares two types of shark scales: linear and geometric. Sibert and Rubin

How they did it — Previously, scientists had a hard time understanding the evolution of vertebrates that lived in the open ocean, also known as pelagic animals.

That’s because sediment records from the ocean floor are often used to analyze ancient marine species. It’s hard to come by sediment records from the deep ocean, where depths can reach 35,000 feet, as in the Mariana Trench.

But Sibert and Rubin pioneered a technique allowing them to extract fossilized shark skin — known as denticles — from the ocean floor. They extracted the skin using deep-sea drills, also known as sediment cores.

“Dermal denticles offer an incredible window into the past of these ancient and elusive marine predators and thus the state of ocean ecosystems through time,” Rubin says.

The study is unusual in the world of shark literature, since it focuses primarily on fossilized skin — shark scales — instead of the animal’s more terrifying teeth, according to Victor J. Perez, an assistant curator of paleontology at the Calvert Marine Museum. Perez is not affiliated with the study.

“Trends in fossil shark diversity are typically defined by dental records [aka teeth]. This study is unique in that it focuses on an open-ocean environment and utilizes dermal denticles to evaluate diversity trends,” Perez tells Inverse.

The researchers, in turn, discovered that ancient pre-extinction sharks possessing unique “geometric” skin patterns or denticles — characterized by their prominent, interlocking ridges — suffered more than sharks with “linear” or straight skin during the extinction.

“There were more geometric denticles before the extinction than after, and the geometric denticle sharks were hit harder than linear denticle sharks — but that doesn’t mean linear denticle sharks were fine,” Sibert says.

Understanding this ancient extinction event could help us save sharks in the present day, researchers argue. Getty

Why it matters — These findings aren’t just a history lesson, the researchers argue. There are two major reasons why you should care about this ancient shark extinction event, which the researchers call a “tipping point” in the evolution of ocean life.

First: These findings can help us understand the evolution of sharks and why their disappearance fundamentally altered the ecosystem of open oceans.

“Before this early Miocene extinction, sharks played a much larger role in the open-ocean ecosystem than they do today,” the study team writes.

This extinction mainly decimated the number of sharks living in the deep sea — and their extinction paved the way for other predators, such as certain types of whales, to rule the open ocean. Today, many sharks spend less time in open oceans and more time in shallower waters — putting them into closer contact with humans.

Second: Sharks are still disappearing — but this time, we know why it’s happening. As it turns out, humans pose a far bigger threat to sharks than vice versa, killing off an alarming number of sharks through overfishing and pollution. The researchers note that the global shark population declined by more than 70 percent in the past 50 years.

“Before this early Miocene extinction, sharks played a much larger role in the open-ocean ecosystem than they do today.”

Perez believes we can learn from this ancient extinction event to possibly save sharks in the present.

“We cannot truly comprehend the magnitude of present-day extinctions without understanding diversity trends prior to human involvement,” Perez says, adding “This study may offer a baseline for comparison with present-day extinctions in open-ocean environments.”

What’s next — We know ancient sharks disappeared in record numbers — but we don’t know how or why they went extinct, according to the researchers.

“Like most research endeavors, this first paper offers more questions than it can answer,” Rubin says.

The researchers can rule out some options. Climate change and threats from larger predators seem unlikely, given the timing of the extinction event.

“The discovery of a major extinction event not correlated with global climate is highly unusual and certain to lead to new research questions,” Perez says.

And although the population size of certain open-ocean predators, such as baleen whales, changed a few million years after the extinction, they were also not a cause of the ancient sharks’ demise. Instead, the emergence of these other, more dominant ocean predators may help explain why shark populations did not rebound.

“While it is unlikely that these groups are what caused the sharks to go extinct in the first place, it is possible that they were better able to evolve quickly and re-diversify in ways the sharks were not,” Sibert adds.

Rubin agrees and hopes that further research will yield more insight into the mysterious disappearance of the Miocene-era sharks — a disappearance that echoes the shark’s present decline.

Abstract: Shark populations have been decimated in recent decades because of overfishing and other anthropogenicstressors; however, the long-term impacts of such changes in marine predator abundance and diversityare poorly constrained. We present evidence for a previously unknown major extinction event in sharksthat occurred in the early Miocene, ~19 million years ago. During this interval, sharks virtually disappearedfrom open-ocean sediments, declining in abundance by >90% and morphological diversity by >70%,an event from which they never recovered. This abrupt extinction occurred independently from any knownglobal climate event and ~2 million to 5 million years before diversifications in the highly migratory,large-bodied predators that dominate pelagic ecosystems today, indicating that the early Miocene was aperiod of rapid, transformative change for open-ocean ecosystems.
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